Download Rocks and Minerals - National Science Teachers Association

Rocks and
Minerals
Reading
Plate Tectonics, Rocks, and Minerals
Earth’s plates consist of rocks composed of many minerals, plus a thin covering of soils.
Developing an understanding of the relationship between the most obvious aspects of
geology—minerals and rocks—and the unifying theory of plate tectonics may seem difficult
at first. Plate tectonics describes how, and by what mechanisms, Earth’s surface has been
and is being transformed on a large scale. We often think of continental movement, mountain building, volcanoes, and earthquakes as those aspects of geology explained through
plate tectonics. However, rock and mineral types, locations of mineral deposits, and rock
formation and transformation also can be understood when studied in the context of plate
tectonics. Plate tectonics provides a link between rock and mineral specimens that can be
studied in the classroom, and the geologic history that led to their development.
Minerals
A mineral is a naturally occurring, inorganic crystalline material with a unique chemical
combination. Different combinations and atomic arrangements of elements give each
mineral its characteristic properties, such as color, hardness, shape, density, and cleavage.
Although about 3,000 different minerals have been identified, most are extremely rare.
About 95% of Earth’s crust is comprised of only 10 to 15 different minerals or mineral
groups. The following relatively short list of minerals and mineral groups comprises the
primary ingredients of rocks:
Feldspar—a group of similar minerals; they make up roughly 60% of Earth’s crust
Quartz—approximately 10% of Earth’s crust, found in many different rocks; sandstones
can be nearly all quartz
Pyroxenes—mineral group with many members; common dark-colored minerals
Amphiboles—another mineral group with many members; common dark-colored minerals
Mica—mineral group; includes biotite and muscovite, common in igneous and
metamorphic rocks
Garnet—another mineral group; diverse in occurrence and color
Clay—mineral group; sediments and sedimentary rocks; produced by weathering of
feldspars
Calcite—major constituent of sedimentary rock limestone, and metamorphic rock marble
Dolomite—similar in occurrence to calcite, but sedimentary rock is called dolostone
Olivine—uncommon at Earth’s surface because it weathers easily; common within Earth
Project Earth Science: Geology, Revised 2nd Edition
Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions.
221
Reading 4
This website from the Smithsonian Institution provides good information on
creating your own classroom exhibit of rocks and minerals:
www.smithsonianeducation.org/educators/lesson_plans/minerals/lesson1_
main.html. From this page, it is possible to go to other useful links about
minerals. The U.S. Geological Survey also provides helpful information about
minerals and their use as resources. This link is titled “Do We Take Minerals
for Granted?” and can be accessed at http://minerals.usgs.gov/granted.html.
Additional interesting mineral information can be found at this web page
as well.
Rocks
Topic: minerals/rocks
Go to: www.scilinks.org
Code: PSCG222
222
A rock normally is an aggregate of mineral grains, although a few rocks are
formed of fossils, fragments of fossils, or fragments of rocks. Different types of
rocks form in different ways.
Igneous rocks result from the solidification or crystallization of molten
rock. They may form at Earth’s surface (where molten rock is called lava) or
below Earth’s surface (where molten rock is called magma). Two common
igneous rocks are granite and basalt. They differ in their mineral composition
and mineral grain size. Granite forms several kilometers below Earth’s surface,
where the insulating properties of the surrounding rocks cause magma to cool
slowly. This allows for the growth of large mineral grains. Basalt forms at
or close to Earth’s surface, where rapid heat loss results in relatively small
mineral grains.
Sedimentary rocks form from rock fragments, minerals, or fossils that are
compressed beneath the weight of overlying sediments. They also form by
chemical precipitation of minerals dissolved by water. Sedimentary rocks form
at low temperatures at or very close to Earth’s surface. Common sedimentary
rocks are sandstone, limestone, and shale.
Metamorphic rocks are those in which the original mineral composition,
grain size, or grain shape has changed as a result of exposure to moderately
high temperatures, high pressure, or both. Most metamorphic rocks form below
Earth’s surface. Marble, slate, and schist are common metamorphic rocks.
Over time, or in response to changing conditions, rocks may change from one
type into another. The rock cycle depicted in Figure R4.1 summarizes the steps
by which one rock type transforms into another. The counterclockwise arrows
indicate changes in rocks and rock materials that may take place. The words
next to the arrows refer to processes by which the changes are accomplished.
