Physical Geology Lecture GEOL 1303 Study Guide for Exam I I. Define Geology “Geology” means: Geos = Greek for “Earth”, and Logos = Greek for “the study of” Geology is considered an “Eclectic” science, drawing upon information from: Chemistry Physics Astronomy Biology Mathematics Ecology Etc. II. Two Main Subdivisions of Geology: 1. Historical Geology (usually 2nd semester) - centered around Earth’s History Stratigraphy – the study of the strata or layers of the earth Sedimentology – study of deposition of eroded earth materials Paleontology – study of ancient life through the interpretation of “Fossils” (remains or indications of past life that must be at least 10,000 years old) Geochronology – the science of dating the earth materials and events throughout earth’s history Geo-ecology – interpreting the past ecosystems of the earth: climate, sea level, plant and animal life, etc. Tectonics & Mountain Building Processes – the study of the movements of the earth’s crustal plates, and how these changes affect surface processes and Life. Historical Geology is concerned with explaining the “history” of the earth in aspects usually concerning the formation of continents, oceans, etc. and how those processes effected and still effect Life on the earth. 2. Physical Geology – centered around the Chemical and Physical aspects of the earth Geochemistry – the chemical makeup of magma, lava, minerals, rocks, etc. Mineralogy – the study of the chemical makeup and occurrence of minerals Petrology – the study of the formation of rocks (which are comprised of minerals) Vulcanology - the study of volcanics Seismology – the study of seismic (earthquake) waves Seismic Tomography – the study of the interior of the earth indirectly by studying the behavior of seismic waves Tectonics – the study of the formation of the continental plates and the mechanics of their movements Oceanography – the study of the chemical and physical aspects of the earth’s oceans Glaciology – the study of the cause and occurrence of glacial episodes Weathering & Erosion –the disintegration or physical and chemical breakdown and subsequent transportation of earth materials Geomorphology – the study of the creations of landforms Soil Sciences – the study of the formation of the various soil types of the world Economic Resources – the study of the formation and usage of natural resources: petroleum, natural gas, coal, stone materials, etc. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ III. The Role of the Geologist : To understand and define: The structure and composition of the earth All facets of magma, lava, and volcanic activity Minerals and rock types Surface processes: rivers, streams, glaciers, etc. The earth’s past: both structure origin and life’s evolution with Paleontological investigations (studying fossil remains) Geologic features of other planets Also: To use information learned to find fossil fuels and ores To learn how to preserve the environment: erosion control, pollution control Geologic information ties in greatly with advances in technology IV. The Development of “Scientific Thought”: Approximately 10,000 years ago, the development of agricultural practices and beginnings of the domestication of livestock set the stage for humans to become more “GREGARIOUS” (= living together in one or two semi-permanent area) This gave rise to the development of villages-towns-cities in which people would share the workload, which increased the amount of personal “LEISURE TIME”. With this extra time came the development of aspects of what we refer to as “CIVILIZATION”. As increased technology developed in certain areas in the world, populations grew, increasing the sharing of the workload, increasing the leisure time, increasing the development of new ideas and technologies, etc. Ancient Grecian, Roman, and Egyptian cultures began to develop rudimentary sciences/technologies, but advances especially in mathematics sped up these changes. Contributions of Ancient Greeks such as Socrates (470 BC – 399 BC), Plato (427 BC – 347 BC), & Aristotle (384 BC – 322 BC) laid down the foundations of Philosophy, Mathematics, and Metaphysics. “Aristotelian Thought” centers on the metaphysical concept that reality (the earth) is surrounded by “spheres of the heavenly bodies” called “Celestial Spheres”. [This idea is thought to have arisen from the impression that the night sky gives an observer even today. It is the feeling of being inside an upside down bowl, or “half of a sphere” that makes up the night sky.] Since heavenly bodies were thought to be the only “true” reality, and the earth’s reality experienced by Aristotle was only a shadow of the “true Celestial Sphere Reality”, Aristotle proposed that there was no need to investigate the earth per se as to its make-up since it was only an illusion of sorts. Circular motion was considered to be restricted to the Celestial Spheres (due to the apparent “circular” motion of the night stars), and therefore impossible to exist on the surface of the earth. Any appearance of circular motion in everyday life was only an illusion according to Aristotle. The Celestial Spheres model of reality is one of the reasons that the “Ancients” (Greeks, Romans, & Egyptians) never formulated advanced studies of chemistry or earth sciences. Since the “lay persons” considered Aristotle somewhat infallible, this concept of Celestial Spheres was not challenged for almost 1200 years. As people ventured out to various parts of the world seeking trade, gold etc., certain new ideas arose to challenge Aristotelian thought. During the so-called “Dark Ages”, Marco Polo (1254 AD – 1324 AD) in his journeys brought back gunpowder from the east, and artillery (cannons and guns) soon developed in the west. As trajectory studies of cannon balls and other projectiles advanced, it was proven that circular motion does indeed exist on the earth. This was the first of many proofs that Aristotle’s Celestial Sphere concept was flawed. As Aristotelian thought gave way to a sophisticated “trial and error” learning process, modern scientific thought had its beginnings. With the rise of great scientific visionaries such as Nicolaus Copernicus (1473 – 1543), Johannes Kepler (1571 – 1630), and Galileo Galilei (1564 – 1642), the basics of planetary motion were developed that began to infringe on some of the statements made in the Bible. This caused the Catholic Church at the time to try to sequester scientific investigation. The “Age of Enlightenment” or “Aufklarung” around the beginning of the 17th century ushered in a rapid growth in the arts, sciences, and mathematics. Sir Isaac Newton (1642 – 1726) developed the Calculus that allowed a greater investigative power in mathematics. There are countless others, but by the mid 18th century scientists and researchers such as James Hutton, Charles Lyell, William Smith, Gottlob Werner, Cuvier, Alfred Wallace, Gregor Mendel, and Charles Darwin laid the foundations of modern geology. The development of the modern “Scientific Method” arose from the trials of these founding fathers of science. V. The Scientific Method: “A way of looking at and describing reality” 1. State the Problem - To solve any problem it must be clear as to what actually needs to be solved. 2. Construction of a Hypothesis - By studying the problem, an educated guess may be formed as to creating a model of investigation. 3. Experimentation, Testing, and Data Gathering – the experimental model is tested and records of the results are kept and compared. The results then may be published and sent worldwide for scrutiny by others. 4. Development of a Theory (Factual Level of Science) If an experimental model tends to be correct after extensive testing and review time and again; it may then be considered a Theory. The Theory of Gravity, the Theory of Light Refraction, Atomic Structure Theory, etc. are all considered to be scientific facts. A fact in science is only considered to be true until it is disproved sometimes in the future. It makes no sense to say that Evolution if not to be believed because that it is “only a theory”. It is “only a theory” that gravity or light behave the way they do. To the Lay Person, theory means a lack of knowledge. This is not the case in science. Theory in science is considered “FACT”. Most religious dogmas, whether they are Judeo-Christian, Buddhist, Hindu, etc., are based upon some form of religious writings: the Bible, Torah, Koran, etc. Religious writings provide a “Fact Level” for describing reality and it is accepted as true because of its Divine nature. Creationist scientists then try to find data to support “religious facts” in the writings. This is tantamount to working the scientific method backwards. End of Introductory Materials Universe Beginnings I. The “Big Bang” Event occurring 10 – 15 BYA attributed with the “creation” of the universe, including all matter, energy, 3-diminsional space, and time. There is no “before” the Big Bang because there was no “time” as we know it until the Big Bang. It starts as an explosion of energy from a single point that expanded outward (and is still expanding outward according to many physicists), creating 3-diminsional space into which energy and subatomic particles were released. Gravity is a function of mass…the greater the mass, the greater the gravitational attraction. 