Rocks

Classifying Rocks

Igneous Rocks

Sedimentary Rocks

Rocks from Reefs

Metamorphic Rocks

The Rock Cycle

Table of Contents

Rocks

• When studying a rock sample, geologists observe the rock’s mineral composition, color, and texture.

Studying Rocks- Classifying Rocks

Rocks

Mineral Composition and Color • Rock – a solid mixture of minerals and

other materials.• Rock-forming minerals – the common

minerals that make up most of the rocks of Earth’s crust.

– Granite – tends to be a light-colored rock that has high silica content

– Basalt – tends to be a dark-colored rock that is low in silica

- Classifying Rocks

Rocks

Texture

• Grains – particles of minerals or other rocks that give a rock its texture.

• Texture – the look and feel of a rock’s surface, determined by the size, shape, and pattern of a rock’s grains.

- Classifying Rocks

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Texture• Grain size

– Coarse-grained – large easy to see grains– Fine-grained – so small they can only be

seen under a microscope

- Classifying Rocks

Rocks

Texture• Grain Shape – varies greatly from sand grains to

large triangular grains

• Grain Pattern – can vary also from layers to swirls to bands

- Classifying Rocks

Rocks

How Rocks Form• Geologists classify rocks into three major

groups: – igneous rock, sedimentary rock, and

metamorphic rock.

- Classifying Rocks

Rocks

How Rocks Form• Igneous rock – type of rock that forms from

the melting then cooling of molten rock at or below the surface.

• Sedimentary rock – type of rock that forms when particles from other rocks or the remains of plants and animals are pressed and cemented together.

• Metamorphic rock – type of rock that forms from an existing rock that is changed by heat and pressure

- Classifying Rocks

Rocks

Classifying Igneous Rocks• Igneous rocks are classified according to

their origin, texture, and mineral composition.

- Igneous Rocks

Rocks

Classifying Igneous Rocks • Extrusive igneous rock – forms from lava on

Earth’s surface.

• Intrusive igneous rock – forms when magma hardens beneath Earth’s surface.

• The faster igneous rock cools, the finer the grains– Extrusive igneous rock tends to be a finer-

grained rock than intrusive igneous rock

Rocks

Mineral Mixture

• Granite is a mixture of light-colored minerals, such as feldspar and quartz, and dark-colored minerals, including hornblende and different types of mica. But granite can vary in mineral composition. This affects its color and texture.

- Igneous Rocks

Rocks

Mineral Mixture

– Feldspar

• Reading Graphs:

– What mineral is most abundant in granite?

- Igneous Rocks

Rocks

Mineral Mixture

– 10%

• Reading Graphs:

– About what percentage of granite is made up of dark minerals?

- Igneous Rocks

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Mineral Mixture

– 100% - (35% + 10%) = 55%

• Calculating:

– If the amount of quartz increases to 35 percent and the amount of dark-colored minerals stays the same, what percentage of the granite will be made up of feldspar?

- Igneous Rocks

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Mineral Mixture

– The overall color would be darker.

• Predicting:

– How would the color of the granite change if it contained less feldspar and more mica and hornblende?

- Igneous Rocks

Rocks

Links on Igneous Rocks

• Click the SciLinks button for links on igneous rocks.

- Igneous Rocks

Rocks

From Sediment to Rock• Most sedimentary rocks are formed

through a series of processes: erosion, deposition, compaction, and cementation.

- Sedimentary Rocks

Rocks

From Sediment to Rock

• Sediment – small, solid pieces of material that come from rocks or organisms.– Usually forms due to erosion

• Erosion – a destructive process in which water or wind loosens and carries away fragments of rock.

• Deposition – process by which sediment settles out of the water or wind that is carrying it.

