Study guide for test 1, GLG101, S05
Chapter 1
You should be able to:
Briefly define geology as a science.
Explain the relationship between geology, people, and the environment.
Discuss the history of geology, including the concepts of uniformitarianism and catastrophism.
Briefly explain relative dating of geologic events and the geologic time scale.
Understand the magnitude and importance of the concept of geologic time.
Briefly discuss the nature of scientific inquiry and scientific methods.
Define and briefly discuss the major “spheres” of the Earth.
Discuss the Earth as a system.
Discuss the major features of the continents and ocean basins.
Briefly discuss the origin and early evolution of Earth.
Compare and contrast the layers of Earth that are defined by composition with those defined by
physical properties.
Explain the concept of the rock cycle.
Answers to review questions:
1.
The traditional divisions are physical and historical geology, often taught as separate,
introductory courses in a one-year sequence. Physical geology deals with the materials (minerals,
rocks, water, etc.) that comprise Earth; with processes of rock formation and decomposition; with
how surface morphology is altered by the various agents of erosion, and with how rocks deform,
lands are uplifted or lowered, continents moved, and ocean basins opened and closed through
tectonic forces and lithospheric plate movements.
Historical geology places origins of rock masses, integrated effects of geologic processes,
interpretations of ancient environments and life forms, and past tectonic movements into the
chronological framework of the geologic time scale. Thus geology is an historical science;
passage of time and evolutionary concepts are vitally important.
2.
Aristotle's explanations of the natural world were not based on keen observations and
experimentation, as modern science is. Instead they were his opinions, based on the limited
knowledge of his day. Unfortunately, many of his wrong interpretations continued to be believed
for many centuries, thus thwarting the acceptance of better ideas based on observations.
3.
They believed Earth to be a very young planet. Accepting such a brief geologic history
forced them to explain Earth's evolution in terms of many, rapid, short-term, catastrophic events.
Stupendous natural features like the Grand Canyon, mountain ranges, the polar ice caps, the
oceans, etc., had to develop quickly. Integrated effects of slow movements, or of slowly operating
processes, were viewed as having had little importance in Earth's geologic history and evolution.
4.
Uniformitarianism basically says that rational observations and analyses of modern
geologic processes and events give an accurate representation of geologic workings in the past.
For example, seemingly inconsequential and barely recognizable stream erosion can cut a Grand
Canyon, given enough time. Lateral movements of a centimeter per year can build oceans and
move continents hundreds of miles, given enough time. In addition to the slow day-to-day
processes, occasional, large-scale, powerful events (volcanic eruptions, earthquakes, meteorite
impacts, etc.) occur as part of the very long, evolutionary history of Earth. Acceptance of the
uniformitarian concept logically forces one to accept a very old age for Earth and a very long
geologic time.
5.
The currently accepted age of the Earth is 4.5 to 4.6 billion years, based on meticulous
experimental measurements of lead isotopes on meteoritic and terrestrial samples. The basic
assumptions and results are supported by rubidium-strontium isotopic age determinations on
meteorite samples. This age gives the time passed since originally dispersed, chemical
constituents of the solar system were assembled into meteorites, asteroids, planetary satellites,
and planets. The oldest rocks yet dated formed about 4 billion years ago. Because Earth is a
dynamic planet, most rocks we see formed much later during Earth's history and thus are much
younger than the age of the Earth.
6.
In a series of horizontal, stratified rocks, younger strata lie above older strata. This is
known as the law of superposition and assumes that all sedimentary strata were originally
deposited as horizontal layers. Fossils (remains of ancient living organisms) changed through
geologic time so that specific fossils or assemblages of fossils are found only in strata of specific
ages and are unique indicators of geologic age; this concept is called the principle of faunal
succession. Relative ages of contacting igneous and sedimentary rocks can be determined by
recognizing cross-cutting relationships and erosional unconformities. These concepts and
relationships enabled geologists to identify and correlate rocks of similar ages anywhere on Earth
and to place these rocks in their proper, chronological order and position as the geologic time
scale was developed.
7.
A hypothesis is a specific idea or explanation, the validity of which can be tested by
observations and experimental studies. It may be one of many, different, competing ideas or
statements purporting to explain some scientific phenomenon. Depending on the outcomes of the
observations and experiments, a hypothesis can be accepted or rejected. Hypotheses usually are
directed to specific, scientific questions and issues. A theory is a useful, currently accepted,
unifying body of concepts and principles in a science. A theory helps to explain what otherwise
might be perceived as disjointed and unrelated observations and phenomena. A theory is based on
far more observations and experiments than a hypothesis and applies to a broader range of
scientific phenomena.
