Lesson 1

 

What is a rock?

Different rocks have different characteristics. Some are

smooth, some are rough, some are striped, and some are

spotted.

 

What are common rocks?

Most buildings and public monuments are made from

common rock. Rock used for building stone often contains

one or more common materials, called rock-forming

minerals. Two rock-forming minerals are quartz and calcite.

 A rock is a mixture of rock-forming minerals and other

materials such as volcanic glass, organic material, or other natural materials.

 

The Rock Cycle

Scientists have created a model to show how rocks slowly

change over time. The rock cycle shows the processes that

create and change rocks. The three types of rocks shown in

the rock cycle are igneous, metamorphic, and sedimentary.

The rock cycle shows how rocks can change from one type

of rock to another.

 

Description: http://www.public.asu.edu/%7Eedimaggi/Pictures/rockcycle.jpg

How do rocks change?

The rock cycle in the figure above shows there are several

processes that change rocks. Weathering breaks down rocks

into tiny mineral grains, or sediments. Erosion moves the

sediments by wind or water. Layers of sediments pile up.

They are compacted, or packed down, by more layers of

sediment piling on top of them. Over time, the pressure of

compaction turns the sediment into sedimentary rock.

Heat and pressure deep inside Earth may change

sedimentary rock into metamorphic rock. The metamorphic

rock can then melt and later cool to form igneous rock. The

igneous rock may then be weathered into mineral grains.

The grains eventually form new sedimentary rock. Any rock

can change into any of the three major types of rock.

A rock can even change into another rock of the same type.

No matter what happens, the mineral material is never lost

or destroyed. It is conserved, or used in other forms.

 

Who discovered the rock cycle?

Scottish scientist James Hutton noticed that some rocks

have straight layers, while others are tilted. He saw that

some rocks are weathered, while others are not. Hutton

observed that rocks change constantly over time.

 

 

Before You Read

Think about an erupting volcano you may have seen on TV

or in the movies. On the lines below, describe what comes

out of an erupting volcano.

 

Formation of Igneous Rocks

When a volcano erupts, red-hot material may flow out of

it. The extremely hot material is melted rock, called magma.

Magma flows like a liquid. When magma flows near or onto

EarthÕs surface, it cools and hardens. Igneous rock forms

from the cooled and hardened magma.

 

How does magma become lava?

Inside Earth, the temperature and pressure in certain

places are just right to melt rocks. As a result, magma

forms. Magma can be found at depths ranging from near

EarthÕs surface to about 150 km below the surface. The

temperature of magmas range from about 650¡C to 1,200¡C.

Where does the heat come from that melts rock inside

Earth?

Some heat comes from the decay of radioactive

elements in rocks. Some heat is left from when Earth was

formed. At first, Earth was very hot, molten material.

Magma is less dense than the solid rock around it.

Because it is less dense, it is forced up toward EarthÕs

surface. When magma reaches EarthÕs surface and flows

from volcanoes, it is called lava.

 

Description: im14

What are intrusive igneous rocks?

Magma is melted rock made up of common elements and

liquids. Magma cools as it rises toward EarthÕs surface. As

magma cools, the atoms and compounds in the liquid

rearrange themselves into new crystals called mineral grains.

As cooling continues, mineral grains grow together to form

rocks. Sometimes this process takes place beneath the

surface. Intrusive igneous rocks form from cooling magma

beneath EarthÕs surface, as shown in the figure below. It

takes a long time for magma beneath EarthÕs surface to cool.

Cooling is so slow, mineral grains grow quite large.

Intrusive igneous rock has large mineral grains.

Intrusive igneous rocks can be found on EarthÕs surface.

After many years, the layers of rock and soil that once

covered them are removed by erosion. Erosion occurs when

the rocks are pushed up by forces inside Earth.

 

What are extrusive igneous rocks?

