Download Section 1: Classifying Rocks Mineral Composition and Color

Rocks
Section 1: Classifying Rocks
Mineral Composition and Color:
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Rocks are made of mixtures of minerals and other materials. Some rocks contain one mineral, others contain several minerals.
About 20 minerals make up most of the rocks of earth’s crust. These minerals are known as rock-forming minerals.
A rock’s color provides clues to the rock’s mineral composition. But color alone does not provide enough information to identify
a rock.
Geologists observe the shape and color of crystals in a rock to identify the minerals the rock contains. Geologists use some of the
same tests used for identifying minerals.
Texture:
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Most rocks are made up of particles of minerals or other rocks which geologists call grains.
Grains give the rock its texture. A rock’s texture is the look and feel of the rock’s surface.
Grain size: rocks with grains that are large and easy to see are called coarse-grained; rocks with grains that are so small they
can only be seen with a microscope are called fine-grained; some rocks have no visible grain even when examined under a
microscope.
Grain shape: grains in a rock vary widely in shape; sometimes grains result from the shapes of crystals that form the rocks, like
granite; in other rocks, grain shape results from fragments of several rocks; fragments can be smooth or rounded or they can be
jagged
Grain Pattern: grains often form patterns; some lie in flat layers like pancakes; others form swirling patterns; some have grains of
different color bands; in others the grains occur randomly throughout
How rocks form:
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Geologists classify rocks into 3 major groups: igneous rock, sedimentary rock and metamorphic rock
Igneous rock forms from the cooling of magma or lava
Sedimentary rock forms when particles of other rocks or the remains of plants and animals are pressed and cemented together;
forms in layers that are buried below the surface
Metamorphic rock forms when an existing rock is changed by heat, pressure of chemical reactions; most metamorphic rock
forms deep underground
Rocks
Section 2: Igneous Rocks
Classifying Igneous Rocks:
Igneous rocks are classified according to their origin, texture and mineral composition
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Origin: Igneous rock may form on or beneath Earth’s surface; extrusive rock is igneous rock formed from lava that
erupted onto Earth’s surface (basalt); intrusive rock is formed when magma hardened beneath Earth’s surface
(granite)
Texture: texture of an igneous rock depends on the size and shape of its mineral crystals; the exceptions to this rule
are the different types of volcanic glass
Intrusive and extrusive rocks usually have different textures; intrusive rocks have larger crystals and are coarse
grained; extrusive rocks have a fine grained or glassy texture
Mineral Composition:
Lava that is low in silica usually forms dark colored rocks, such as basalt.
Lava that is high in silica usually forms light colored rocks, such as granite.
Using thin slices of rock, geologists can study crystals under a microscope to determine the rock’s mineral composition
Uses of Igneous Rocks:
People throughout history have used igneous rock for tools and building materials.
Building Materials: Granite has a long history as a building material; used to build bridges and public buildings and for
paving streets with cobblestones, polished granite is used in flooring, countertops and curbstones; Basalt is crushed to make
gravel used in construction
Other Uses: pumice is a good abrasive for cleaning and polishing; ancient native Americans used obsidian to make sharp
tools for cutting and scraping; perlite (formed from heating obsidian) is often mixed with soil for starting vegetable seeds
Rocks
Section 3: Sedimentary Rocks
From Sediment to Rock:
Sediment: small, solid pieces of material that come from rocks or living things
Most sedimentary rocks are formed through a series of processes: erosion, deposition, compaction and cementation
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Erosion: destructive forces (heat and cold, rain, waves, grinding ice) are breaking up and wearing away (weathering) the rocks
on Earth’s surface; in erosion, running water, wind or ice loosen and carry away fragments of rock
Deposition: when the moving water, wind or ice slows down, it deposits the sediment in layers; if water is carrying the sediment
rock and other materials settle at the bottom of lakes or oceans; deposition is the process by which sediment settles out of the
water or wind carrying it
Compaction: the process that presses sediments together; thick layers of sediment build up over millions of years; the weight of
new layers further compacts sediment squeezing them tightly together; layers often remain visible is sedimentary rock