300,000 years later, the universe was still expanding but was cool enough to form atoms. As a few subatomic particles collided and fused, a denser, more massive structure was formed that had more gravity that attracted more particles, increasing its mass and gravity…and so on…to the point that the first atoms of the element Hydrogen was created. At first, the universe was 100% Hydrogen. Today, by weight, Hydrogen and Helium comprise 98% of the known universe, the remaining 2% comprising the rest of the elements As more and more hydrogen atoms were created, the hydrogen began to accrete forming larger and larger masses of hydrogen, having larger and larger gravitational forces. As gravitational forces increased, hydrogen atoms began to be forced (fused). 4 hydrogen atoms would fuse to form 2 helium atoms (atomic fusion) releasing a tremendous amount of energy in the form of heat, light…the entire electromagnetic spectrum. This is the birth of the first stars. This fusion reaction is the core reaction of today’s hydrogen bombs. In essence, stars are huge balls of hydrogen that are constantly converting hydrogen into helium and other elements. As stars “use up” or convert most of their hydrogen into helium, the helium began to fuse forming nitrogen, carbon, oxygen, etc…the 92 naturally occurring elements. If the star is like the size of our sun, a “main sequence star” (run of the mill, common type) and begins to run out of hydrogen (or starts to “die”), it collapses inward from its own gravity and then begins to swell to a very large size. This event is called a Nova. It then collapses under its own mass and forms a small “Brown Dwarf” star, and fizzles out. The entire process from start to finish of the life of a main sequence star is about 10 billion years. Another type of star is the “Red Giant”, many times the size of a “main sequence” star. It also fuses hydrogen, but at a much faster rate – twice as fast or faster. When a Red Giant star dies, it collapses in on itself and then violently explodes in an event known as a Super Nova. The elements formed during the life of a Red Giant are then thrown outward into space forming clouds of dust called Nebulae. The particles of elements in the clouds of stardust then start to accrete as in the Big Bang. The hydrogen can accrete and form a Protostar that warms the clouds of dust. Later, the other masses of elemental materials are pulled inward toward the new star forming globs of hot, molten matter called Planetesimals, the first stage of the birth of a planet. II. The Birth of our Solar System Our sun, “Sol” formed from nebular stardust as mentioned above around 5 billion years ago, and has enough hydrogen left for another 5 billion years. As our sun was forming, other smaller clumps of matter accreted forming the planetesimals that began to circle around the sun. These would eventually become the 9 planets of our solar system: 1-Mercury, 2- Venus, 3- Earth, 4- Mars, 5- Jupiter, 6- Saturn, 7- Uranus, 8- Neptune, and 9- Pluto. The first 4 planets are called “Rocky” or “Terrestrial Planets.” The next 4 planets are called “Gas Giants” or “Jovian Planets” The 9th planet Pluto is an anomaly. It is the smallest of all planets and may have been trapped by our solar system at a later time. III. The Effects of Solar Winds on our Solar System Solar Wind is the accumulative force of all of the electromagnetic energy (light, magnetism, radiation) released from the sun (or any other star) It is thought that all of the planets started out possessing thick atmospheres of hydrogen, helium, methane, ammonia, CO2, and water vapor. Solar winds are thought to have blown away the volatile thick atmospheres of the 4 inner planets leaving behind their rocky cores (Mercury, Venus, Earth, & Mars). It is thought that the Jovian Gas Giants are far enough away not to have been effected as much by solar winds, as they still possess their volatile atmospheres. It is also thought that at the center of the gas giants is a rocky core (Jupiter, Saturn, Uranus, & Neptune). The actions of solar winds is the reason that the solar system of today has 4 Inner Terrestrial (Rocky) Planets, and 4 Outer Jovian Planets, with the 9th planet being the anomalous Pluto. Structure of the Modern Earth I. The Earth consists of three concentric layers: 1. the core 2. the mantle 3. the crust They are formed as a result of: density differences between the layers variations in composition differences in temperature and pressure II. The Characteristics of the Core: The Core is thought to be composed of iron with some nickel. It is spherical in shape with its outer surface lying 2900 km below the outer surface of the earth. The total diameter of the core is 3470 km. It has an average density of about 10 to 13 grams/cm3 and comprises 16% of the earth’s volume. Seismic Tomography data (studying the earth’s interior indirectly by studying the behavior of earthquake waves) indicate that the core has a small Solid Inner Region (1220 km in diameter) surrounded by an apparently Liquid Outer Region (2250 km thick). III. The Characteristics of the Mantle: The Mantle surrounds the core and comprises about 83%of the earth’s volume. It is less dense than the core with an average density of approximately 3.3 – 5.7 grams/cm3. It is composed largely of Peridotite, a dark, dense, igneous rock containing high amounts of iron and magnesium. The Mantle can be divided into three regions: 1. The Lower Mantle – This is solid and comprises most of the volume of the earth’s interior. 2. The Upper Mantle – This consists of the Asthenosphere and the overlying solid mantle rocks up to the base of the crust. The asthenosphere surrounds the lower mantle and has the same peridotite composition. It behaves plastically and slowly flows. Partial melting within the asthenosphere generates Magma, molten rock material, some of which rises to the surface because it is less dense than the material from which it was derived. 3. The Lithosphere – This is the solid portion of the upper mantle and the overlying crust. The lithosphere is broken into numerous pieces called Plates that move over the asthenosphere as the result of underlying Convection Cells (or Mantle Plumes generated from heat). IV. The Characteristics of the Crust: The Crust is the outermost layer of the earth. It consists of two types of rock materials: 1. Continental Crust – (20 – 90 km thick) this material comprises most of the continental plates. It has a density of 2.7 grams/cm3 and is rich in silica and aluminum. This type of rock material is referred to as being “Sialic” or “Felsic”. 2. Oceanic Crust – (5 – 10 km thick) has a density of 3.0 grams/cm3 and is largely comprised of the igneous rock Basalt, which is rich in iron and magnesium. This type of rock material as referred to as being “Mafic” or “Basaltic”. V. The Refinement of the Earth’s Crust The early outer crust of the earth was a mixture of sialic and mafic material. The following processes occurred to separate the sialic materials from the mafic. This allowed for the formation of the continents that are mostly sialic (less dense granitic materials) in composition, from the denser, more mafic materials that today compose the oceanic crust. A. Separation of Mafic Materials from Sialic 1. Partial Melting – This is the process whereby hot mantle plumes rising up from the upper mantle heats (“partially melts”) the overlying mixture of mafics and sialics. This causes the denser mafic materials to separate downward, leaving the less dense sialic materials above…. Think of the early crust as being a block of wax (representing the less dense sialic material) with marbles (representing the denser mafic material) suspended in it. If this mixture of wax and marble is placed into a pan and heated up, the marbles (mafics) will sink to the bottom, leaving the wax (sialics) on top. 2. Fractional Crystallization – Mafic materials, being high in iron and magnesium, will crystallize at a higher temperature than sialic material, that is high in