8.
The four major spheres of our living environment are: 1) the atmosphere - the gaseous
envelope surrounding our planet; 2) the hydrosphere - those environments (oceans, rivers, lakes,
ice, groundwaterand water vapor in the atmosphere) involved in the hydrologic cycle; 3) the
biosphere - the diverse, surficial and near-surface environments that include all living organisms
and their habitats; and 4) the solid earth - the soils, regolith, and crustal bedrock layers of Earth; it
hosts most of the hydrosphere, forms the inorganic substrate for the biosphere, and interacts
extensively with the atmosphere.
9.
In an open system both energy and matter flow into and out of the system. Closed
systems, however, are self-contained with regards to matter.
10.
Positive feedback mechanisms tend to enhance or drive changes in a system. By contrast,
negative feedback mechanisms work to maintain a system as it is (e.g., maintain the status quo).
11.
The Earth system is driven by energy from two sources. The Sun provides the energy that
drives the external processes that occur in the atmosphere and on the Earth’s surface. Internal
processes, such as plate tectonics and volcanism, are driven by energy from the Earth’s interior.
This internal energy is the result of leftover heat from the origin of the Earth and also heat from
the decay of radioactive elements.
12.
The theory for the origin of the solar system, called the nebular hypothesis, states that
approximately 5 billion years ago the bodies of the solar system condensed from an enormous
cloud. As the cloud contracted and began to rotate, the protosun began forming. The protoplanets
(planets in the making) formed from material that had condensed and accreted inside the cloud.
The inner planets, Mercury, Venus, Earth, and Mars, were unable to retain appreciable amounts
of the lighter components of the primordial cloud; while the outer planets (Jupiter, Saturn,
Uranus, and Neptune) accumulated large amounts of hydrogen and other light materials because
of their much colder temperatures.
13.
Earth’s compositional layers include 1) the crust, Earth’s comparatively thin outer skin;
2) the mantle, a solid rocky shell that extends to a depth of about 2900 kilometers (1800 miles);
and 3) the core, which can be further divided into the outer core, a molten metallic layer, and
inner core, a solid iron-rich sphere.
14.
These terms describe the outer two layers or “shells” of the Earth. The lithosphere is the
surface (outermost) shell and the asthenosphere is the name of the shell directly under the
lithosphere. The two shells differ significantly in their mechanical responses to stress.
Lithospheric rocks under stress fail (deform) by brittle fracturing (faulting). In contrast, deeper
rocks of the asthenosphere deform by ductile flowage, in which the rock gradually changes shape
and form without ever being physically cracked or broken. Ductile flowage is enhanced by higher
temperatures while brittle fracturing is typical of "colder" rocks.
15.
Earth’s youngest mountains tend to occur in two major zones. The first zone is the
circum-Pacific belt, which includes the mountains of the western Americas and volcanic island
arcs of the western Pacific. The second zone extends eastward from the Alps through Iran into the
Himalayas. Note that the younger mountain belts on Earth generally occur as long, topographic
features at the margins of continents.
16.
Shields are relatively flat expanses of metamorphic rocks and associated igneous plutons
found near the center or cores of the continents. The crystalline rocks in shields are typically
Precambrian in age and highly deformed. Stable platforms are areas of the stable interior where
the highly deformed rocks of the shield are covered by a thin veneer of sedimentary rocks. The
sedimentary rocks are nearly horizontal except where they have been deformed to form large
basins or domes.
17.
The three major topographic units of the ocean floor are the continental margins, deepocean basins, and oceanic (mid-ocean) ridges.
18.
Light-colored, coarse-grained intrusive rock = granite
Detrital rock rich in clay-size particles = shale
A fine-grained black rock that makes up the oceanic crust = basalt
Nonfoliated rock, for which limestone is its parent rock = marble
19.
May be intrusive or extrusive = igneous
Lithified by compaction and cementation = sedimentary
Sandstone is an example = sedimentary
Some members of this group are foliated = metamorphic
This group is divided into detrital and chemical categories = sedimentary
Gneiss is a member of this group = metamorphic
20.
Sedimentary rocks are composed of constituents derived from the disintegration and
decomposition of other rocks (igneous, metamorphic, or sedimentary). Metamorphic rocks were
once igneous, sedimentary, or metamorphic rocks that have since changed in texture and/or
mineral composition in response to elevated temperatures, or elevated temperatures and pressures
(deep burial). Igneous rocks form by cooling and crystallization of magmas; magmas form by
melting of other igneous, sedimentary, or metamorphic rocks. Therefore, all rocks are the result
of various processes acting upon pre-existing rocks.