Extrusive igneous rocks form as lava cools on the surface

of Earth, as shown in the figure above. When lava reaches

the surface, it is exposed to air and water, which cools it

quickly. The atoms in the liquid do not have time to

arrange into large crystals. Therefore, the mineral grains in

extrusive igneous rock are quite small.

 

 

What is volcanic glass?

Sometimes, lava that comes out of a volcano cools so

quickly that few or no mineral grains form. A rock that

forms from this quickly cooling lava is called volcanic glass.

Volcanic glass has few or no crystals because the atoms are

not arranged in an orderly pattern.

Obsidian is a volcanic glass that looks like shiny black

glass. Pumice and scoria are also volcanic glasses, but they

do not look like glass. They have lots of holes, or pores.

These materials form from a gooey liquid that contains

pockets of gases. Some of these gases escape and holes are

left where the rock formed around the gas pocket.

 

Classifying Igneous Rocks

Igneous rocks can be grouped as either intrusive or

extrusive depending on how they are formed. Igneous rocks

can also be grouped according to the type of magma they

come from. An igneous rock can form from basaltic,

andesitic, or granitic magma. The type of magma that cools

to form an igneous rock affects the properties of that rock.

Some of the chemical and physical properties of a rock are

its mineral composition, density, color, and melting

temperature.

 

What are basaltic rocks?

Igneous rocks that are dense and dark-colored are

basaltic (buh SAWL tihk). They form from magma

containing a lot of iron and magnesium, but little silica,

which is made of silicon and oxygen. Basalt gets its dark

color from the iron and magnesium it contains. Basaltic

lava is fluid and flows freely.

 

What are granitic rocks?

Granitic igneous rocks are light-colored and not as dense

as basalt. They form from thick, stiff magma that contains

lots of silica, but smaller amounts of iron or magnesium.

Stiff granitic magma can build up lots of gas pressure. This

pressure is released in violent volcanic eruptions.

 

What are andesitic rocks?

Andesitic igneous rocks have mineral compositions

between those of basalt and granite. Like granitic magma,

andesitic magma can produce violent volcanic eruptions.

 

Lesson 3

 

Before You Read

Think about a time you packed a sandwich for lunch and

placed a can of juice or soda on top of it. Describe how the

sandwich looked at lunchtime and why it looked that way.

 

Formation of Metamorphic Rocks

Like a sandwich that has been flattened by a can of soda,

rocks can be affected by changes in pressure. Changes in

temperature also can affect rocks. Metamorphic rock is rock

that has been changed because of changes in temperature

and pressure, or the presence of hot, watery fluids. These

conditions may change the rockÕs form, the material it

contains, or both.

Metamorphic rock may form from sedimentary rocks,

igneous rocks, and even other metamorphic rocks.

 

How do heat and pressure change rock?

Rocks deep beneath EarthÕs surface are under great

pressure from the layers of rock above them. Temperature

also increases with depth. In some areas, the pressure and

the temperature are just right to melt rock. The melted rock

forms magma. Different types of metamorphic rock may

form from the magma.

In other places deep inside Earth where there is a lot of

liquid, rocks do not melt. Instead, some mineral grains

dissolve in the liquid and then form new crystals. Under

these conditions, minerals sometimes exchange atoms with

surrounding minerals and new minerals form.

 

How does shale change to gneiss?

Depending on the amount of pressure and the

temperature under Earth, one type of rock can change into

several different types of metamorphic rock. For example,

shale, a sedimentary rock, will change into slate, a

metamorphic rock. As the temperature and pressure on it

increase, the slate can change into phyllite, then into schist,

and finally into gneiss (NISE).

 

How do hot fluids change rock?

Hot fluids from magma flow through spaces in and

between underground rocks. The hot fluids are mostly

water, but they also contain dissolved elements and

compounds. These fluids can react with the rock they flow

through and change its composition. As shown in the figure

below, the hot fluid flows into the rock and chemically

changes it into a type of metamorphic rock.