Cementation: is the process in which dissolved minerals crystallize and glue particles of sediment together; dissolved minerals
seep into the spaces between particles and the harden
Types of Sedimentary Rock:
There are three major groups of sedimentary rocks: clastic rocks, organic rocks and chemical rocks
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Clastic rocks: a sedimentary rock that forms when rock fragments are squeezed together; grouped by the size of the rock
fragments or particles of which they are made; shale, sandstone, conglomerate and breccia; if fragments have rounded edges
they form conglomerate; if there are large fragments with sharp edges they form breccia
Organic rocks: form where the remains of plants and animals are deposited in thick layers; “organic” once part of a living thing
or were made by living things; coal and limestone
Chemical rocks: minerals that are dissolved in solution crystallize and form chemical rocks; also form from mineral deposits left
when seas or lakes evaporate; limestone and rock salt
Uses of Sedimentary Rocks:
People have used sedimentary rock throughout history for many different purposes including building materials and tools; people
made arrowheads out of flint; sandstone and limestone have been used as building materials the White House is made of sandstone;
limestone is used in making cement and steel
Rocks
Section 5: Metamorphic Rocks
“Meta” means change…”morphosis” means form…metamorphic rocks are rocks that change form
Heat and pressure deep beneath Earth’s surface can change any rock into metamorphic rock. The appearance, texture,
crystal structure and mineral content of the rock can change. Metamorphic rock can form from igneous, sedimentary and
other metamorphic rock
Heat and pressure under Earth’s surface cause rocks to change form.
Types of Metamorphic Rocks:
Foliated Rocks: grains are arranged in parallel layers; foliated rocks may split apart along these bands; common foliated
rocks are slate, schist and gneiss
Slate is a common foliated rock. Heat and pressure changed the sedimentary rock shale into slate. Slate is a denser, more
compact version of shale. During the change, new minerals such as mica formed in the shale
Nonfoliated Rocks: grains are arranged randomly; they do not split into layers; common nonfoliated rocks are marble and
quartzite
Quartzite forms out of sandstone. The weakly cemented quartz particles in sandstone recrystallize to form quartzite, which is
extremely hard
Uses of Metamorphic Rock:
Certain metamorphic rocks are important materials for building and sculpture. Marble and slate are two of the most useful
metamorphic rocks.
Marble forms when limestone is subjected to heat and pressure. It can be cut into thin slabs or carved into many shapes.
Marble is easy to polish. The Lincoln Memorial in Washington D.C. is carved from marble.
Slate comes in a variety of colors red, gray, black, and purple. It also splits into flat pieces. The pieces can be used for
flooring, roofing, outdoor walkways, chalkboards and trim for buildings.
Rocks
Section 6: The Rock Cycle
Forces deep inside the earth and at the surface produce a slow cycle that builds, destroys and changes the rocks in the
crust
Rock Cycle: a series of processes on Earth’s surface and in the crust that slowly change rocks from one kind to another
There are many ways a rock can go through the rock cycle. Here is one way…
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Igneous rock on the surface is turned into sediment by erosion. The sediment is deposited and slowly becomes
sedimentary rock
The sedimentary rock is buried by more sediment. Heat and pressure slowly change the sedimentary rock into
metamorphic rock.
The metamorphic rock is forced into the mantle. The metamorphic rock melts to form magma. The magma erupts
and cools to form igneous rock again
The Rock Cycle and Plate Tectonics:
Plate movements start the rock cycle by helping to form magma, the source of igneous rocks. Plate movements also cause
faulting, folding and other motions of the crust that help form sedimentary and metamorphic rock.
Igneous rocks:
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Where oceanic plates move apart and magma moves upward and fills the gap with new igneous rock.
Where oceanic plate is subducted beneath a continental plate magma forms and rises. The result is a volcano made
of igneous rock.
A collision of continental plates may push rocks so deep they melt and form magma. This magma slowly cools and
hardens to form igneous rock
Sedimentary and Metamorphic rock:
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Collision of continental plates could push up a mountain range. Erosion begins and the mountains are eventually
worn away leading to the formation of sedimentary rock
Collision between plates can also push rock deep into the mantle. Heat and pressure there could change the rocks
into metamorphic rocks
The rock cycle continues, for hundreds of millions of years.