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Chapter 4 Rocks
Section 1 Classifying Rocks
Geologists observe a rock’s mineral composition, color, and texture.
1. Mineral composition and color
a. Rocks are made of mixtures of minerals and other materials
i. Some rocks contain one material; others contain several
ii. About 20 minerals make up most of Earth’s crust
1. Rock-forming Minerals: the common minerals that make up most of the rocks of
Earth’s crust
b. A rock’s color provides clues to the rock’s mineral composition
i. Granite: a light-colored rock that has high silica content
ii. Basalt: a dark-colored rock that is low in silica
iii. Colors alone are not enough to identify a rock
iv. Geologists observe rock’s shape and color of crystals to identify the minerals in the rock.
1. Geologists also use some of the same tests used to identify minerals to identify rocks
a. Ex: Testing the surface of the rock with acid determines if the rock contains
minerals made of compounds called carbonates
2. Texture
a. A rock’s texture is very useful in identifying a rock
b. Most rocks are made up of particles of minerals or other rocks called grains
i. Grains: give rock its texture
ii. Texture: the look and feel of the rock’s surface
c. Geologists use size, shape, and pattern of the grains to describe texture
i. Grain size
1. Some rocks have large grains that are easy to see—coarse-grained
2. Some rocks have grains so small a microscope needs to be used—fine-grained
3. Some rocks have no grain, even under a microscope
ii. Grain Shape
1. Some grains look like particles of sand
2. Some look like small seeds or exploding stars
3. The grain results from the shapes of the crystals in some rocks
4. Can be smooth and round or can be jagged
iii. Grain Pattern
1. Some grains lie in flat layers like pancakes
2. Other grains form swirling patterns
3. Grains can be in bands of different colors
4. Grains can occur randomly throughout a rock
3. How Rocks Form
a. Geologists can use color, texture, and mineral composition to classify a rock by its origin, or how
the rock formed
b. Geologists classify rocks into three major groups
i. Igneous rock: forms from the coding of magma or lava
ii. Sedimentary rock: forms when particles of other rocks or the remains of plants and animals
are pressed and cemented together.
1. Form in layers below the surface
iii. Metamorphic rock: forms when an existing rock is changed by heat, pressure, or chemical
reactions
1. Most forms deep underground
Section 2 Igneous Rock
1. Classifying Igneous Rock
a. Igneous rocks are classified according to their origin, texture, and mineral composition
i. Origin
1. Extrusive Rock: igneous rock formed from lava that erupted onto Earth’s surface
a. Basalt is the most common extrusive igneous rock
i. One of the most common rocks on Earth; forms much of Earth’s
ocean floors.
2. Intrusive Rock: igneous rock that formed beneath Earth’s surface
a. Granite is the most common
i. Forms the core of many mountain ranges
ii. Texture
1. Rapidly cooling lava forms fine-grained igneous rock with small crystals
2. Slowly cooling magma forms coarse-grained rock with large crystals
3. So, intrusive and extrusive rocks usually have different textures
a. Intrusive rocks have larger crystals than extrusive rocks
b. Extrusive rocks have a fine-grained or glossy texture
i. Obsidian is an extrusive rock that cooled rapidly without forming
crystals
iii. Mineral Composition
1. Most of Earth’s minerals contain silica
a. Silica: a material formed from oxygen and silicon
2. Lava that is low in silica usually forms dark-colored rocks
a. Ex: Basalt
3. Magma that is high in silica usually forms light-colored rocks
a. Ex: Granite
4. Geologists can make thin slices of a rock
a. Makes it easier to view under a microscope
2. Uses of Igneous Rock
a. Many are hard, dense, and durable so they are used in tools and building materials
b. Building Materials
i. Ancient Egyptians used granite for statues over 3500 years ago
ii. Incas of Peru used granite and other igneous rocks to build a fortress
iii. In the U.S. in the 1800’s and 1900’s, granite was used to build bridges and public buildings
and cobblestone streets
iv. Today-thin, polished sheets of granite are used in curbstones, floors, and kitchen counters
v. Basalt is crushed to make gravel used in construction
c. Other uses
i. Pumice has a rough surface
1. Good for cleaning and polishing
ii. Obsidian
1. Ancient Native Americans used it to make sharp tools for cutting and scraping
iii. Perlite is formed from the heating of obsidian
1. Mixed with soil for starting vegetable seeds
Section 3 Sedimentary Rocks
1. From sediment to rock
a. Sediment: small, solid pieces of material that come from rocks or living things
i. Includes shells, bones, leaves, stems, and other remains
ii. Sedimentary rocks form when sediment is deposited by water and wind
iii. Most are formed through a series of processes
b. Erosion: the process by which water, ice, wind, or gravity moves weathered rock or soil
i. The forces of erosion form sediment
1. Forces include cold, rain, waves, and grinding ice
ii. Water, wind, or ice loosen and carry away rock fragments
c. Deposition: the process by which sediments settles out of the water or wind carrying it
i. Eventually, the moving water, wind, or ice slows and deposits the sediment in layers
d. Compaction: the process that presses sediments together
i. Thick layers of settlement build up gradually
ii. The heavy layers press down on the layers beneath them
iii. The weight of new layers compacts the sediments
iv. The layers often remain visible in sedimentary rock
e. Cementation: the process in which dissolved minerals crystallize and glue particles of sediment
together
i. During compaction, the minerals in the rock dissolve in water
ii. The dissolved minerals seep into the spaces between particles and harden
2. Types of Sedimentary rock
a. Sedimentary rocks are classified by the type of sediments that make up the rock
b. There are 3 major groups
i. Clastic Rocks: a sedimentary rock that forms when rock fragments are squeezed together