 

Description: http://gamediv1.weebly.com/uploads/9/8/4/6/9846625/427418953_orig.jpg

Classifying Metamorphic Rocks

Metamorphic rocks form from igneous, sedimentary, or

other metamorphic rocks. Heat, pressure, and hot fluids

cause these rocks to change. The types of metamorphic

rocks that form can be classified based on their composition and texture.

 

What are foliated rocks?

As some metamorphic rocks form, their mineral grains

line up in parallel layers. Metamorphic rocks with a foliated

texture have parallel layers of mineral grains. For example,

slate is a metamorphic rock that forms from shale, a

sedimentary rock. When shale is exposed to heat and

pressure, it changes. Its mineral grains line up in parallel

layers to form slate, a foliated metamorphic rock.

Slate SlateÕs parallel layers of mineral grains are pressed so

tightly together that water cannot pass between them easily.

Slate also breaks into smooth, flat pieces. Because it sheds

water and splits smoothly, slate is often used for paving

stones and roofing tiles.

Gneiss Gneiss is a foliated rock that forms when granite

and other rocks are changed by heat and pressure. The

foliated texture of gneiss is easily seen in its light and dark

bands. As gneiss forms, the movement of atoms separates

the dark minerals in the rock from the light-colored

minerals in the rock.

 

What are nonfoliated rocks?

Some metamorphic rocks are formed without layers. In

these rocks, the mineral grains grow and rearrange, but do

not form layers. Nonfoliated rocks are metamorphic rocks

that form without a layered texture.

Sandstone Sandstone is a sedimentary rock made mostly of

quartz grains. When it is heated under a lot of pressure,

sandstone is changed into quartzite. Heat and pressure cause

the sandstoneÕs quartz grains to grow larger and lock

together like pieces of a jigsaw puzzle. The quartz grains in

quartzite are not in layers, so quartzite is a nonfoliated rock.

Marble Another nonfoliated metamorphic rock is marble.

Marble forms from limestone that is under great heat and

pressure. Limestone contains the mineral calcite. Heat and

pressure change the calcite into marble, which does not have

a layered texture. In fact, marbleÕs fine, smooth texture

makes it the perfect material for sculptures and buildings.

 

Lesson 4

 

Before You Read

Imagine you are stacking slices of bread, one on top of the

other. Then you put a heavy book on top of the stack and

leave it there overnight. Describe how the slices of bread

might look the next day.

 

Formation of Sedimentary Rocks

Weathering breaks down rocks into sediment. Sediment is

the loose material, such as tiny pieces of rock, mineral

grains, and bits of shell, that are moved by wind, water, ice,

or gravity. Sediments come from already-existing rocks that

are weathered and eroded.

Sedimentary rock forms when sediments are pressed and

cemented together, or when minerals form from solutions.

About 75 percent of the rocks you see on EarthÕs surface

are sedimentary rocks.

 

What do sedimentary rocks look like?

Sedimentary rocks often form as layers, like a stack of

papers. The older layers are at the bottom because they

were deposited first. The newer layers are at the top because

they were deposited later. If sedimentary rock is not disturbed,

the layers will remain in place, with the oldest at the bottom

and youngest at the top.

Sometimes, though, forces within Earth overturn layers of

sedimentary rock. Then, the oldest layers are no longer on

the bottom. The order of the layers is changed.

 

Classifying Sedimentary Rocks

Sedimentary rocks can be made of just about any material

in nature. Sediments come from weathered and eroded

sedimentary, metamorphic, and igneous rock. Sediments

also can come from the remains of some organisms. The

composition of a sedimentary rock depends on what types

of sediments formed it.

Sedimentary rocks are classified by what they are made of.

They are also classified by the way in which they formed.

Sedimentary rocks are classified as detrital, chemical, or

organic.

 

Detrital Sedimentary Rocks

The word detrital (dih TRI tul) comes from the Latin

word detritus, which means Òto wear away.Ó Detrital

sedimentary rocks are made from the broken pieces of other

rocks. The tiny pieces are compacted and cemented together

to form solid sedimentary rock.