silica and aluminum. If the entire mixture of mafic and sialic material is heated to the point of melting and then allowed to cool, the mafic minerals will crystallize first, and being denser than the sialic material, will separate downwards in the melt from the still molten sialic materials on top…. Think of this as being similar to placing a cup of lead B-B’s (representing the mafic materials) and a cup of plastic B-B’s (representing the sialic materials) randomly in a vessel. Lead has a melting temperature of around 8000 F and plastic’s melting temperature, let’s say, is about 2000 – 3000F dependant upon the type of plastic. If the vessel is heated, as the temperature reaches the 3000F mark, the plastic B-B’s would become liquid, but the lead BB’s would remain solid. As the temperature surpasses 8000F, all of the B-B’s would be molten. Now allow the vessel to cool. As the temperature drops below 8000F, the lead will begin to solidify (or crystallize first) but the plastic would remain molten. Since lead has a greater density, it would move downward in the vessel. As the temperature drops below 2000F, the plastic would begin to solidify on top of the lead, completing the separation. B. Formation of Continental (Sialic) Plates: Continental Accretion As more crustal movement occurred, as the less dense sialic material was pushed against the denser mafic materials, the denser mafic material would become subducted or pushed downwards, underneath the sialic materials, thereby melting as it was subducted, the hot magma rising upwards through the sialic materials forming island arcs (“clumps” of sialic material). The formation of the island arcs perpetuated the refining processes of partial melting and fractional crystallization. As the less dense sialic “clumps” formed on the earth’s surface, spreading centers (divergent “cracks” in the earth’s surface) pushed the sialic materials together forming larger masses of sialic “chunks” in a process known as Continental Accretion. This caused a fusion of the early sialic materials into Sialic or Granitic Continental Plates. This also accounts for the composition of the continental plates as being High Grade Metamorphic Terranes (metamorphic rocks are formed under intense heat and pressure, the conditions during continental accretion). Other Aspects of the Earth VI. The Atmosphere Prior to 4.5 BYA – The atmosphere consisted of hydrogen, methane, ammonia, hydrogen sulfide, nitrogen, argon, and water vapor. 4.5 BYA to 3.0 BYA – The atmosphere consisted of nitrogen, argon, water vapor, CO2, and sulfur dioxide. 3.0 BYA to Today - The atmosphere consisted approximately of 78% nitrogen, 20% oxygen, with the remaining 1-2% argon, water vapor, CO2, and minor gasses. How is it known that the early atmospheres were composed as mentioned above? Where did the other gasses go? Where did oxygen come from? Evidences of the early atmospheres: 1. It is thought that all planets had at their formation atmospheres similar to the Jovian planets of today. Because of solar winds, the volatile gasses (hydrogen, methane, ammonia, hydrogen sulfide, sulfur dioxide, etc.) were blown off of the inner terrestrial planets, leaving the rocky core. By studying the composition of the atmospheres of the Jovian planets today, geologists can derive the conditions of earth’s early atmosphere. 2. Banded Iron Formations – In certain areas there have been igneous activity resulting in formations of layers of iron interspersed between layers of other materials (i.e. silica) that have formed at the earth’s surface. Those banded iron formations that date before 3.0 BYA are composed of elemental, un-oxidized iron, indicating that they formed in an atmosphere devoid of free oxygen. Those banded iron formations that date younger than 3.0 BYA consist of iron that is oxidized throughout. This indicates that these younger iron layers formed in an atmosphere rich enough in oxygen to cause the complete oxidization of the iron. Hence, prior to 3.0 BYA there was not much free oxygen in the earth’s atmosphere, yet after 3.0 these was. 3. Oxygen is not given off in substantial amounts by volcanic activity today…so where did it come from? One source is photochemical dissociation. This occurs whenever oxygen-bearing chemical compounds in the upper atmosphere are subjected to cosmic radiation (background radiation from the Big Bang, solar radiation, etc.) and break apart releasing their oxygen atoms. This accounts for some of the “free” oxygen in the atmosphere, but not all. The other great source of oxygen is photosynthesis. This is the process whereby plants or plant-like organisms take in CO2 and H2O, and in the presence of sunlight convert these compounds into sugars thereby releasing free O2 into the atmosphere. Photosynthesis is a series of chemical reactions that convert sunlight energy into chemical energy. The processes occur in the chloroplasts of plants and algae. The components for raw photosynthesis are water, CO2 , and light energy. The formula for photosynthesis is: light energy & chlorophyll 6CO2 + 12H2O ----------------------- C6H12O6 + 6O2 + 6H2O The oldest known photosynthetic organisms, and the oldest known fossils, are Stromatolites. These are inter-tidal bluegreen algae with the oldest to date is 3.6 BYA VII. The Hydrosphere and the Hydrologic Cycle Where did the free water on earth come from? As magma is formed within the earth, chemical compounds begin to bond eventually forming various compounds and minerals. Many of these compounds contain water – H2O as part of their makeup. Sometimes a crystal lattice (a tinker-toy like structure of bonding atoms) contains enough space within its 3-diminsional structure for mater molecules to “fit”. As magma rises to the earth’s surface and is released on the surface as lava, the water escapes as steam in a process known as out gassing. As the steam cools in the atmosphere, water precipitates into clouds of water vapor. As these clouds cool, they loose their water as rain or other forms of water precipitation upon the surface of the earth. This volcanic out gassing is the source of most of the free water that comprises the oceans, lakes, rivers, etc. Over the time of earth’s existence, volcanism has out gassed enough water to fill the low-lying areas forming the ocean basins. Rainwater is naturally acidic, having a pH of about 5.5 to 6.5. As it hits the rocks and minerals on the surface of the earth, it is a major source of weathering and erosion of earth materials. As it runs down to the low-lying areas, it accumulates. As the sun evaporates the water, it rises as water vapor, forms clouds and this Hydrologic Cycle continues again and again. VIII. The Biosphere: The Organization of Life on Earth The biosphere is the term for all of the living aspects of the earth. All life as we know it is composed of atoms of various elements. These atoms bond in various ways to form molecules. Certain molecules make up cells, or the basic unit of life. This is called so because the cell exhibits all of the aspects that we consider to be living (atoms and molecules are not considered to be alive). Certain cells work together to form tissues, and various tissues together form organs. Organs work in conjunction to form systems (circulatory, respiratory, muscular, etc.), and all of the systems together form the organism, the entity. All organisms of the same species in a geographic area are called a biologic community. All of the biologic communities in a geographic area are called a biologic population. All of the populations in an area interact with the abiotic (non-living aspects – soil, air, sunlight, etc.) to form an ecosystem. All of the ecosystems on earth interact to collectively form the ecosphere or biosphere. When an organism dies, certain bacteria and fungi (ecological decomposers) break down the organism back into molecules and atoms that are put back into the ecosystem for other organisms to use. The calcium in your bones came from foodstuffs (i.e. milk) consumed during your life. The milk containing the calcium came from the cow…the cow got the calcium from the grass consumed…the grass got the calcium from absorbing it from the soil…the soil formed from the weathering and erosion of calcium containing rocks and minerals, that came from the earth in the form of cooling magma or lava. Or the calcium in the soil could have come from the decomposition of the skeleton of some previous living organism…You may have calcium in your bones that once was incorporated into the skeleton of a dinosaur!!! Geochemistry and the Formation of Minerals Mineral – a naturally occurring crystalline solid substance with a definite (specific) chemical