Chapter 2
You should be able to:
Briefly discuss the evidence used by Alfred Wegener to support his theory of continental drift.
Explain why continental drift was not accepted by most scientists when the theory was originally
proposed.
Explain the theory of polar wandering and how it helped to renew interest in the idea of
continental drift.
Discuss geomagnetic reversals and seafloor spreading; and how each contributed to the
development of the theory of plate tectonics in the 1960s.
Briefly explain the theory of plate tectonics.
Compare and contrast the distribution and geologic characteristics of tectonic plate boundaries,
including divergent, convergent, and transform boundaries.
Discuss the evidence used to test the plate tectonics model including ocean drilling and hot spots.
Briefly explain how plate motions are measured.
Discuss mantle convection and the various mechanisms proposed to explain plate motion.
Briefly discuss the importance of plate tectonics in providing a unified explanation of Earth’s
surface features and major processes.
Answers to the Review Questions
1.
Alfred Wegener is credited with developing the continental drift hypothesis in the early
1900s.
2.
Speculations about the apparent “nice fit” between the west coast of Africa and the east
coast of South America date from the sixteenth century, when the first reasonably accurate maps
of the Americas were compiled. This observation led some scientists to suspect that the
continents had once been joined together based on their similar coastlines.
3.
Pangaea was the supercontinent that existed in late Paleozoic time when Gondwanaland
(the Southern Hemisphere landmass composed of Africa, India, South America, Australia, India,
and Antarctica) collided with North America to form one, super-large landmass. Pangaea was a
relatively short-lived continent as it began breaking up during the Triassic period.
4.
If the continents were once together, they must have drifted apart. Thus Wegener (Fig.
19.8) had to prove that now widely separated continents and/or pieces of continents were once
close together or contiguous. The evidence he used to support continental drift included apparent,
geometrical fits between edges of continents; similar, late Paleozoic and early Mesozoic
stratigraphic, geologic, and paleoclimatic records from different continents; and organisms,
identified by their fossil remains, that would have had serious trouble migrating from one
continent to another across oceans the size of those today. These latter organisms include landdwelling amphibians and reptiles and numerous species of plants.
5.
If Mesosaurus was able to swim well enough to cross the vast ocean currently separating
Africa and South America, its remains should also be found on other continents. Since this is not
the case, we conclude that South America and Africa were joined during the time period that
these animals existed.
6.
Animals were thought to have migrated from continent to continent using island chains as
stepping stones, floating on logs, crossing on temporary continental links such as today’s isthmus
of Panama, or swimming. Plants (seeds and spores) floated on currents or were rafted by the
wind. In retrospect, these mechanisms were not satisfactory, and oceanographic studies had pretty
much debunked them by the early 1950s. For example, Mesosaurus, an early Permian, aquatic
reptile known only from South Africa and Brazil, lived in freshwater and coastal, salt water
habitats, much like those of the modern crocodile or salt water crocodile. Such animals would
have had great difficulty migrating between continents if the Atlantic Ocean was the same size in
early Permian time as it is today. It seems more sensible to believe that the land areas were once
together and have since drifted apart as the ocean opened and widened.
7.
Wegener accounted for glaciers in southern landmasses with his supercontinent of
Pangaea. The large landmass would provide the necessary conditions to generate large expanses
of glacial ice over much of the Southern Hemisphere. This same geography would also place
today’s northern landmasses nearer the equator and account for their vast coal deposits.
8.
Paleomagnetism is the study of remnant magnetic characteristics of rocks and of Earth’s
magnetic field through geologic time. The inclination or dip of the remnant magnetization gives
the latitude at which the magnetization was acquired when the rock originally formed. If the rock
is carried to a different latitude by continental drift, its original paleomagnetic inclination (a direct
measure of the latitude at which it formed) will not change so long as the rock is not heated above
the Curie temperature, about 550°C.
9.
Seafloor spreading is the divergent movement of two oceanic plates away from a midocean ridge, accompanied by addition of new basalt to the trailing edges of the diverging plates.
Partial melting of a rising, mantle-peridotite plume supplies the basaltic magma. Professor Harry
Hess of Princeton University laid out the basic idea early in the 1960s using new topographic data
on the ocean floor acquired during and after World War II. Sea-floor spreading occurs today
along the oceanic ridges.
10.