 

What are weathering and erosion?

Weathering is the process in which air, water, or ice breaks

down rocks into smaller and smaller pieces. The movement

of weathered material is called erosion.

 

What is compaction?

Erosion moves sediments to a new place, where they are

deposited in a thin layer. Over time, layer upon layer of

sediment builds up. The weight of the top layers pushes

down on the lower layers. Downward pressure causes small

sediments to stick together and form solid rock. The process

in which layers of sediments are pressed together to form rock

is called compaction. The figure below shows how rock pieces

are compacted to form sedimentary rock.

 

Compaction of Sediments

 

Cementation

 

 

What is cementation?

Large sediments, like sand grains and pebbles, cannot

form rock from pressure alone. These large sediments form

rock only if something helps them stick together.

As water moves through rock and soil, it picks up materials

released by the weathering of minerals. The resulting solution

of water and dissolved minerals moves through open spaces

between larger sediments. The solution acts as a kind of glue

that holds the large sediments together. Cementation is the

process in which sediments are held together by dissolved

minerals produced when water moves through rock. Minerals

such as quartz, calcite, and hematite make the best cement

for holding large sediments together.

 

What are the shapes and sizes of sediments?

Detrital rocks have a grainy texture, like grains of sugar.

They are named according to the shapes and sizes of the

sediments that form them. For example, conglomerate and

breccia (BRECH uh) are detrital rocks that form from large

sediments. If the sediments are rounded, the rock is called

conglomerate. If the sediments have sharp angles, the rock

is called breccia. The farther sediments are carried by wind,

water, or ice, the more rounded they become.

 

What materials are found in sedimentary rocks?

Conglomerate and breccia are formed from gravel-sized

sediments that are cemented together by quartz or calcite.

These sediments may come from the minerals quartz or

feldspar, or may contain chunks of other rocks, such as

gneiss, granite, or limestone.

Sandstone forms from small sediments. The sand-sized

sediments in sandstone can come from almost any mineral,

though they usually come from quartz and feldspar. Shale

is a detrital sedimentary rock that is made from the smallest

clay sediments.

 

Chemical Sedimentary Rocks

When water evaporates from a salt solution, salt grains

remain. In a similar way, when the water in a lake evaporates,

its minerals remain. The remaining mineral deposits form

sediments which, in turn, form rocks. Chemical sedimentary

rocks form when dissolved minerals come out of solution and

form sediments that become rocks.

 

How does limestone form?

Calcium carbonate is found dissolved in ocean water.

Calcium carbonate comes out of solution as the mineral

calcite. Calcite forms crystals, which bond to form limestone,

usually on the bottom of lakes and shallow seas. Long ago,

the central United States was covered with a shallow sea.

Over time, the water evaporated. As a result, much of the

central United States has limestone bedrock.

 

How does rock salt form?

Some bodies of water contain a lot of dissolved salts. When

the water evaporates, it deposits the mineral halite, or rock

salt. Rock salt is mined. It is used in manufacturing glass,

paper, and soap. It is also made into table salt.

 

Organic Sedimentary Rocks

Rocks made of materials that were once living things are

called organic sedimentary rocks. One of the most common

organic sedimentary rocks is fossil-rich limestone. It is made

of the remains of once-living ocean organisms. Ocean animals,

such as clams and snails, make their shells out of calcium carbonate,

which eventually becomes calcite. When the animals die, their

shells pile up and become cemented together to form fossil-rich limestone.

 

What are other organic sedimentary rocks?

Chalk Chalk is an organic sedimentary rock that is made

up of extremely tiny bits of animal shells. When you write

with chalk, you are crushing and smearing the calcite shell

remains of once-living ocean animals.

Coal Coal is a useful organic sedimentary rock that forms

when pieces of dead plants are buried under other sediments in

swamps. The plant material is chemically changed. The resulting

sediments are compacted to form coal. Today, coal is a fuel used

in power plants to make electricity.