composition (or slight range of compositions), and a crystalline structure that reflects its atomic or molecular arrangement. Mineraloid – a substance that almost fits the definition of a mineral, and many times considered a mineral, but has a chemical composition that is a little too variable. (i.e.): Bauxite – (Hydrous aluminum oxide) is a mineraloid because of its variability of water content in its chemical composition. Some oxides of iron such as Limonite having a variable composition Snowflakes (frozen water) are technically true minerals. Rocks are solid materials comprised of minerals, so, technically, ice is a rock. Window Glass is not a mineral because it is first of all not naturally occurring, and secondly, it is amorphous, not having a crystalline structure at all. It is really a very viscous quasi-liquid. Minerals are comprised of elements bonded together. Element – a substance composed of all of the same atoms; it cannot be changed into another element by ordinary chemical means. There are 92 naturally occurring elements in nature, each with their own unique physical properties. Atom – the smallest fundamental unit of an element that still retains the unique properties of that element. Phases of Matter: Solid – a rigid substance that retains its shape unless distorted by a force. i.e. – minerals, rocks, iron, wood, ice Liquid – flows easily and conforms to the shape of the containing vessel, has a well-defined upper surface and a greater density than a gas. i.e. – water, lava, wine, blood, gasoline Gas – flows easily and expands to fill all parts of a containing vessel; lacks a well defined upper surface, and is compressible. i.e. – helium, nitrogen, air, water vapor Plasma – matter composed of charged ions. i.e. – the matter comprising solar flares. The Neils Bohr Model of the Atom The physicist Neils Bohr formulated the following model of the atom in the early 20th century. It consists of a central nucleus composed of proton(s) (positively charged sub atomic particles), neutrons (neutrally or non-charged particles that add to the mass of the atom), surrounded by electrons or negatively charged particles that encircle the nucleus in various energy fields, or levels. The Periodic Table of Elements Elements are arranged on the Periodic Table of Elements according to their atomic make up and physical properties. They are numbered according to their atomic number and grouped into “families” according to their reactivity. Atomic Number – (of an atom) – This is the number of protons (positively charged particles) in the nucleus of an atom. Hydrogen has an atomic number of “1” because it has only one proton in its nucleus. Helium has an atomic number of “2” because it has two protons in its nucleus….Uranium has an atomic number of 92 because it has 92 protons in its nucleus. The elements with atomic numbers past 92 are elements that are only made under special circumstances usually under laboratory conditions. Some of the elements past atomic number 92 may exist in nature but are extremely rare. Mass – pertains to the quantity of matter that an object contains. Weight - a function of gravity in formulating and measuring the attraction of an object towards another…Weight can vary due from the gravitational attraction of one mass to another. Mass does not vary. I.e. – if you weigh 120 lbs on earth, you would weigh 1/6th of that on the moon (20 lbs) because the moon has less mass than the earth, therefore less of a gravitational attraction, but your mass would remain the same. Atomic Weight – It has been determined that hydrogen, the lightest of all elements, has a weight of 1.67 X 10-24 grams, or 0.000,000,000,000,000,000,000,001,67 grams. Since this measurement is inconvenient, the relative weights of atoms is used, rather than the actual weights. The relative weights of the atoms of different elements are known as the ATOMIC WEIGHTS and are proportional to the actual weights of the atoms, when compared to the atomic weight of the common element carbon-12 isotope, which is 12.011, on an arbitrary scale. Hence, carbon atoms “weigh” about 12 times that of hydrogen atoms. The atomic weight of oxygen atoms is 15.999, or about 12 times that of hydrogen, having an atomic weight of 1.0079. Atomic Mass Number – is the sum of the protons and neutrons in the nucleus (the mass of the electrons is negligible to the mass of an atom). The atomic mass number is primarily used in higher chemical reactions and will not be utilized here. Equilibrium and Entropy One of the physical aspects of our reality is entropy. This is the tendency of matter to “want” to be at its lowest energy level, or at equilibrium (rest). This can also be described as the tendency of water “wanting” to flow downhill, a stacked, ordered pack of playing cards thrown into the middle of a room to “scatter randomly”…things in our reality tend to “seek” their lowest energy level. Atoms seek the same. They usually seek the lowest energy level possible…closest to being at rest. Atomic Structure and Bonding Atomic Structure – The central core of an atom is called the nucleus. It is composed of: protons (positively charged particles) neutrons (electrically neutral particles) Common hydrogen has no neutrons. Surrounding the nucleus in various energy levels or shells are: electrons (negatively charged particles) Except for hydrogen, which has only one proton and one electron, all other atoms have only two electrons in the first or innermost energy shell. The other shells contain various numbers of electrons, but the outermost shell never has more than eight electrons. It is the electrons in this outermost shell that are usually involved in chemical bonding. The Atomic Number of an Element – is the number of protons in its nucleus. Hydrogen has an atomic number of 1, it has 1 proton in its nucleus…Helium has an atomic number of 2, and it has 2 protons in its nucleus…and so on. Naturally Occurring elements – There are 92 naturally occurring elements in nature, and are numbered accordingly as to their atomic number: hydrogen – 1, helium – 2, …uranium – 92. The elements on the periodic table past 92 (93 – 109?) were discovered under laboratory conditions and are not thought to occur in abundance in nature. Native Element – Some elements are found in nature in their pure, elemental form such as gold, silver, sulfur, and others. In geology, these are called Native Minerals. Entropy – The randomness or amount of disorder in a system. Things in our universe tend to want to be at rest, or at their lowest equilibrium…water runs downhill, a stacked deck of playing cards thrown into the air scatter… Atoms, also being matter, “want” to be at their lowest energy state. Since we live is a dynamic, changing universe, this complete “rest” cannot always be the case. So think of the actions of atoms as doing the “best they can do” to reach the lowest equilibrium for the time being. If an atom has 2 protons (each a positive charge) in its nucleus (this would be helium), it would be “at rest” with 2 electrons in its shells, giving it an overall charge of “0”. If it were to loose an electron (a negative charge) its overall charge would be changes to +1. Conversely, if it gained an extra third electron, its overall charge would be –1. An atom that has lost or gained an electron is called an Ion. If the ion is (+) it is called a Cation. If the ion is negative, it is called an Anion. Ion– This is an atom that has either lost or gained electrons, changing its overall charge. The periodic table has the known elements arranged in order as to their atomic number, and to their types or to their reactivity. Look at the periodic table in your text. The elements in the upper right hand side of the periodic table, centered around the element F (or fluorine) have atomic configurations that tends to allow them to receive an extra outer shell electron, causing them to have an overall negative charge under certain circumstances. This tendency to become negative is called electronegativity and the resulting ion is an anion. The elements in the lower left and left side of the table centered around Fr (or francium) have atomic configurations that allow them to loose an outer electron causing them to become positively charged. This tendency to become positive is called electropositivity forming ions that are cations. Atomic Bonding – the interaction of electrons around atoms can result in two or more atoms joining together. Ionic Bonds Positively charged cations are attracted to negatively charged anions because of the charge difference. Ionic Bonds are formed by this attractiveness between the cation and the anion (i.e. sodium chloride