1. Fragments can be too small to be seen or too heavy for you to lift
2. Clastic rocks are grouped by the size of their particles
3. Common clastic rocks include shale, sandstone, conglomerate, and breccia
a. Shale forms from tiny particles of clay
b. Sandstone forms from sand
c. Conglomerate has particles with rounded edges
d. Breccia is made of large fragments with sharp edges
ii. Organic Rocks: forms where the remains of plants and animals are deposited in thick layers
1. Organic: substances that were part of living things or made by living things
2. Two important organic rocks are coal and limestone
a. Coal forms from the remains of swamp plants buried in water
i. As the layers build up, the weight squeezes the decaying plants which
slowly change into coal
b. Limestone forms in the ocean from coral, clams, and oysters
i. When these animals die, their shells pile up
ii. These layers of sediment can grow to a depth of hundreds of meters
iii. Compaction and cementation change the sediment to limestone
3. Chemical Rocks: rock formed when minerals that are dissolved in a solution
crystallize
a. Ex: Limestone can form when calcite that is dissolved in lakes, seas, or
underground water comes out of solution and forms crystals
i. This kind of limestone is considered a chemical rock
b. Chemical rocks may also form from mineral deposits left when seas or lakes
evaporate
i. Ex: Rock Salt-forms by evaporation
3. Uses of Sedimentary Rock
a. Used for building materials and tools for years
i. Flint was used for arrowheads
ii. Sandstone and limestone have been used as building material
1. The White House is made of sandstone
2. Limestone is used to make cement and steel
Section 4 Rocks from Reefs
1. Coral Reefs: structure formed from the skeletons of coral animals
a. How coral animals live
i. Most coral animals are smaller than your fingernail
ii. They look like a small sack with a mouth surrounded by tentacles
iii. Tiny algae grow within the body of each coral animals
1. Algae provides coral with substances they need to live
2. Coral provide a framework for the algae to grow on
iv. Algae need sunlight so almost all coral growth occurs within 40 meters of the water’s
surface
b. How a coral reef forms
i. Coral animals absorb calcium from the ocean water to form their skeletons
1. The calcium combines with carbon and oxygen to form calcite—calcite is a mineral
ii. When coral animals die, their skeletons remain. More coral builds on top of them, gradually
forming a coral reef
iii. Coral animals form only in warm, shallow water of tropical oceans
iv. Reefs are most abundant around islands and along the eastern coasts of continents
1. In the U.S., only the coasts of Hawaii and Florida have coral reefs
v. Over thousands of years, reefs may grow to be hundreds of kilometers long and hundreds
of meters thick
vi. Reefs usually grow toward the open ocean
2. Limestone from coral reefs
a. A coral reef is really organic limestone
i. Limestone from coral reefs has been forming in Earth’s oceans for over 400 million years
ii. Limestone is exposed when seas retreat
1. In the U.S., reefs that formed millions of years ago are exposed in Wisconsin, Illinois,
Indiana, Texas, New Mexico, and others.
2. Today, these reefs are thick deposits of sedimentary rock.
3. Deposits help geologists learn about past environments
Section 5 Metamorphic Rock
Heat and pressure deep beneath Earth’s surface can change any rock into metamorphic rock.
1. Types of Metamorphic Rock
a. While metamorphic rocks are forming, high temperatures change the size and shape of the grains,
or mineral crystals, in the rock.
i. Extreme pressure squeezes rock so tightly that the mineral grains may line up in flat,
parallel layers
b. Metamorphic rocks are classified according to the arrangement of the grains that make up the
rocks
i. Foliated rocks
1. Metamorphic rocks that have their grains arranged in parallel layers or bands
2. Includes slate, schist, and gneiss
3. Foliated rocks may split along the bands
ii. Nonfoliated rocks
1. The mineral grains in these rocks are arranged randomly
2. Do not split into layers
3. Include marble and quartzite
a. Quartzite forms out of sandstone
i. The weakly cemented quartz particles in sandstone recrystallize to
form quartzite
ii. Quartzite is much smoother than sandstone
2. Uses of Metamorphic Rock
a. Marble forms when limestone is under great heat and pressure
i. It has a fine, even grain
ii. It can be cut into thin slabs or carved into many shapes
iii. Easy to polish
iv. Marble is used for many buildings and statues
1. Ex: Abraham Lincoln
b. Slate also comes in a variety of colors
i. Can be used for flooring, roofing, outdoor walkways, chalkboards, and as trim for stone
buildings
Section 6: The Rock Cycle
Forces deep inside 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 and mantle that slowly changes rock from
one kind to another
1. A cycle of many pathways
a. Beginning the Rock Cycle—Stone Mountain
i. Millions of years ago, a huge mass of granite formed deep beneath Earth’s surface
ii. The forces of mountain building slowly pushed the mountain upward
iii. Over millions of years, water and weather began to wear away the granite of Stone
Mountain, forming sediment
iv. Today, particles of granite sediment still break off the mountain and become sand which is
carried to the ocean
b. Continuing the Rock Cycle
i. Over millions of years, layers of sandy sediment will pile up on the ocean floor
ii. This sediment will be compacted by its own weight
iii. Calcite in the ocean will cement the particles together
iv. The quartz that once formed Stone Mountain will become sandstone, a sedimentary rock
v. As more sediment piles up, the pressure will increase
vi. The rock will become hot and the particles will be compacted in the sandstone until no
spaces are left
vii. Silica, the main ingredient in quartz, replaces calcite as the cement holding the rock
together
viii. The rock’s texture will change from gritty to smooth
ix. After millions of years, the sandstone will have changed into quartzite, the metamorphic
rock
c. The future of the rock cycle
i. We can trace alternative pathways