By the early 1970s, age-dating and paleomagnetic studies of basaltic lavas had produced
a detailed chronology of Earth’s magnetic field from the late Tertiary to the present and had
conclusively documented several intervals of reversed polarity. Vine and Matthews recognized
two fundamental characteristics of the stripe-like, seafloor magnetic patterns: they were
symmetrically arranged about mid-oceanic ridges; and the ages, lateral positions with respect to
the ridge axis, and remnant magnetic polarities of the seafloor basalts conformed to the polarities
and time intervals evident in the newly developed, paleomagnetic time scale. They concluded that
the magnetic patterns were acquired when the basalts were erupted along the axis of the midocean ridge. As new magma was erupted, the older basalts split, forming roughly equal-sized
strips attached to the trailing edges of diverging, lithospheric plates. These strips retained the
rock’s original magnetic polarity, accounting for the rough, lateral symmetry observed in the
magnetic patterns on both sides of the ridge axis.
11.
New, oceanic lithosphere is formed at the mid-ocean ridges and an equivalent area of old
lithosphere sinks into the mantle along a subduction zone. Most geologists believe that Earth’s
radius and the area of lithosphere have been constant over geologic time; thus the production and
subduction rates of lithosphere have to be roughly equal over time.
12.
Oceanic lithosphere, being composed of basalt (rich in Fe- and Mg-rich minerals), is
dense and readily sinks into the mantle at subduction zones. However, continental lithosphere
(andesitic to granitic in composition) is not dense enough to sink into the mantle and be
subducted. In some instances, small pieces or chips of continents may be subducted if they are
carried down with unusually old, dense, oceanic lithosphere.
13.
The Himalaya Mountains formed when the subcontinent of India collided into Asia
approximately 40 million years ago. During this continental-continental collision, the crust
buckled, fractured, and was generally shortened and thickened, resulting in the highest mountains
on Earth today.
14.
Transform boundaries are long, vertical, deep faults along which two plates move in
opposite directions horizontally and parallel to the boundary. Plate motions at divergent and
convergent plate boundaries have their major components of motion perpendicular to the
boundary. Volcanism is prominent along convergent and divergent plate boundaries but is
generally absent along transform fault boundaries.
15.
No, California is not sinking into the ocean. The sliver of California west of the San
Andreas fault is slowly moving northwest as part of the Pacific plate and the movement is
essentially horizontal. Far in the geologic future, the sliver may eventually arrive in Alaska or the
Aleutian Islands.
16.
The age of the oldest sediments recovered by deep-ocean drilling is about 160 million
years. These sediments are geologically quite young when compared to the oldest continental
crust, which has been dated at 3.9 billion years.
17.
The Emperor Seamounts are aligned roughly north-south and are older to the north,
proving that when they formed, the Pacific plate was moving northward with respect to a
stationary hot spot. The Hawaiian Islands are oriented west-northwest and east-southeast and are
youngest to the southeast; thus the plate was moving west-northwest over the same hot spot when
the Hawaiian Seamounts were growing. Loihi, a rapidly growing seamount off the southern coast
of Hawaii, is the newest active volcano to develop over the hot spot. Its location supports
continuing northwest movement of the Pacific plate.
18.
Himalayas - These have formed along a convergent, continent-continent, collisional
boundary between the Indian subcontinent and Eurasia.
Aleutian Islands - These islands are the oceanward part of a volcanic island arc situated
on the northwestern margin of the North American plate; the volcanoes lie above the subducting
Pacific plate.
Red Sea - The Red Sea occupies a major rift zone and very young seafloor spreading
center that has opened between Africa and the Arabian block.
Andes Mountains - The Andes are a volcanic and plutonic arc resting on the western
margin of the South American plate; they lie above subducting, oceanic lithosphere of the Nazca
and Antarctic plates.
San Andreas fault - This is a transform fault that forms the boundary between the North
American and Pacific plates. The crustal sliver composed of westernmost California and the Baja
California peninsula on the eastern edge of the Pacific plate is moving northwestward with
respect to North America.
Iceland - Iceland and nearby smaller islands comprise a major zone of basaltic volcanism
that probably overlies a mantle hot spot located directly beneath the Mid-Atlantic Ridge, the
divergent boundary between the Eurasian and North American plates.
Japan - The Japanese Islands lie on the eastern margin of the Eurasian plate, above
subducting parts of the Pacific and Philippine oceanic plates.
Mount St. Helens - This is a very young stratovolcano in the state of Washington; it is
part of the Cascade Range, a continental-margin, volcanic arc extending from the Canadian
border to northern California.
19.