salt – the sodium is electropositive giving up an electron, symbolized Na+1, and the chlorine is electronegative, symbolized Cl-1, receiving the extra electron. Ionic bonds are commonly between a metal cation and a nonmetal anion such as NaCl – table salt (Halite), Fe2O3 – iron oxide (Hematite), etc. Covalent Bonds If certain nonmetallic atoms bond, such as Si (silicon) and O (oxygen), they tend to have their outer electron shells overlap, resulting an a sharing of electrons called a Covalent Bond in the mineral quartz or SiO2. Covalent bonds are generally much stronger bonds and contribute to the strength and overall hardness of the mineral. Metallic and Van der Waals Bonds Metallic Bonding – results from an extreme type of electron sharing. The electrons of the outermost electron shells of metals such as gold, silver, and copper readily move about from one atom to another. This electron mobility accounts for the fact that metals have a “metallic luster”, they provide good thermal and electrical conductivity, and are malleable or easily reshaped. (i.e. Copper is a good conductor of electricity and is easily made into wire.) Only a few minerals have metallic bonding. Van der Waals Bonds – some electrically neutral atoms and molecules do not have electrons available for ionic, covalent, or metallic bonding. Nevertheless, they have a weak attractive force between them when in proximity. This weak attractive force is the Van der Waals or residual bond. The atoms of carbon in the mineral graphite are covalently bonded to form sheets that are attracted to each other by Van der Waals bonds. This accounts for the softness of the mineral graphite and its ease of use in pencil leads or lubricants. Chemical compound - If two or more different elements bond together (ionic or covalent), the resulting substance is called a chemical compound. The Formation of a Mineral I.e. Halite – NaCl – sodium chloride As the sodium and chlorine atoms begin to bond, they are stacked to provide the smallest space possible. This threedimensional framework results is an overall neutral charge on the crystal. In halite, the sodium atoms are bonded in all directions with the chlorine atoms on all sides, with the chlorine atoms surrounded by the sodium atoms. The smallest 3-dimensional framework formed is called a unit cell of a crystal. As more and more unit cells connect to each other, a framework called a crystal lattice is formed. This continues to grow until there are no more unit cells in solution or other chemical conditions change. The resulting smooth planer surface formed at the terminus is called a crystal face. The shape of the crystal and the crystal habit (the usual crystal shape of a certain mineral) is set. In addition to containing atoms of a single element, many times minerals contain tightly bonded, charged groups of other elements known as radicals. Even though the radical is composed of different elements, it behaves as single units in a mineral. A good example is the carbonate radical formed when one carbon atom bonds with three oxygen atoms forming CO3-2, which acts as an ion with a minus 2 charge. Other common radicals include: the sulfate radical – SO4-2 (having a minus 2 charge) the hydroxyl radical – OH-1 (having a minus 1 charge) the silicate radical – SiO4-4 (having a minus 4 charge) As the magma or lava is cooling and containing many minerals starting to form, the crystals do not have a chance to form wellformed crystals because of the proximity of the other minerals crowded together. This results in the mosaic look of igneous rocks such as granite. Interpreting Chemical Formulas Some minerals have simple compositions such as NaCl, sodium chloride, where there is a one-to-one ratio of sodium atoms to chlorine atoms. Others have a more complex composition such as orthoclase feldspar – KAlSi3O8. This means that orthoclase is comprised of potassium, aluminum, 3-silica atoms (hence the “Si3” subscript), and 8 – oxygen atoms. It is read as “potassium aluminum silicate” with the “Si3O8” being the radical, acting as an anion in itself. The definition of a mineral states “minerals have a specific chemical composition, or a slight range of compositions”. Sometimes other atoms can be substituted in a formula such as iron sometimes can inter-substitute for magnesium because of their similar sizes and charges. The mineral Olivine is (Mg,Fe)2SiO4 meaning that it may be found as magnesium silicate (called Fosterite), iron silicate (called Fayalite), or a combination of both. Common Elements in the Earth’s Crust (Listed in order of abundance) Element Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium All others Symbol O Si Al Fe Ca Na K Mg % by weight 46.6 27.7 8.1 5.0 3.6 2.8 2.6 2.1 1.5 % by atoms 62.6 21.2 6.5 1.9 1.9 2.6 1.4 1.8 0.1 Oxygen and silicon together constitute more than 74% by weight of the atoms of the earth’s crust, and nearly 84% of the atoms available to form compounds. Mineral Groups Each mineral group contains members that share the same type of negatively charged ion or radical. Mineral group Composition Carbonates (CO3)-2 Halides Cl-1, F-1 Native Element ------- Oxide O-2 Silicate (SiO4)-4 Sulfate (SO4)-2 2H2O Sulfide S-2 Anion Examples Calcite Dolomite Halite Fluorite Gold Silver Sulfur Diamond Graphite Hematite Magnetite Quartz K-Spar Olivine Anhydrite Selenite CaCO3 CaMg(CO3)2 NaCl CaF2 Au Ag S C C Fe2O3 Fe3O4 SiO2 KAlSi3O8 (Mg,Fe)2SiO4 CaSO4 CaSO4 + Galena Pyrite PbS FeS2 The Silicate Minerals A combination of silicon and oxygen is known as “silica”. Quartz is pure silica (SiO2) because it is entirely composed of oxygen and silicon. Silicate minerals include about one-third of all known minerals Approximately 95% of the earth’s crust is composed of silicates. The basic building block of all silicates is the silica tetrahedron consisting of one silicon atom and four oxygen atoms. This forms a four-faced pyramidal structure called a tetrahedron. The silicon atom has a positive charge of +4, and each of the four oxygen atoms has a negative charge of –2. Positive 4 plus (negative times 4) = negative 4…So, the silica tetrahedron is written as: (SiO4)-4 Because of its negative charge, it does not exist in nature as an isolated group. It combines with various positively charged cations. Other silicate minerals have the addition of one or more additional elements: orthoclase – KAlSi3O8 olivine – (Mg,Fe)2SiO4 mica – KAl2(AlSi3)O10(OH)2 Five Arrangements of Silica Tetrahedra I. Isolated Tetrahedra Olivine, (Mg,Fe)2SiO4, an ultramafic mineral of the earth’s mantle with the negative charge of minus 4 of the radical (SiO4)-4 being offset by the positive 2 charge of iron (Fe) and magnesium (Mg), also positive 2 charge. II. Single Chains Silica tetrahedra may also be arranged in continuous chains whereby each tetrahedron shares two of its oxygen atoms with the adjacent tetrahedra. This results in a silicon to oxygen ratio of 3:1 as seen in the pyroxene mineral Enstatite, MgSiO3. The resulting net charge of each chain is -2, so to offset this, the parallel chains are linked together by magnesium atoms, Mg+2. III. Double Chains Double chains of silica tetrahedra in which alternate tetrahedra, in which two parallel rows, are cross-linked characterize the amphibole group. This results in a silicon to oxygen ratio of 4:11. The resulting net charge of the double chain is a minus six (-6) electrical charge. Mg+2, Fe+2, and Al+2 are usually the cations that link the double chains together, creating a net charge of zero. i.e. Hornblende(Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5(Al,Si)8O22(OH)2 IV. Sheet Silicates (Continuous Sheets) In sheet structure silicates, three oxygens of each tetrahedron are shared by adjacent tetrahedra. This results in continuous “sheets”, with a silicon to oxygen ratio of 2:5. The sheets also have a net negative electrical charge that is offset by positive ions located between the sheets. The micas, such as biotite and muscovite, and clay minerals are examples of sheet silicates. i.e. Muscovite Mica - KAl2(AlSi3)O10(OH)2 V. Three-Dimensional Network Silicates These form whenever all four oxygens of the silica tetrahedron are shared by adjacent tetrahedra. This sharing of oxygen atoms results in a silicon to oxygen ratio of 1:2, which is electrically neutral. i.e. Quartz – SiO2 The Two broad groups of Silicate Minerals 1. Ferromagnesium Silicates are those silicate minerals rich in iron (Fe) and magnesium (Mg). I.e. olivine, amphibole, pyroxene, and biotite mica. These are commonly dark colored and more dense (3 grams/cm3) than non-ferromagnesium silicates. Common in MAFIC ROCKS. 