- Sedimentary Rocks

Rocks

From Sediment to Rock• Compaction – process by which

sediments are pressed together under their own weight.– Occurs over millions of years as

layers build• Cementation – process by which

dissolved minerals crystallize and glue particles of sediment together into one mass.– Takes place when sediment is in

the presence of water

- Sedimentary Rocks

Rocks

Types of Sedimentary Rock• There are three major groups of

sedimentary rocks: – clastic rocks, organic rocks, and

chemical rocks.• Clastic rock – sedimentary rock that

forms when rock fragments are squeezed together under high pressure. – Can range in size from microscopic

clay particles to large boulders – Ex: shale, sandstone, conglomerate,

and breccia

- Sedimentary Rocks

Rocks

Types of Sedimentary Rock• Organic rock – sedimentary rock that

forms from remains of organisms deposited in thick layers.– Ex: Coal and Limestone– Coal forms from the remains of swamp

plants buried in water. – Limestone forms in the ocean, where

many living things, such as coral, clams, and oysters, have hard shells made of calcite that collect on the ocean floor.

- Sedimentary Rocks

Peat

Bituminous

Lignite

Anthracite

Rocks

Types of Sedimentary Rock• Chemical rock –

sedimentary rock that forms when minerals crystallize from a solution. (Reacts to HCl)– Ex: Limestone forms

from calcite deposits dissolved in lakes, rivers, streams.

- Sedimentary Rocks

Rocks

Uses of Sedimentary Rock• Sedimentary rock has

been used for thousands of years in building structures, tools, and statues.

– Ex: The White House in Washington D.C. is constructed of sandstone.

- Sedimentary Rocks

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Links on Sedimentary Rocks

• Click the SciLinks button for links on sedimentary rocks.

- Sedimentary Rocks

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Coral Reefs• Coral reef – a structure of calcite skeletons

built up by coral animals in warm, shallow ocean water. – When coral animals die, their skeletons remain.

More corals build on top of them, gradually forming a coral reef.

– Only form in ocean water between 30°N and 30°S latitude

- Rocks From Reefs

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Coral Reefs(not in notes)

• Coral reef is really organic(from living things) limestone.

• Deposits of organic limestone help geologists figure out what the environment was like long ago.

• If they find a fossil of coral reef, they would know that location used to contain warm, shallow ocean water.

Rocks

More on Coral Landforms

• Click the PHSchool.com button for an activity

about coral landforms.

- Rocks From Reefs

Rocks

Metamorphic Rock• Metamorphic rock –

forms when heat and pressure beneath Earth’s surface changes the appearance, texture, crystal structure, and mineral content of either igneous, sedimentary, or other metamorphic rock.

- Metamorphic Rocks

Rocks

Types of Metamorphic Rocks • Geologists classify

metamorphic rocks according to the arrangement of the grains that make up the rocks. – Foliated rocks – metamorphic

rocks that have grains arranged in parallel layers or bands.

– Ex: Slate – metamorphic shale (denser and more compact than shale)

- Metamorphic Rocks

Shale

Slate

Gneiss

Rocks

Types of Metamorphic Rocks• Nonfoliated rocks – metamorphic rocks

that have randomly arranged metamorphic grains that do not split into layers– Ex: Quartzite – metamorphic sandstone

Marble – metamorphic limestone

- Metamorphic Rocks

Rocks

Links on Metamorphic Rocks

• Click the SciLinks button for links on metamorphic rocks.

- Metamorphic Rocks

Rocks

A Cycle of Many Pathways• Forces deep

inside Earth and at the surface produce a slow cycle that builds, destroys, and changes the rocks in the crust.

- The Rock Cycle

Rocks

• Rock cycle – processes on the surface and inside Earth that slowly change rocks from one kind to another.

- The Rock Cycle

Rocks

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 to form sedimentary and metamorphic rocks.

- The Rock Cycle

Rocks

Rock Cycle Activity

• Click the Active Art button to open a browser window and access Active Art

about the rock cycle.

- The Rock Cycle

Rocks

Magma

Rock Cycle

Igneous

Sedimentary

Metamorphic

- The Rock Cycle

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Sedimentary

includeinclude include

can be

IntrusiveNon-

foliatedClastic

Graphic Organizer

Igneous Metamorphic

Extrusive Organic Chemical Foliated