MINERALS, ROCKS, AND VOLCANOES
Geology is the study of the structure, composition,
processes and history of the Earth. The outer layer of
the Earth is called the crust and is composed of
minerals and rocks.
A mineral is a naturally occurring, inorganic substance
that possesses a definite chemical composition and a
distinctive set of physical characteristics. Minerals such
as diamonds or rubies can be very valuable as are gold
and silver ore. Minerals found in sand or limestone,
however are not valuable.
Your body needs minerals. These minerals are generally
found as compounds in food and can actually be toxic if
taken in amounts that are too large.
The most common minerals found in the Earth's crust
are the silicates which are compounds containing silicon
and oxygen. Silicon dioxide or silica is the fundamental
silicon-oxygen compound. It actually exists as an SiO4
tetrahedron, but oxygen atoms are shared so that the
actual ratio of oxygen to silicon atoms is 2 to 1.
Most rocks contain aluminum, calcium, sodium or
potassium in addition to silicon and oxygen. Feldspars
which contain calcium or sodium are said to be
plagioclase. If potassium is present instead, it is called
orthoclase.
Nonsilicates compose less than 10% of the mass of the
Earth's crust. They include pure elements such as gold
or silver, gemstones, and ores of iron, copper, nickel
and tin. Carbonates (CO3-2) such as calcium carbonate,
oxides (O-2) such as iron oxide, and sulfides (S-2) such as
galena which is lead sulfide, are the most common non
silicate groups.
Classification of minerals is based on physical and
chemical properties because this method distinguishes
among different form of the same element or
compound. Graphite, charcoal and diamond are all
composed of carbon but have very different properties
and value. Some of the main properties used to identify
minerals follow.
Crystal form refers to the size and shape assumed by
the crystal faces when a crystal is given time and space
to grow. Small pieces of sodium chloride are observed to
be shaped like boxes and calcite crystals are shaped like
boxes that have been distorted from their normal
rectangular shape.
Hardness refers to the ability of a mineral to resist
scratching. Minerals are arranged in order of hardness
on the Mohs' scale which has representative minerals in
order from 1(softest) to 10(hardest). Talc is the softest
mineral on the Mohs' scale and Diamond is the hardest.
Cleavage is the ability of a mineral to break along
definite smooth planes. Minerals can have definite
cleavage along one or more planes or no cleavage.
Fracture refers to the way a mineral breaks. It may
splinter, break into rough, irregularly shaped pieces, or
break into shell shaped forms known as conchoidal.
Color is determined by the frequencies of light reflected
by the mineral. Color may vary within samples of the
same mineral.
Streak is the color of a powder of a mineral. Color of
streak will not change from sample to sample thus
showing the mineral's true color.
Luster is the shininess of a mineral's surface. It can
range from the appearance of a polished piece of metal
to none at all such as clay.
Specific gravity refers to a mineral's density compared
to the density of water. Each mineral has a
characteristic specific gravity that does not vary among
pure samples.
Rocks
A rock is defined as a solid, cohesive natural aggregate
of one or more minerals. The three major categories of
rocks are igneous, sedimentary and metamorphic.
Igneous rocks are formed by the solidification (freezing)
of magma. Igneous rocks may be formed deep under the
Earth's surface from magma. If the magma reaches the
surface of the Earth we say the rocks are formed from
lava.
Sedimentary rocks are formed at the Earth's surface by
compaction and cementation of layers of sediment. They
are aggregates of
1. Rock fragments resulting from the weathering of
older rocks.
2. Minerals precipitated from a solution.
3. Altered remains of plants or animals.
Metamorphic rocks are formed when existing rock is
changed by pressure, temperature, and/or a change in
chemical components. Metamorphism occurs below the
surface of the Earth but above the depths at which rock
is melted.
The Rock Cycle is a description of the processes by
which rocks are formed, broken down, reformed and
recycled into the Earth's magma. The idea that these
processes have been the same throughout the history of
the Earth is called uniformitarianism. This idea was
developed by an 18th century geologist named James
Hutton. He is called "the father of geology" for his
development of these two ideas that are the foundation
of modern geology.