The three models proposed to explain mantle convection are a shallower, two-layer
convective model, a whole mantle convective model, and a deeper layer model. The shallow,
two-layer model envisions a thin convective layer above 660 kilometers and a deeper layer
below. The whole mantle model proposes a single convective cell where rising, hot mantle
plumes originate near the core-mantle boundary and subducted lithosphere descends into the
lower mantle. The deeper, layered model has two layers that swell and shrink in the lower mantle
without substantial mixing. None of the models fit all of the available data, thus the exact nature
of the convective flow in the mantle remains unknown.
Chapter 3
You should be able to:
List the definitive characteristics that qualify certain Earth materials as minerals.
Explain the difference between a mineral and a rock.
Discuss the basic concepts of atomic structure as it relates to minerals.
Compare and contrast the different types of chemical bonding.
Explain what is an isotope and how it relates to radioactive decay.
Discuss the internal structures of minerals.
List and discuss in some detail the various physical properties of minerals.
Explain the structure and importance of silicate minerals.
List the common rock-forming silicate minerals and briefly discuss their physical properties.
List other minerals groups and give an example of the important nonsilicate minerals.
1.
A rock is a more or less hardened (lithified) aggregate of minerals and/or amorphous
solids such as natural glass and organic matter.
2.
The particles are electrons, protons, and neutrons. The latter two are heavy particles
found in the nucleus of an atom. Electrons are tiny, very lightweight particles that form a “cloud”
surrounding the nucleus. The mass and charge data are as follows:
proton - one atomic mass unit, 1+ electrical charge
neutron - one atomic mass unit, electrically neutral
electron - tiny fraction of one atomic mass unit, 1- electrical charge
3.
(a) The number of protons - A neutral atom with 35 electrons has 35 protons. (b) The
atomic number - The atomic number is 35, equal to the number of protons in the nucleus.
(c)
The number of neutrons - The mass number (80) is the sum of protons (35) and neutrons. Thus
the nucleus contains 45 (80 - 35) neutrons.
4.
Valence electrons are those outermost few electrons in an atom or molecule that
participate in chemical reactions and bond formation. Valence electrons are the bonding
electrons.
5.
Ionic bonds are strong attractive forces between closely spaced ions of opposite (+ and -)
electrical charges. The ions are formed by chemical reactions in which valence electrons are
removed from a donor atom or molecule, producing a positively charged ion (+ ion) and acquired
by another atom or molecule, producing a negatively charged ion (- ion). These reactions
(ionizations) enable both ions to achieve much higher chemical stability (more stable valence
electron configurations) than the respective neutral atoms.
In covalent bonding, the more stable, outer, electron configurations are achieved by
sharing of valence electrons among two or more neighboring atoms in a molecule or crystalline
compound. Charged atoms (ions) do not form.
6.
One or more valence electrons are simultaneously gained and lost by atoms participating
in a chemical reaction. The atoms that gain electrons are negative ions; those that lose electrons
are positive ions.
7. Isotopes are atoms of the same element (same atomic number) that differ in mass number
(numbers of neutrons are different). Thus natural uranium includes a small fraction of atoms with
mass 235 (143 neutrons and 92 protons) together with the more abundant atoms with mass 238
(146 neutrons). In general, isotopes of the same element have very nearly identical chemical
characteristics.
8.
Crystal form refers to the geometrically regular, external growth shape that minerals can
exhibit if crystal growth is free and unobstructed by other minerals (the crystal grows into a fluidfilled cavity, for example). Most crystal growth in nature is obstructed (not free), so crystals
showing their characteristic, geometric forms are not that common. Mineral samples broken from
larger masses have their shapes determined by fractures and cleavage, not by crystal growth.
9.
A particular mineral may exhibit many different colors. Thus by itself, color is seldom
definitive in mineral identification, but it may be helpful. Mineral color is highly sensitive to
relatively small changes in chemical composition and also to changes in bulk chemical
composition, such as in plagioclase feldspar.
10.
A hardness comparison with quartz would establish that the grain was above 7 in the
Mohs scale. So are many other minerals. A jeweler could quickly determine the refractive index,
thus verifying or dashing your hopes. Diamond has the highest refractive index of any mineral.
11.
Any mineral listed in Mohs scale (Table 3.2), corundum for example, will scratch softer
minerals (those with lower hardness values) and will not scratch harder minerals. Corundum
would scratch virtually all other minerals, diamond being the lone exception. Thus corundum is
widely used in abrasives and polishing compounds.
12.
