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The Rock Cycle
Igneous rock is any rock that forms from magma or lava. The name “igneous” comes from the Latin word
ignis, meaning “fire.” Igneous rocks are classified according to their origin, texture, and mineral
composition. Extrusive rock is igneous rock formed from lava that erupted onto Earth’s surface. Basalt is the
most common extrusive rock. Igneous rock that formed when magma hardened beneath Earth’s surface is
called intrusive rock. Granite is the most abundant intrusive rock in continental crust.
The texture of an igneous rock depends on the size and shape of its mineral crystals. The only exceptions
to this rule are the different types of volcanic glass—igneous rock that lacks a crystal structure. Igneous rocks
may be similar in mineral composition and yet have very different textures.
Rapidly cooling lava forms fine-grained igneous rocks with small crystals. Slowly-cooling magma forms
coarse-grained rock with large crystals.
Intrusive rocks have larger crystals than extrusive rocks. Extrusive rocks have a fine-grained or glassy
texture. Basalt, for example, is an extrusive rock whose crystals are too small to be seen without a microscope.
Sediment is 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.
Erosion occurs when running water, wind, or ice loosen and carry away fragments of rock. Eventually, the
moving water, wind, or ice slows and deposits the sediment in layers. Deposition is the process by which
sediment settles out of the water or wind carrying it. The process that presses sediments together is
compaction. Over millions of years, thick layers of sediment build up. These heavy layers press down on the
layers beneath them. While compaction is taking place, the minerals in the rock slowly dissolve in the water.
Cementation is the process in which dissolved minerals crystallize and glue particles of sediment together.
There are three major groups of sedimentary rocks: clastic rocks, organic rocks, and chemical
rocks. Most sedimentary rocks are made up of broken pieces of other rocks. A clastic rock is a sedimentary
rock that forms when rock fragments are squeezed together. Clastic rocks are grouped by the size of the rock
fragments, or particles, of which they are made. Common clastic rocks include shale, sandstone,
conglomerate, and breccia.
Organic rock forms where the remains of plants and animals are deposited in thick layers. Two important
organic sedimentary rocks are coal and limestone. Coal forms from the remains of swamp plants buried in
water. Over millions of years, they slowly change into coal. Limestone forms in the ocean, where many living
things, such as coral, clams, and oysters, have hard shells or skeletons made of calcite. As they die, their
shells pile up in layers on the ocean floor. Over millions of years, compaction and cementation change the
sediment to limestone.
Organic rock forms when minerals that are dissolved in a solution crystallize. Limestone can form when
calcite that is dissolved in lakes, seas, or underground water comes out of solution and forms crystals.
Chemical rocks can also form from mineral deposits left when seas or lakes evaporate.
Every metamorphic rock is a rock that has changed its form. In fact, the word metamorphic comes from the
Greek words meta, meaning “change,” and morphos, meaning “form.” Heat and pressure deep beneath
Earth’s surface can change any rock into metamorphic rock. When rock changes into metamorphic rock,
its appearance, texture, crystal structure, and mineral content change. Metamorphic rock can form out of
igneous, sedimentary, or other metamorphic rock.
Collisions between Earth’s plates can push rocks down toward the heat of the mantle. Magma rising
through the crust can also provide heat to produce metamorphic rocks. Rock buried deep in the crust is under
pressure hundreds or thousands of times greater than at Earth’s surface, and that pressure can change rock
into metamorphic rock.
When metamorphic rock is forming, high temperatures can change the size and shape of grains in the
rock. Tremendous pressure can also squeeze rock so tightly that the mineral grains may line up in flat, parallel
layers.
Geologists classify metamorphic rocks according to the arrangement of the grains that make up
the rocks. Metamorphic rocks that have their grains arranged in parallel layers or bands are said to be
foliated. Foliated rocks— including slate, schist, and gneiss—may split apart along these bands. One common
foliated rock is slate, which is basically a denser, more compact version of shale.
Some metamorphic rocks are nonfoliated. The mineral grains in these rocks are arranged randomly.
Nonfoliated metamorphic rocks—including marble and quartzite—do not split into layers. Quartzite forms out of
sandstone. The weakly cemented quartz particles in the sandstone recrystallize to form quartzite, which is
extremely hard.
Name_______________________________
1. What is the most common extrusive rock? Where is it found?
2. What is the most common intrusive rock? Where is it found?
3. Igneous rock formed from lava that erupted onto Earth’s surface is called____________________ rock.
4. Igneous rock formed from magma below Earth’s surface is called____________________ rock.
Classify the following sedimentary rock by writing Clastic, Chemical, or Organic in the blank beside it.
________________ 5. Limestone made from
precipitated calcite
________________ 6. Coal
________________ 9. Shale
________________ 7. Breccia
________________ 11. Limestone made from shells
________________ 8. Rock salt
________________ 12. Conglomerate
________________ 10. Sandstone
Match each term with its definition by writing the letter of the correct definition on the line beside the term.
____ 13. Erosion
a. small, solid pieces of material from
rocks or living things
____ 14. clastic rock
b. the process that presses sediments
together
____ 15. Sediment
c. sedimentary rock that forms from
remains of plants and animals
____ 16. Cementation
d. the process in which running water,
wind, or ice loosen and carry away
____ 17. organic rock
rock fragments
e. the process in which dissolved
____ 18. Compaction
minerals crystallize and glue
sediments together
____ 19. chemical rock
f. sedimentary rock that forms when
rock fragments are squeezed
____ 20. deposition
together
g. the process by which sediment
settles out of wind or water
h. sedimentary rock that forms when
minerals dissolved in a solution
crystallize
Classify each of the following metamorphic rocks by writing either Foliated or Nonfoliated in the
blank beside it.
________________ 21. Marble
________________ 22. Slate
________________ 23. Gneiss
Fill in the blanks in the flowchart below.
Collisions between Earth’s plates push rock down toward the heat of Earth’s 24.
________________. ! As the rock is buried deeper in the crust, 25. ___________________ also
increases on the rock. ! The rock is squeezed so tightly that the 26. ___________________ of the
rock change, creating metamorphic rock.