Igneous Rocks
Igneous rocks constitute about 80% of the Earth's
surface. There are two main classifications of igneous
rock. When magma makes its way to the surface, as in a
volcanic eruption, the rock that is formed is said to be
extrusive. If the magma solidifies underground, the
resulting rock is said to be intrusive.
When one is studying volcanoes, it is important to
understand plate tectonics, the idea that the surface of
the Earth is composed of many large pieces that are
moving past, over, and under each other. Volcanoes are
mostly found along converging plate boundaries.
When plates converge, one must move under the other
in an area called a subduction zone.
The melting of rock and vaporization of water in these
regions causes pressure to build up and eventually
volcanic eruptions.
The Pacific Rim is sometimes called "the ring of fire"
because of all the volcanoes surrounding the Pacific
Ocean.
Igneous rocks are classified in terms of their texture
(grain size) and mineral composition. Grain size is
determined primarily by the rate at which molten
material cools to form rock. Material exposed to the
atmosphere cools quickly and only small grains have
time to form. Material that is deep under the surface of
the Earth loses heat more slowly and much larger
crystals have time to form. Granite generally forms at
some depth and has large crystals that are easily seen.
Basalt forms near or at the surface and has much
smaller crystals that are difficult to see. Obsidian forms
from lava shot into the air and cools so rapidly that
practically no crystals form. This gives obsidian its
characteristic glassy appearance.
Plutons are defined as intrusive igneous rocks formed
by the solidification of magma. They are classified
according to their size and shape and their orientation
with respect to the surrounding rock.
Discordant means the pluton cuts across the grain of the
surrounding rock and concordant means it lies in the
same direction as the grain of the surrounding rock. A
batholith is a discordant pluton of enormous size. The
minimum size for a batholith is 103 square kilometers.
Dikes are discordant plutons formed when magma fills
a vertical fracture in existing rock. A dike will be thin in
one dimension and large in the other two.
A sill is similar to a dike but is concordant. A laccolith is
a concordant pluton shaped like a blister. It forms from
intrusive magma.
A volcano is a hill or mountain formed from lava and
rock fragments ejected through a vent in the surface of
the Earth. There are three products of volcanic
eruptions.
1. Gasses such as steam, carbon dioxide, and hydrogen
sulfide make up 99% of the gasses. Steam accounts for
almost 90%.
2. Lava that can be very fluid or nearly solid depending
on the temperature and the composition of the lava. The
more fluid lava which is mainly basaltic is very hot and
has a low silica content. Cooler, high silica lava is called
rhyolitic or andesitic and moves much more slowly.
3. Solid material ranging from the size of dust particles
to boulders may be expelled. Collectively these particles
are called tephra.
Volcanoes may erupt peacefully or explosively. The
Hawaiian volcanoes tend to erupt peacefully since the
very fluid lava allows gasses to escape. Other volcanoes
like Mount St Helens tend to explode since the viscous
lava blocks the escape of gasses and allows pressure to
build up.
There are three types of volcano classifications by
structure.
1. Shield - a shield volcano is built up from many fluid
lava flows over time. Its sides are relatively gently
sloping and it forms a low, mounded profile. Mauna
Loa in the Hawaiian chain is a shield volcano.
2. Stratovolcano - a stratovolcano or a composite cone
volcano is built up from a combination of lava and
tephra to form a more steeply sloping profile. Examples
are Mount Fuji and Mount St Helens.
3. Cinder Cone - a cinder cone volcano is composed
almost entirely of tephra with very little lava flow. The
profile of a cinder cone shows very steep sides.
Many volcanoes have a large central depression called a
caldera. This roughly circular area will be surrounded
by steep walls and results from the collapse of the peak
of the volcano when the underlying lava chamber is
partially emptied. Crater Lake in Oregon occupies a
caldera on top of Mount Mazama.