2. Non-ferromagnesium silicates are those silicate minerals lacking Fe and Mg. I.e. quartz, microcline, and muscovite mica. These are commonly light colored and less dense (2.7 grams/cm3) than ferromagnesium silicates. Common in SIALIC ROCKS. The Two Feldspar Groups of Silicate minerals 1. Potassium-Rich – The potassium feldspars represented by microcline and orthoclase (KAlSi3O8) are common in igneous, metamorphic, and some sedimentary rocks. They are typically light colored (pink, blue, light-green, or creamcolored). If they are in abundance in igneous rocks, this is an indication that the original magma (or lava) was NOT rich in abundant Fe and Mg, making the rock SIALIC or FELSIC. 2. Calcium/Sodium Rich – The plagioclase feldspars range from calcium rich (CaAl2Si2O8), to sodium rich (NaAlSi3O8) creating several varieties. They are typically white, cream colored, to medium gray. They can be distinguished from the potassium feldspars by parallel lines on the crystal faces called striations. When in abundance in an igneous rock it is an indication that the parent magma was intermediate to mafic in origin. The Carbonate Minerals These minerals contain the negatively charged radical (CO3)-2. Calcite (rhombohedral crystals) and aragonite (elongated prismatic crystals) are both CaCO3, but aragonite usually changes into calcite. These are the main component of the sedimentary rock LIMESTONE. There are many carbonate minerals but we will be concerned with only one other: dolomite (Ca,Mg) (CO3)2. This forms the rock DOLOSTONE from the conversion of calcite into dolomite by the addition of magnesium. Other Mineral Groups Oxides – an element is combined with oxygen. I.e. iron oxides – hematite Fe2O3 or magnetite Fe3O4 form major iron deposits in the world. Aluminum oxides (bauxite) form aluminum ores Sulfides – a positively charged ion bonds with sulfur (S-2). I.e. galena (PbS), iron pyrite (FeS2), sphalerite (ZnS), etc. Many of these sulfides of metals are important ore minerals. Sulfates – an element is combined with the anion (SO4)-2 forming minerals such as gypsum CaSO4 +2H2O. Halides – These minerals contain halogen (meaning “salt formers”) elements such as chlorine (Cl-1), fluorine (F-1), etc. Halite (NaCl) and fluorite (CaF2) are common halide minerals. Properties of Minerals Used for Identification Of the 3500 known minerals, no two minerals have the exact set of identifying properties. Crystal Habit – the typical form of the crystal of a mineral that is reflective of its internal atomic arrangement – For example: Halite, Pyrite, Galena form cubic crystals Micas form “book” crystals of thin sheets Quartz forms hexagonal di-pyramidal prisms Fluorite forms octahedra Cleavage – a property of minerals whereby they split or break along closely spaced, smooth planes. The number of these cleavage planes can vary from one mineral type to another mineral, but is consistent within an individual mineral: Quartz – No cleavage planes at all Muscovite, biotite – 1 cleavage plane (a sheet silicate) Feldspars, amphiboles – 2 cleavage planes Calcite – 3 planes not at right angles Halite, galena – 3 planes at 900 (“cubic cleavage”) Fluorite – 4 cleavage planes Hardness – a test for the relative hardness of one mineral compared to another. This uses the Moh’s Hardness Scale: 10 – diamond 9 – corundum 8 – topaz 7 – quartz 6.5 steel nail 6 – feldspar 5.5 window glass 5 – apatite 4 – fluorite 3 – calcite 3.0 copper penny 2.5 – 3.0 fingernail 2 – gypsum 1 – talc Luster – the description of how light is reflected from the surface of the mineral: Dull, earthy – i.e. ochre hematite Glassy – i.e. quartz Metallic – i.e. pyrite Pearly – i.e. talc Color – the visible color of the mineral to the eye. This is the least reliable property due to the fact that just a small amount of impurities can change the color of the mineral, but not the chemical composition. i.e. Varieties of Quartz – all being SiO2 rock quartz – clear amethyst – purple citrine – yellow rose quartz - pink Streak – the color of the powdered form of the mineral accomplished by rubbing the mineral across a streak plate (an unglazed ceramic tile). This is a more reliable indicator of the mineral in question. Specific Gravity – This is a ratio of the weight of a mineral to the weight of its volume in water; a measure of density. A mineral with a SG of 3.0 has an SG 3 times that of water. I.e. a cm3 of galena (lead sulfide) has a higher specific gravity than a cm3 of quartz. Special Properties: Magnetism – Some iron compounds are naturally magnetic such as magnetite. Reaction with Acid (effervescence) - Carbonate minerals react (fizz) in the presence of acid releasing CO2 Phosphorescence under UV light – Under UV light, many minerals phosphoresce or glow. Optical Properties – Minerals such as calcite doubly refract light passing through it. Igneous Rocks and Intrusive Igneous Activity Magma – a mobile, silicate melt formed in the upper mantle or lower crust, as much as 100 to 300 kilometers below the surface. It accumulates at depths in reservoirs called magma chambers. Magma chambers may be only a few km below the surface at spreading centers and below the oceanic crust or only a few tens of km below oceanic subductions or continental plates. Magma may slowly cool in place forming intrusive igneous rocks. Or, magma may breech the surface in the form of a volcano forming extrusive igneous rocks. Lava is the term for magma that has breeched the earth’s surface. Magma Types: Ultramafic Comprises the upper mantle (asthenosphere) Very low in silica (45% or less) & low viscosity Does not breech the earth’s surface usually Only forms intrusive igneous rocks High in ultramafic minerals such as olivines Forms dark greenish-colored rocks Basaltic or Mafic Magma Low viscosity; fluid-like and flows easily Low silica content (47 – 50% silica) Temperature range of 900 – 12000C High in mafic minerals: amphibole, pyroxene, olivine Forms dark colored rocks Intermediate Magma A “mixture” of mafic and sialic magmas Medium viscosity Silica Content of 50 – 59% Temperature range of 800 – 10000C May contain both mafic and sialic intermediate minerals Forms medium, grayish-colored rocks Felsic or Sialic Magma Higher viscosity, thicker, gooier High silica content (65 – 70% silica) Temperature range of less than 8000C High in sialic minerals: quartz, microcline, muscovite Forms light colored rocks Bowen’s Reaction Series A series of reactions based upon fractional crystallization. Mafic minerals have a higher point of crystallization and crystallize first, followed by intermediates, followed by sialics, as the magma cools. Discontinuous Branch – a succession of ferromagnesian silicates crystallize as the temperature of the magma drops: Olivine to pyroxene to amphibole to biotite. Continuous Branch – Plagioclase feldspars with increasing amounts of sodium crystallize. This leaves the higher silicate minerals to crystallize last, at the lowest temperatures: potassium feldspars, muscovite, and quartz. Effects of Silica Content on Magma Silica tetrahedra in magma link together to form polymers The more silica polymers, the thicker, gooier the magma The thicker, gooier the magma, the more explosive a volcano may be The more silica polymers, the more sialic the magma creating lighter colored rocks The more sialic the magma, the lower the temperature at which it crystallizes (less than 8000C) The reverse is true for more mafic magmas… The fewer the silica polymers, the thinner, less viscous the magma and the more free-flowing the magma The less viscous the magma, the less explosive the volcano The more mafic the magma is creates darker ferromagnesian minerals and darker rocks The more mafic the magma, the higher the temperature at which it crystallizes (800 – 10000C) Effects of Pressure on Magma In the earth, pressure on rocks (or a magma body) from the surrounding rocks (or overburden – the weight of the rocks above the structure) keeps it from expanding and prevents melting. A drop in pressure causes hot rocks to melt. An increase in pressure causes melting rocks to slow their melting. A decrease in pressure can arise from the erosion or removal of the overburden, causing the rocks to melt. An increase in pressure can arise from tectonic activity or an increase in pressure from the magma chamber. As magma is rising upwards through the rocks, pressure is decreasing as it nears the surface, preventing it from solidifying and thus possibly forming a volcano. Effects of Water Content on Magma Water lowers the melting point of magma “Dry Magmas” have a water content of less than 10% “Wet Magmas” have a water content of 10 – 15% water Water at high temperatures is very volatile At high temperatures water tends to