The specific gravity of water is one by definition. Thus equal volumes of water and gold
would have their weights in the ratio 1:20. Since the 25 liters of water weigh 25 kilograms, the 25
liters of gold will weigh almost 500 kilograms (25 liters x 2O kg/l = 500 kg).
13.
Silicon is the name for the element with atomic number 14; the chemical symbol is Si.
Elemental silicon is a semiconductor and is widely utilized today in computer chips. Silicate
refers to any mineral that contains the elements silicon and oxygen bonded together as the SiO4
molecule, typically with additional elements present. Most rock-forming minerals are silicates.
Silicon as a native element does not occur naturally. It is manufactured from quartz, silicon
dioxide, at high temperatures under strongly reducing conditions.
14.
“Ferromagnesian” is a word derived from the chemical elements magnesium and iron
(ferro, ferrous, ferric, etc.). The term refers to rock-forming silicate minerals that contain some
iron (Fe) and/or magnesium (Mg) in addition to silicon and oxygen. Additional elements such as
aluminum, sodium, and calcium may be present without changing the designation.
Ferromagnesian minerals comprise most of the dark-colored (dark green and black) mineral
grains in igneous rocks. Common examples include olivine, pyroxenes, amphiboles, and biotite.
15.
They are both micas with layered (sheet-silicate), internal, crystalline structures and one
direction of perfect cleavage. Muscovite is the light-colored, potassium aluminum (K and Al)
mica; and biotite is the darker-colored, ferromagnesian mica (contains Mg and Fe).
16.
Twinning striations are the definitive characteristic for identifying plagioclase. They are
generally visible in most hand samples, but a microscope may be necessary for positive
identification. Orthoclase possesses the other physical properties of plagioclase (hardness and 2
directions of cleavage at 90°, but it doesn’t have striations. Both feldspars can be white or
colorless, but pale-pink or tan colors usually indicate orthoclase. Ca-rich plagioclase may be
fairly dark gray to black. Thus color alone is not definitive; however, in rocks with pinkish
orthoclase and white plagioclase, color is very helpful in telling the two feldspars apart.
17.
(a) hornblende
(SiO2)
(d) olivine - green
(e) plagioclase feldspar with twinning striations
(b) muscovite
(c)
quartz
(f) clay minerals
18.
Both minerals are carbonates. Calcite reacts vigorously with dilute, strong acids such as
hydrochloric (HCl), with the formation of carbon dioxide (CO2) gas bubbles. In contrast,
dolomite must first be finely powdered before reacting vigorously enough with the same dilute
acid to produce visible bubbling.
Chapter 4
You should be able to:
Discuss the physical and chemical characteristics of magma.
Explain the process of crystallization and how it relates to the formation of igneous rocks.
List the various igneous textures and explain their origins.
Compare and contrast the various igneous compositions.
Discuss silica content and how it relates to the chemical composition of igneous systems.
Explain the classification system used for igneous rocks.
List and discuss the various names for felsic, intermediate, mafic, and pyroclastic igneous rocks.
Explain Bowen’s Reaction Series and how it relates to the composition of igneous rocks.
Discuss the evolution of magmatic systems.
Briefly explain the concept of partial melting and how it relates to magma formation.
Discuss the origin of magma from solid rock.
Answers to the Review Questions
1.
Magma is a general term that refers to any molten-rock melt on or beneath Earth's
surface. Magmas usually include some solid mineral grains and/or dissolved gases in addition to
the molten liquid.
2.
Magma is a general term that refers to any molten-rock melt on or beneath Earth's
surface. Magmas usually include some solid mineral grains and/or dissolved gases in addition to
the molten liquid. Lava is a much more restricted term to describe magma extruded on the
surface. Thus all rock melts are magmas, but only those extruded at the surface are lavas.
3.
Crystallization (growth of solid mineral grains from a magma) depends on the rate at
which the constituent ionic groups move through the melt and attach to the growing mineral
grain. Slow rates of transport and/or short cooling intervals (fast cooling) inhibit in-melt transport
and contribute to slow grain growth and small grain size. Natural glasses like obsidian (rhyolitic
glass) cool so quickly that mineral grains do not have time to grow. Slower cooling allows for a
longer period of crystal growth and therefore larger grain sizes. Also, a high water content in the
magma favors higher in-melt transport rates and more rapid grain growth than would occur in a
“dry” magma of equivalent composition and temperature.
4.
