Rocks

Earth Science Chapter 4 1

Rocks

Chapter 4


Rock• Material that makes up solid parts of Earth• Can be made of

• one or more minerals• Solid organic matter• Noncrystalline matter, such as glass


Rock-forming minerals

• About twenty minerals that make up most rocks on Earth’s crust


Granite

• Made of quartz, feldspar, hornblende, and mica

• May also contain other minerals• Generally light-colored• High silica content


Basalt

• Dark-colored• Low in silica• Very small mineral crystals


Grains

• Small particles of minerals or other rocks that make up a rock

• Grain size• Coarse, fine, or not visible

• Grain shape• Rounded or jagged

• Grain pattern• Nonbanded or banded


Texture

• The look and feel of a rock’s surface


Igneous rock

• From Latin term meaning “from fire”• Hardened magma or lava


Sedimentary rock

• Compressed or cemented sediment• Sediment – rocks, minerals, and organic

matter that has been broken into fragments


Metamorphic rock

• Means “changed form”• Existing rock is altered by tremendous

pressure, extreme heat, or chemical processes


Discuss

• What three characteristics do geologists use to identify rocks?

• What are the grains of a rock?• What are the three main groups of rocks?• Gneiss is a kind of rock that forms when

heat and pressure inside Earth change granite. To what group of rocks does gneiss belong?


Igneous rock

• Formed from lava or magma• Classified by origin, texture, and mineral

composition


Origin

• Extrusive rock• From lava• Basalt is most common

• Intrusive rock• From magma• Granite is most common


Texture

• Depends on size and shape of mineral crystals• Some igneous rocks cool so quickly that they

lack crystal structure• Intrusive rocks have larger crystals

• Cooled more slowly


Silica

• Material formed form oxygen and silicon


Mineral composition• Lava low in silica forms dark-colored rocks

• Basalt has feldspar and dark-colored minerals, but no quartz

• Magma high in silica forms light-colored rocks• Granite rich in feldspar is speckled pink• Granite rich in hornblende and mica is light gray with

dark specks• Quartz crystals add gray or smoky specks to granite


Building materials from igneous rocks

• Granite is used for• Statues (over 3,500 years ago)• Fortresses (about 600 years ago)• Bridges, public buildings, and cobblestones

(1800s and 1900s)• Curbstones, floors, and countertops (today)

• Basalt is crushed and used in construction


Other uses of igneous rocks

• Pumice’s rough surface makes it a good abrasive for cleaning and polishing.

• Obsidian was used by ancient peoples to make sharp tools.


Discuss

• How are igneous rocks classified?• Compare and contrast granite and basalt in

terms of their origin and texture. Which is extrusive? Which is intrusive?

• Would pumice be a good material to use to make a floor? Explain.


Sediment

• Small, solid pieces• Come from rock, shells, bones, leaves, etc.


Sedimentary rock formation

• Sediment is deposited by water and wind.• Most sedimentary rocks are formed by

erosion, deposition, compaction, and cementation.


Erosion

• Running water, wind, or ice loosen and carry away fragments of rock


Deposition

• Sediment settles out of the water or wind carrying it

• Sinks to the bottom of the water


Compaction

• Sediments are pressed together by weight of layers above

• Minerals dissolve in the water


Cementation

• Dissolved minerals crystallize and glue particles together


Discuss

• What is sediment?• In layers of sedimentary rock, where would

you expect to find the oldest layers? Explain.

• Place these steps in the formation of sedimentary rock in the proper sequence: compaction, erosion, cementation, deposition.


Major groups of sedimentary rocks

• Clastic rocks• Organic rocks• Chemical rocks


Clastic Rocks• Formed when rock fragments are squeezed

together• Shale

• Clay particles in thin, flat layers• Sandstone

• Forms from sand• Mostly quartz

• Conglomorate• Rounded rock fragments

• Breccia• Jagged rock fragments


Organic Rocks

• Remains of plants and animals in thick layers• Coal

• Remains of swamp plants (takes millions of years to form)

• Limestone• Shells from coral, clams, and oysters (millions of

years to form)


Chemical Rocks

• When minerals dissolved in solution crystallize• Limestone

• When dissolved calcite comes out of solution• Rock salt

• When seas or lakes evaporate


Uses

• Flint arrowheads• Hard, yet can be shaped• Formed when silica settles out of water

• Sandstone and limestone building materials• Soft enough to be easily cut

• Limestone used in making cement and steel


Discuss• What are the three main types of

sedimentary rock?• What process causes deposits of rock salt to

form? What type of sedimentary rock is rock salt?

• The particles of sediment that make up shale are not usually well cemented. Would shale be a good choice of building material in a wet climate?


Metamorphic rock

• Can be made from any other rock being changed by heat and/or pressure

• May become smoother• Minerals in the rock may change into other

minerals• Classified according to the arrangement of

the grains that make up the rocks


Foliated rocks

• Grains arranged in parallel layers or bands• May split apart along these bands


Nonfoliated rocks

• Mineral grains arranged randomly


Uses

• Marble• Limestone subjected to heat and pressure• Has fine, even grain• Can be cut thin or carved into shapes• Easy to polish• Flooring, statues, buildings

• Slate• Foliated – splits easily into flat pieces• Flooring, roofing, sidewalks, chalkboards


Discuss

• What does metamorphic mean?• How do geologists classify metamorphic

rocks?• Describe foliated and nonfoliated rocks.• Which might be more useful for carving

chess pieces, marble or slate?


Rock cycle

• Series of processes on Earth’s surface and in the crust and mantle that slowly change rocks from one kind to another

• Any of the three major types of rock can be changed into another of the three types

• There are many possible pathways a rock can follow

• See diagram on page 115


Discuss

• Website• Define the rock cycle in your own words.• What must happen in order for any rock in

the rock cycle to become a sedimentary rock?

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