escape as a gas (superheated water vapor) High pressure keeps water from escaping Cracks in the overburden may allow water to escape Sialic (Granitic) magmas usually solidify below the surface intrusively Mafic magmas being less viscous and many times reach the surface as lava The water content in most mafic magmas is very low: 12% Pressure keeps water from expanding Near “mafic” melts usually contain 1 – 2 % H2O, causing it to remain molten and it easily reaches the surface. Igneous Rock Textures The texture of an igneous rock refers to the size, shape and arrangement of the constituent mineral grains and reflects the rate of cooling. IGNEOUS ROCK TEXTEURES: I. Glassy Texture – “resembling man-made glass” having “concoidal fracture” possessing a “randomness” of crystal lattices Ions have no time to migrate to form crystals Indicates a rapid rate of extrusive cooling Example = Obsidian, Volcanic Glass II. Aphanetic Texture – “a” meaning “without”; “phaneros” meaning “visible” – overall meaning is that the rock has crystals, but they can not be identified with the naked eye. Tiny crystals have formed, but require magnification to identify Indicates relatively “quick” extrusive cooling but at a rate slower than that required for a glassy texture Cooled at, or near the earth’s surface. Example = Rhyolite, Andesite, or Basalt III. Phaneritic Texture – “Phaneros” meaning “vivible” – whereby there are large crystals present that are easily identified with the naked eye. An individual large crystal is referred to as a Phenocryst. The presence of phenocrysts indicates a slow rate of intrusive cooling occurring deep within the earth’s crust. Pegmatites are igneous rocks that cool extremely slow resulting in “giant” phenocrysts. Examples = Granite, Diorite, Gabbro, or Peridotite IV. Porphyritic Texture – This texture indicates that the igneous rock had two cooling periods – the first one slow and the second one quicker. The larger phenocrysts form during the first cooling period while the magma is at an intrusive depth in the crust. If it were to continue cooling at this deprt, it would have formed a phaneritic texture. Something occurred to move the still molten magma (containing the first formed phenocrysts) closer to the surface whereby the still molten material cools at a faster rate. This gives the rock two distinct crystal sizes: the larger phenocrysts that formed first are set in a finer “groundmass” (matrix) of either porphyritic, aphanetic, or glassy textures. Rock texture may be porphyritic/aphanetic, meaning that phenocrysts formed first and the still molten ground mass rapidly cooled. Or, the rock may be porphyritic/phaneritic meaning the the phenocrysts formed and the ground mass cooled having a phaneritic texture. Examples: Basalt Porphyry, Granite Porphry V. Pyroclastic Texture – Means “fire-broken” – formed from volcanic ejecta: the ash, cinders, and “bombs” expelled from eruptions of volcanoes. All pyroclastic textured rocks are extrusive being produced by explosive volcanic eruptions. Aside from any pre-formed crystals, pyroclastics are generally categorized as to the particle size: fine ash X < 0.06 mm coarse ash - 0.06mm – 2.0mm cinders 2.0mm – 64.0mm “bombs” X > 64.0mm Being thrown into the air and later settling out on the ground, they may cool before they settle out forming ”unwelded” pyroclastics, or they may remain glowing hot as they settle out forming “welded” pyroclastics. “Tuffaceous rocks” is the term for the rocks resulting from the settling of pyroclastic particles, and may result in the formation of “unwelded tuffs” or “welded tuffs”. Igneous Intrusive Bodies – “Plutons” Pluton – an intrusive igneous body that cools and crystallizes deep within the earth’s crust. The geometric shape of plutons may be: massive or irregular tabular cylindrical mushroom shaped Plutons are also described as to whether they are concordant or disconcordant. Concordant pluton - has boundaries parallel to the to the layering of the country rock (the surrounding rock) Disconcordant pluton – has boundaries that cut across the layering of the country rock. Pluton Types: I. Dikes – discordant intrusive bodies usually emplaced in preexisting fractures cutting across the country rock as the magma rises. Characteristics: They are discordant cutting across the layering of the country rock in zones of weakness, such as cracks. Most are 1 to 2 meters wide, but they can range from a few centimeters to more than 100 meters thick. They form whenever magma is forced into pre-existing fractures of the country rock, or when the fluid pressure in the dike itself creates its own fractures. Many can form “wall-like” structures radiating outward from some volcanoes like the spokes on a wheel. II. Sills – concordant intrusive bodies that are sheet-like emplaced between layers of the country rock. Characteristics: Sills are concordant emplaced whenever fluid pressure is so great that it lifts the overlying rocks, filling in with magma in a horizontal manner. They are tabular or disk-like in shape with many usually a meter or less thick. Some are much thicker, up to 300 meters or more (i.e. the Palisades of NY and New Jersey) Most have intruded into sedimentary rock, but many are also commonly found injected into piles of volcanic rock. Sill inflation prior to a volcano erupting may account for volcanoes swelling just before exploding. III. Laccoliths – sill-like in that they are concordant, but with a “mushroom shape”. Characteristics: Laccoliths are concordant mushroom-shaped intrusive bodies. They tend to have a flat floor with a domed up center. Like sills, they lift up the overlying strata of the country rock, but usually being larger than sills, the overlying strata bends to conform to the curved shape. They also are relatively shallow intrusions. IV. Volcanic Pipes and Necks – discordant cylindrical conduits of volcanoes. Characteristics: A volcanic pipe is the term for the actual conduit of magma upward from the magma chamber deep below. Through this structure magma rises to the surface. When a volcano ceases to erupt, surface processes begin to erode the cone while the once molten volcanic pipe solidifies. Whenever the solidified volcanic pipe is exposed by erosion, it is termed a volcanic neck. (i.e. ”Shiprock” in northwestern New Mexico) V. Batholiths and Stocks – are the largest of all plutons. Characteristics: These are very large intrusions created by repeated, forceful injections and voluminous intrusions of magma in the same area. Many times these intrusions continue for millions upon millions of years (i.e. the coastal batholiths of Peru took about 60 million years; and the Llano Uplift of Central Texas). To be called a batholith, the body must be greater than 100 km2 of total surface area. A stock is similar in formation but has a surface area less than 100 km2. Some stocks are simply parts of large plutons that once exposed by erosion are batholiths (the (tip of the iceberg”) Most are granite in composition, but some may be diorite. (mostly sialic magmas, with some intermediate magmas) Most are formed near continental margins during episodes of mountain building or great uplift (during an orogeny or tectonic activity). As the solutions at the tops of the intrusions penetrate cracks in the overlying strata, concentrations of minerals dissolved in the solutions may become concentrated. (i.e. gold, copper, silver) Granitization – the process whereby the surrounding country rock is transformed into granite in a severe form of metamorphism. This may account for the great amounts of granite formed in some batholiths that shows a gradation from granite into some other rock at the borders. Some batholiths show a direct igneous origin of its granite since its borders are “sharp” at the transition from granite to country rock. The presence of inclusions of country rock especially at the top of batholiths indicates that it was igneous in origin. Volcanism Volcanism – the process whereby lava and its contained gasses, and pyroclastic materials are expelled upon the earth’s surface, or into the earth’s atmosphere. Volcano – a conical mountain formed around a vent where lave, pyroclastic materials, and gasses have erupted. One purpose of volcanoes is to help rid the interior of the earth of excess heat buildup. Volcanoes and Religion – Native Americans of the northwest tell of a titanic battle between the volcano gods Skel and Llao accounting for the huge volcanic eruptions that occurred ca. 6600 BP. In Hawaii, Pele is the goddess residing in the crater of Kilauea responsible for the eruptions and earthquakes there. Ancient Greeks believed that the god Pluto or Vulcan was