As noted above, the in-melt transport properties of the magma have very important
effects on crystallization. In general, in-melt transport rates are enhanced by lower magma
viscosity and slowed by higher viscosity. Magma viscosities increase (transport rates decrease)
with lower temperature and higher silica (SiO2) content. Thus natural rhyolitic glasses, formed by
rapid cooling of relatively low temperature, silica-rich lavas, are much more common than
basaltic glasses formed by rapid cooling of hotter, lower silica content lavas. Large quantities of
magmatic volatiles (such as water) can profoundly increase transport rates and crystal growth
rates. For this and other reasons, geologists believe magmas that form pegmatites contain very
large percentages of water and other volatiles.
5.
The two criteria are texture and mineral composition. Texture describes the sizes, shapes,
and mutual contact relationships of the constituent mineral grains and other physical features of
the rock. The mineral composition is also a definitive factor. The names used for the common
igneous rocks are based mainly on the percentages of three major minerals; quartz, potassium
feldspar, and plagioclase feldspar. For the latter mineral, the ratio of sodium (Na) to calcium (Ca)
basically differentiates diorite from gabbro. In diorite, the plagioclase composition is intermediate
(sub-equal amounts of Na and Ca) and in gabbro, the plagioclase is dominantly calcic (Ca > Na).
Plagioclase in granites is dominantly sodic (Na > Ca).
6.
(a) vesicles (b) glassy (not crystalline) (c) porphyritic
porphyritic texture (f) phaneritic texture (g) pegmatite
(d) aphanitic texture
(e)
7.
Very large silicate mineral grains (crystals) indicate extremely fast, in-melt transport of
the mineral constituents (atoms and molecules) to the growing crystals. We know that pegmatite
magmas are small volume, relatively low temperature melts that are extremely rich in water and
other dissolved volatiles (gases). The volatiles promote very fast rates of molecular transfer, thus
accounting for rapid growth of very large crystals.
8.
Two distinctively different sizes of mineral grains in the same igneous rock (a porphyritic
texture) usually indicate that crystal growth occurred in two stages. First, the larger grains
(phenocrysts) grew over a prolonged period of crystallization at a slow cooling rate in a magma
chamber at some depth below the surface of the Earth. Then the magma rose closer to the surface;
and the smaller, groundmass grains grew in a second, shorter, crystallization episode during
which the cooling rate was much faster. If this second stage of crystallization is extremely rapid,
than an aphanitic texture may develop in the groundmass surrounding the larger phenocrysts.
9.
Both are igneous rocks with quartz and potassium feldspar as major minerals. Granite is
the phaneritic-textured rock crystallized slowly at depth from intrusive, granitic magma. Rhyolite
is the aphanitic, rapidly cooled, volcanic rock that forms when granitic magma is extruded during
a volcanic eruption.
10.
(a) Granite and diorite - both are phaneritic igneous rocks. Granite has quartz and
potassium feldspar as dominant minerals and is light in color. Diorite has plagioclase (sub-equal
amounts of sodium and calcium) as the definitive mineral and is darker than granite in color.
Biotite, hornblende, and augite are ferromagnesian minerals that are commonly found in diorite.
(b) Basalt and gabbro - both rocks are dark in color and have the same mineral compositions.
Calcium-rich plagioclase is the definitive feldspar and quartz is absent. Olivine and augite are the
main ferromagnesian minerals in both rocks. Basalt is an aphanitic volcanic rock and gabbro has
a phaneritic texture, reflecting its origin at depth from a slow-cooling intrusive magma.
(c) Andesite and rhyolite - both are aphanitic-textured rocks, usually of volcanic origin. Rhyolite
has the same dominant minerals (quartz and potassium feldspar) as granite while andesite has the
same mineral composition as diorite. Typically, rhyolites are light in color and andesites are
somewhat darker. Whereas biotite is the only common ferromagnesian mineral in rhyolite,
andesite often contains hornblende or augite in addition to biotite.
11.
Tuff and breccia have pyroclastic textures as opposed to the crystalline textures of granite
and basalt. In pyroclastic textures, the rock is composed of solidified magma fragments and/or
fragments broken from other volcanic rocks. In crystalline textures, the minerals grow from the
magma into mutually interlocking grains. Clastic means fragmental or broken, and “pyro” means
fire. Tuffs and volcanic breccias are products of explosive volcanism.
12.
General knowledge gained from deep mines and drill holes tells us that rock temperatures
gradually increase with depth below a relatively shallow zone wherein rock temperatures are
dominated by circulating groundwaters and surface climatic conditions. The change in rock
temperature with depth is called the geothermal gradient. Although the rate of temperature change
varies from place to place, the average value is between 20°C and 30°C per kilometer in the
upper crust. Areas of active volcanism and/or shallow, still-hot intrusive masses exhibit high
geothermal gradients; thus rock temperatures increase rapidly with depth below these regions.