Historically significant volcanic eruptions include
Mount St Helens, Mount Vesuvius(Pompeii), Paricutin,
Mount Pelee', Surtsey, and Tambora. The eruption of
Tambora was the largest in written history. Mt Pelee'
was the most deadly with 40,000 deaths.
Sedimentary Rock
The initial idea of the nature of sedimentary rock is
credited to James Hutton, a Scottish geologist also
known as the "Father of Geology" who lived in the late
eighteenth century. He surmised that rock fragments,
sand, and other material would be deposited as layers of
sediment in rivers, lakes and on the seafloor. These
layers would then be naturally cemented together to
form sedimentary rock.
The layers in sedimentary rock are called strata. The
idea of superposition (new material is the top layer,
older material is the bottom layer) was developed before
Hutton.
The change of sediment into rock is called lithification.
During this process, weight from top layers forces the
water out of the lower layers. Silica, calcium carbonate,
and/or iron oxides which are dissolved in groundwater
precipitate out and form bonds with the sediment. This
is the cementing process that results in solid
sedimentary rock.
Detrital sediments are composed of solid fragments
derived from preexisting rock. Shale is composed of fine
particles from mud. Sandstone is composed of larger
particles that typically are found as sand. Conglomerate
is made from rounded, pebble sized rocks embedded in
silica, calcium carbonate or iron oxide. Breccia is
similar to conglomerate but the component pebbles are
not rounded.
Chemical sediments are composed of minerals that
originally were in solution and precipitated out to form
rock. They can be organic or inorganic.
Organic limestone consists of seashell material that has
been lithified. They are sometimes known as chalk
deposits and consist primarily of calcium carbonate.
Bituminous coal (soft coal) is considered an organic
sedimentary rock since it consists of partly decayed
plant remains. Anthracite (hard coal) is considered to
be a metamorphic rock since it is formed from
bituminous coal.
Remember that our coal supply is limited. We will
eventually use up this source of energy and will need to
find other ways to satisfy our energy needs.
Inorganic rock forms when water evaporates and
dissolved inorganic material precipitates out to form
rock. Rock salt and gypsum are examples of rock
formed in this manner. Cave dripstone or stalactites
and stalagmites are formed when water drips from the
ceiling in a cave and evaporates.
Bedding, or stratification, describes the layers that
develop at the time sediment is deposited. Normally the
layers are close to horizontal and concordant.
Sometimes the layers are tilted with respect to each
other. This is called cross-bedding and occurs most
often at the mouth of a river or stream.
Fossils are found in sedimentary rock. A fossil consists
of the remains or traces of a prehistoric organism. It
may be an imprint in the rock or lithified remains of a
living organism.
Metamorphic Rock
Metamorphism is the process during which the
structure and/or mineral content of a rock are changed
while the rock remains solid. Igneous, sedimentary and
metamorphic rocks can undergo metamorphism. Heat,
pressure and chemically active fluids are the agents of
metamorphism.
Limestone to marble is an example of metamorphism
that involves a structural change but no chemical
change. The appearance of marble is much different
from limestone, but chemically they are the same.
An example of metamorphism that involves a chemical
change as well as a physical change is shale to slate to
schist to gneiss.
There are three types of metamorphism.
Contact metamorphism occurs mainly because of heat
with very little pressure. This occurs in surface rock
located near fissures where magma makes its way to the
surface.
Shear metamorphism occurs mainly because of
pressure and occurs in active fault zones. When one side
of the fault slides past the other, very large forces cause
changes in the crystal structure of the rock in contact.
Regional metamorphism occurs when pressure and heat
cause changes in the nature of the rock. It seems to
affect large areas and be associated with mountain
building.
Metamorphic rock may be foliated. When the grains
that make up the rock are aligned in a direction
perpendicular to the compressive forces causing the
metamorphism, the rock is said to be foliated. Distinct
layers are visible and may even lead to cleavage as in
slate.