responsible for eruptions there. Active Volcanoes – There are approximately 550 active volcanoes on earth today. (i.e. Mt. St. Helens, Mauna Loa, Kilauea, etc.) At any one time there are about 12 volcanoes erupting somewhere on earth. Dormant Volcanoes – There are numerous volcanoes that have erupted in the recent geologic past and probably will erupt again in the future. Mt. Vesuvius in Italy, Mt Pinatubo in the Philippines Extinct Volcanoes – These are volcanoes that have erupted in historic times but show no sign of erupting again. Volcanic Gasses – The gasses released from the magma as it moves upward. In sialic magma, expansion if restricted due to the high viscosity (high silica content) and gas pressure increases greatly causing explosions releasing ash and other pyroclastics. In mafic magmas, expansion occurs due to the low viscosity (low silica content) allowing gasses to expand and escape easily…creating a quieter eruption. Composition of volcanic gasses – 50 – 80% of all volcanic gasses are water vapor, with lesser amounts of CO2, N2, and sulfur gasses – sulfur dioxides and hydrogen sulfides. Very small amounts of CO, H, and Cl are released. The Blue Haze Famine – Iceland, 1783 – gasses (probably sulfur dioxide) escaped from the Laki Fissure, causing 75% of livestock to die. The gas caused the overall temperature to drop causing crop failures causing 25% of the human population to die. Cameroon, Africa, 1986 – A cloud of volcanic CO2 was emitted from under Lake Nyos (that sits atop a volcano) killing by asphyxiation ALL ANIMAL LIFE for miles around, including 1746 humans. ++++++++++++++++++++++++++++++++++++++++++++++++++ Lava Flows I. Sialic (high viscosity flows) - tend to be thicker, “lobe-shaped flows with distinct margins. II. Mafic (low viscosity flows) – tend to be comparatively thinner, fluid flows that are widespread. III. Types of Lava Flows – (Hawaiian Terms) 1. Aa – a flow characterized by a surface consisting of rough, angular, jagged blocks and fragments. This is due to a higher silica content than: 2. Pahoehoe – a flow characterized by a smooth, ropy surface, almost like taffy. This is due to relatively lower silica content than Aa. Speed of Lava Flows Not very quick…the fastest low viscosity in Hawaii ever measured had a speed of 9.5 kilometers/hour. Faster speeds occur whenever the lava flow is insulated on all sides forming a lava tube or conduit. After the eruption and the lava tube drains, sometimes the roof of the tube collapses forming a skylight into the tube. IV. Pressure Ridges and Spatter Cones Pressure Ridges – As the surface of a flow begins to solidify, pressure from the flow causes the surface to buckle, forming ripples of sorts called pressure ridges. Spatter Cones – Gasses escaping from the flow hurl globs of lava into the sir. These globs fall back to the surface and stick together forming small, steep-sided spatter cones. These may rise several meters above the flow. V. Columnar Jointing Common in flows that are relatively “thick” – several tens of meters to 100 meters thick. They form because of differential cooling of the flow – the outer surfaces “freeze” while the inside is still molten. This results in the vertical “splitting” of the flow into roughly pentagonal prism-like columns – hence “columnar jointing”. Examples include: Devil’s Postpile National Monument, California, Devil’s Tower, Wyoming. VI. Pillow Lava – Bulbous masses of basalt that resemble “pillows” result from mafic magma cooling under water. This is VERY common at the submarine spreading centers such as at the MOR (mid-oceanic ridge) Anywhere pillow lava is found on the surface of the earth indicates that there was mafic, basaltic lava that cooled underwater. VII. Pyroclastics Meaning “fire-broken” (pyros = fire; clastic = broken) – these are all of the volcanic ejecta or materials thrown from the volcano during the eruption. These pyroclastics are categorized as to size: fine ash – X < 0.06mm coarse ash 0.06mm – 2.0mm cinders 2.0mm – 64.0mm bombs X > 64mm Sometimes ejecta in the size range of 2.0mm – 64.0mm is termed lapilli. Ash in the atmosphere is dangerous to the flight of commercial jetliners. Since 1980 about 80 jetliners have been damaged by flying into volcanic ash clouds. It causes their engines to seize up. In 1989 over the Redoubt Volcano Alaska, KLM Flight 867 developed clogged engines from ash and fell 3 km before the crew restarted the engines! VIII. Distinguishing Flows from Sills in Cross-section Chilled Margin – The solidified outer edge of a lava flow or an intrusive body such as a sill. As a lava flow flows across the surface of the earth, the first to solidify is the “margin that touches the relatively cold ground. In a sill, the “chilled margin” encircles the intrusion since it “froze” at all contacts with the country rock. Gasses that are released in a flow tend to form “bubbles called “vesicles”. In a flow, these vesicles accumulate at the top of the solidification much like the foam of a beer poured into a glass. Vesicles usually are not as abundant in a sill since it cooled intrusively. Altered Country rock – Upon coming into contact with the surface of the ground, a flow usually alters the country rock on the bottom surface only, whereas a sill alters the country rock on all edges. IX. Flood Basalts – Occur as high volume, low viscosity, mafic flows over a broad, flat area, many times resulting in “flood basalts’ up to 100 meters thick or greater. Columnar jointing is common. Lava “Plateaus” may be formed. X. Anatomy of a Volcano Supplying the volcano is a magma chamber deep beneath the surface vent where lave begins to pour forth.. Magma reaches the volcano via a volcanic pipe or conduit. The depression at the top of the volcano is the crater or main vent. Along the side, there may be smaller conduits forming side vents or side cones. Long slits may occur along the sides of the cone forming fissures or fissure eruptions. XI. Types of Volcanoes 1. Shield Volcano Characteristics: Usually mafic resulting in forming basaltic rocks Low viscosity of the magma due to low silica content Low slope angle of the cone – usually 6 - 120 Many fissure eruptions, lava tubes, conduits, etc. Typical of the Hawaiian Islands or MOR volcanoes. 2. Cinder Cone Characteristics: Quickly formed (sometimes overnight) Symmetrical cone with relatively steep sides Usually less than 300m high Composed entirely of pyroclastic materials (i.e. cinders and ash) Generally mafic to intermediate in composition Erodes away easily and quickly 3. Stratovolcano or Composite Cone Characteristics: Formed by an alternating series of lava flows followed by pyroclastic flows. Usually intermediate to sialic magmas. Very explosive due to their high silica content and viscosity. I.e. Mt. St. Helens Many times after going extinct, the cone erodes away leaving the volcanic neck or volcanic pipe exposed. This may contain the rock Kimberlite associated with the formation of diamonds and other gems. XII. Violent Magmas – Ash Flow Tuffs and Calderas Characteristics of Violent Magmas: “Dry”, granitic magmas less than 10% H20 Will not solidify before reaching the surface Volatiles separate from magma and form a frothy mixture of super hot gasses and magma Ensuing eruption is extremely violent “Nuee ardente” super hot gas and ash flow clouds are produced Many pyroclastics are produced – pumice, tuffaceous rocks, etc. Calderas are formed. This occurs after the eruption, whereby the top of the volcano falls back into itself creating a circular depression. These create very good mining districts for precious metals. I.e. Creede, Colorado. XIII. Hot Spots Characteristics: Bodies of magma that have risen near the surface creating a localized zone of melting below the lithosphere. This results in surface eruptions over long periods of time. I.e. Hawaiian Islands, Yellowstone National Park The Hawaiian Island chain is composed of islands dating from: Hawaii (The Big Island) – 0.7 MYA to today Maui – 0.8 – 1.3 MYA Molokai – 1.3 – 1.8 MYA Oahu – 2.3 – 3.3 MYA Kauai – 3.8 – 5.6 MYA These islands are also listed as to size: Hawaii the largest, and Kauai the smallest. Hawaii is over the hot spot and is active. The other islands have either dormant or extinct volcanoes. The Pacific Plate has been slowly moving to the northwest where 5.6 MYA Kauai was over the hotspot and was active while the other islands had not even formed yet. There is a new volcanic seamount (an underwater volcanic mountain) called Loihi is forming and is 940 meters below the surface. It will someday take the place over the hotspot and become the largest island, while Hawaii erodes smaller as the other islands have done.