13.
Melting of rocks is thought to be caused by the addition of heat from a body of magma, a
decrease in pressure without the addition of heat (known as decompression melting), and the
introduction of volatiles (mainly water) which lowers the melting temperatures of minerals.
14.
Magmas contain many different chemical constituents. Minerals that crystallize from
magma almost always have different compositions from the magma, and, during any given
portion of the crystallization history, only a fraction of the magma crystallizes into minerals.
Physical separation of melt and crystals can produce rocks enriched in the early-formed minerals
and a magma enriched in those components excluded from the early-formed minerals. Rocks
different in composition from the original parent magma can then crystallize from the remaining,
compositionally changed (compositionally differentiated) magma. This process, known as
magmatic differentiation, operates throughout the crystallization history of a parent magma and
later derivative magmas. Thus accumulations of early-formed minerals and crystallization of
later-stage derivative magmas can result in different igneous rocks being derived from a single
batch of an original parent magma.
15.
Bowen's reaction series depicts the order in which major minerals crystallize at low
(crustal) pressures from a hot, basaltic magma and how that magma changes composition
(differentiates) as it graduallycools. In terms of rock classification, the reaction series predicts
that Ca-rich plagioclase, olivine, and pyroxene will crystallize first (basalt or gabbro), followed
by hornblende and plagioclase with Na to Ca ratios of about one (diorite and andesite). At lower
temperatures, quartz and potassium feldspar (granite and rhyolite) crystallize from fractionated
magmas strongly enriched in silica and potassium.
16.
Partial melting denotes the fusion behavior of multi-component solids (rocks are mixtures
of minerals of different compositions) that melt over a range of temperatures. Melt fractions
produced at lower temperatures are enriched in the more fusible components and unmelted,
residual solids are enriched in refractory components. Just think of the reverse of Bowen's
Reaction Series. Low temperature, small volume, partial melts of basaltic rocks would be
enriched in K and Na-rich feldspar components and silica. At higher temperatures, pyroxenes and
olivine would be the last minerals to melt. In general, the compositions of partial melt fractions
are more felsic than the solid parent and the residual solid rocks are more mafic.
17.
As a generalization, partial melts are enriched in chemical components from minerals
with lower melting temperature ranges and depleted in components from the more refractory
minerals. Thus, basalt partial melts are enriched in alkalis and silica and depleted in magnesium
compared to the parental mantle peridotite. Partial melting of mafic rocks in the lower crust is
expected to yield more felsic liquids such as andesite and rhyolite, depending on the extent or
degree to which the basaltic parent is melted. The smaller the percentage of the parent rocks that
melts, the more felsic the derived liquid. Of course, complete melting without any fractionation
produces a liquid with the same composition as the solid rock.
18.
Basaltic magmas are generated in slowly rising mantle-rock plumes. In this case, melting
temperatures decrease as the plume rock decompresses. As the plume rises, little heat is lost and
its temperature remains essentially constant; however its melting temperature range gradually
decreases due to lowered pressures. Eventually, partial melting begins and continues for as long
as the plume keeps rising without significant heat loss. Hot spot, flood basalt, and mid-oceanic
ridge volcanism are thought to be driven by partial melting of rising, mantle peridotite plumes.
19.
Because melting temperatures of dry rock increase with depth, basaltic magmas
generated deep in the mantle have temperatures well above their range of crystallization
temperatures at the surface. As basaltic magmas migrate upward, the confining pressure steadily
diminishes and reduces the melting temperature. Therefore, those environments where basaltic
magmas ascend rapidly enough that the heat loss to the surrounding environment is offset by the
drop in the melting temperature result in large outpourings of basaltic lava at Earth’s surface.
20.
Basaltic magma is the main product of partial melting at depth in the mantle below the
ocean basins. Andesite is the common magma type of island and continental margin volcanic
arcs, and granite is an important magma of the continental lithosphere. Much, if not all granite is
derived by partial melting of middle and upper crustal rocks that are more felsic than basalt,
including older intrusive rocks, gneisses, and other igneous rocks of intermediate to felsic
composition. Basalt magma carries the heat necessary for melting into the lower and middle crust,
but partial melting and magmatic differentation combine to favor production of magmas more
felsic than basalt. In the ocean basins, basaltic magma only encounters a thin mafic crust on its
way to the surface. Thus, it induces little partial melting and undergoes little change in
composition due to assimilation and or mixing with other more felsic rocks and magmas.