Shortcuts, indicated by interior arrows, may occur in this cycle. For example,
igneous rocks can be metamorphosed to form metamorphic rocks by heat and
pressure, or sedimentary and metamorphic rocks may undergo weathering to
become sediments and/or dissolved mineral material. (The dissolution of rock
material during weathering and the possibility of that material being precipitated
to form sedimentary rock is not included on Figure R4.1 in order to keep the
diagram relatively simple; examples of such chemically precipitated rocks are
some limestones that form in oceans and salt [halite] deposits that can form in
desert lakes or in an isolated part of an ocean.)
National Science Teachers Association
Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions.
Reading 4
Compaction and
Cementation
SEDIMENTARY
ROCK
Weathering, Erosion,
and Deposition
SEDIMENT
Figure R4.1 Heat and
Pressure
Weathering, Erosion,
and Deposition
METAMORPHIC
ROCK
Melting
Heat and
Pressure
MAGMA
Weathering, Erosion,
and Deposition
IGNEOUS
ROCK
Cooling and
Crystallization
The rock cycle. Rock
types plus magma and
sediment are in capital
letters. Processes that alter
rocks or sediments are in
capital and lower case.
Arrows indicate how one
type of rock material may
change as geological
conditions change. These
changes need not occur,
or they could take many
millions of years to happen.
Strictly speaking, before you can accurately identify a rock, you must know
what minerals are in it. If a rock has decomposed or weathered, even geologists
may have a difficult time recognizing those minerals and determining what rock
it is—or what it used to be. It is usually easy to tell when rocks and minerals
have been weathered; they may crumble when handled and their surfaces may
be dirty or covered with what appear to be brownish stains. Typically, geologists
examining a weathered rock will break it with a hammer to expose a fresh or
unweathered surface. The geologists will then study the rock’s minerals with a
magnifying glass and scratch grains with a knife blade to determine hardness. By
identifying minerals, they can give the rock a provisional name. If the rock needs
further study, geologists might use a specialized (petrographic) microscope using
polarized light or analytical instruments that provide chemical data to identify
the minerals and thus definitively name the rock.
The identity of a rock or mineral usually provides clues as to the type of
environment where it formed, which is often quite different from Earth’s surface.
For example, a typical granite that is now exposed at Earth’s surface probably
crystallized at a temperature of about 700°C, at high pressures, and at a depth
within Earth of at least 4–10 km. That rock, now on the surface of Earth, was
exposed when the overlying rocks were eroded and weathered away. This action
is part of the uplifting process on Earth’s crust. Limestone provides another
example of a rock that indicates what an environment was like in the past:
A coral-containing limestone that is found today on land in a relatively cold
climate originally formed in a warm, shallow ocean. (That is where we find
most corals living today.) Assuming the limestone formed where it is now found,
Project Earth Science: Geology, Revised 2nd Edition
Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions.
223
Reading 4
the environment at that location must have changed significantly since the time
the rock formed. (An alternate explanation could be that something, perhaps a
glacier, transported the rock a long distance to a new location with a different
environment.)
The identity of rock and mineral specimens also may provide information
on the location of ancient plate boundaries. Unusual rocks and minerals may
form at specific types of plate boundaries. For example, peralkaline volcanic
rocks, which have an unusual chemical composition, currently are forming at
several active divergent plate boundaries where continents are pulling apart
(e.g., East Africa Rift). Rocks of similar composition that are approximately
700 million years old occur in the central and southern Appalachian
Mountains. These rocks are believed to mark the time and location where an
early supercontinent began to split into North America and Africa, with the
Atlantic Ocean gradually forming between them.
Many of the rock cycle processes occur as a result of plate motion and
interaction. At subduction zones, where one plate slides beneath another,
seafloor sediment on the underlying plate is carried along with oceanic crust
down into the asthenosphere where some of the rock may remelt. Some of
this material will recrystallize into new igneous rock. The heat that is released
as the molten rock cools and recrystallizes metamorphoses adjacent rocks.
High pressures created when two plates push together or “collide” also can
metamorphose any sedimentary, igneous, and metamorphic rocks at the
plate boundary.
Conclusions
Specific minerals and rocks form in response to specific environmental
conditions. Locating and recognizing these minerals and rocks is how past
environmental conditions on Earth are determined.
224
National Science Teachers Association
Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions.