ROCKS:

You probably don’t realize it,

but we depend on many rocks in our everyday lives. Rocks have different

chemical and physical properties that make them quite useful to us. Besides providing a

nice hard place for us to stand on Earth, we use rocks for the construction of our homes, for fuel, and for the creation of statues and art. Go home today and look around your house. You may be

surprised by what rocks you find.

ROCKS:

A ROCK is composed of one or more minerals. Rocks composed of only

one mineral are said to be MONOMINERALLIC.

This is a photo of a slice of Dunite, a monominerallic igneous

rock, under a microscope. Almost all of the grains in this rock are olivine. Note the high

order interference colors of olivine and the minor secondary calcite which occurs as veinlets

through the sample.

ROCKS: Most rocks are POLYMINERALLIC, that is, made of many different minerals. There are over 2,500 different minerals. However, there are 10-15 minerals known as the

rock-forming minerals, which compose about 90% of all the rocks in Earth’s crust.

METHOD OF FORMATION:

Rocks are classified by their METHOD OF FORMATION, into

three groups:

1) IGNEOUS2) SEDIMENTARY3) METAMORPHIC

Let’s take a more detailed at each.

IGNEOUS ROCKS:

Igneous rocks form from the MELTING and SOLIDIFICATION (hardening) of

molten material from within the Earth.

IGNEOUS ROCKS:Molten material within the Earth is

called MAGMA. When this molten material reaches Earth’s surface it’s

called LAVA. LAVA:

ON OR NEAR EARTH’S SURFACE

MAGMA:INSIDE OF

EARTH

IGNEOUS ROCKS:

Igneous rocks are solid, hard, and compact and are composed of randomly arranged

INTERGROWN MINERAL CRYSTALS.

GRANITE IS AN IGNEOUS ROCK WITH COARSE GRAINED CRYSTALS.

MAYBE YOU CAN SEE THEM BETTER CLOSE

UP?

TEXTURE:

Igneous rocks are best identified by their texture, environment of

formation, and mineral composition

Texture describes a rock’s “grain appearance” which is determined by

the size of the crystal and the arrangement of the crystals.

There are FOUR textures: glassy, fine, coarse, and very coarse.

TEXTURE:

1) GLASSY: A glassy texture forms when a mineral cools so quickly that no crystals are visible. Glassy textures occur when there is a rapid drop in

temperature and pressure. This condition exists when magma reaches Earth’s surface through a volcano.

OBSIDIAN

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

TEXTURE:

There are TWO types of glassy texture:

a) VESICULAR: A vesicular texture exhibits gas pockets and the rock appears holey. Ex: Pumice

b) NON-VESICULAR: A non-vesicular texture looks like glass. Ex: Obsidian

PUMICE

OBSIDIAN

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

TEXTURE:

2) FINE: A fine texture

forms when a mineral cools quickly at or near Earth’s surface. The grain size is less than 1mm. Ex: Rhyolite

RHYOLITE

SCORIABASALT

ANDESITE

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

TEXTURE:

3) COARSE: A fine texture

forms when a mineral cools SLOWLY WITHIN the Earth under high pressure. The grain size ranges from 1-10 mm. Ex: Granite

GRANITE

DIORITE GABBROPERIDOITE

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

TEXTURE:

4) VERY COARSE: A course

texture forms when a mineral cools very slowly within the Earth, producing grain sizes 10 mm or larger. Ex: Pegmatite

PEGMATITE

A CLOSER LOOK!!!

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

TEXTURE:

TEXTURE IS DETERMINED BY THE

LENGTH OF COOLING!

Rocks that cool FAST have SMALLER crystals.

Rocks that cool SLOW have LARGER crystals.

SMALL CRYSTALS

COOLED FAST

LARGE CRYSTALS

COOLED SLOWLY

RATE & DEPTH OF COOLING:

SUPER IMPORTANT FACT!

The grain size of igneous rocks depends on the RATE and DEPTH at which magma cools.

The diagram shows the

relationship between grain

size and rate/depth of

cooling.

Basalt 0.1 mm

Diabase 1 mm

Granite 10 mm

ROCK NAME GRAIN SIZE ENVIRONMENT

Surface

Near Surface

Deep

Slow Moderate Fast COOLING RATE

A

B

C

RATE & DEPTH OF COOLING:

COOLING RATE COOLING TIME DEPTH

CR

YS

TA

L S

IZE

CR

YS

TA

L S

IZE

CR

YS

TA

L S

IZE

THESE ARE GRAPHING RELATIONSHIPS THAT YOU SHOULD KNOW.

RATE & DEPTH OF COOLING:

COOLING RATE COOLING TIME DEPTH

CR

YS

TA

L S

IZE

CR

YS

TA

L S

IZE

CR

YS

TA

L S

IZE

THESE ARE GRAPHING RELATIONSHIPS THAT YOU SHOULD KNOW.

ENVIRONMENT OF FORMATION:

By studying a rock’s texture, you can infer a rock’s environment of formation. There are TWO different environments of formation.

1) EXTRUSIVE or VOLCANIC: This is when an igneous rock hardens NEAR the Earth’s surface or ON the Earth’s surface to form a rock with small or no crystals. These rocks are found by volcanoes.

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

ENVIRONMENT OF FORMATION:

2) INTRUSIVE or PLUTONIC: This is when

an igneous rock hardens within the Earth to form large crystals. When magma doesn’t erupt it can flow underground. Underground magma flows are called INTRUSIONS because they can move into and through fractures (cracks) in rock. If an intrusion hardens, it produces a large mass of igneous rock called a PLUTON.

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

CHEMICAL COMPOSITION:

A rock’s chemical composition refers to a rock’s mineral make-up. Igneous rocks are generally

described as having either a FELSIC composition or a MAFIC composition.

1) FELSIC COMPOSITION: Rocks with a felsic composition tend to contain a high percentage of the minerals potassium feldspar and quartz. They are RICH IN ALUMINUM (Al) and low in both iron (Fe) and magnesium (Mg). These rocks are usually LIGHTER IN COLOR and LOWER IN DENSITY than mafic rocks. Felsic rocks make up a large portion of continental crust. The granite family of rocks is felsic.

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

NOTE: Even

though OBSIDIAN is dark in color, it is considered FELSIC, because it is low in density.

This is the one

exception to the rule!

CHEMICAL COMPOSITION:

2) MAFIC COMPOSITION: Rocks with a mafic composition tend to contain a high percentage of the minerals plagioclase feldspar and pyroxene. They are RICH IN IRON (Fe) and MAGNESIUM (Mg) and low in ALUMINUM (Al). These rocks are usually DARKER IN COLOR and HIGHER IN DENSITY than felsic rocks. Mafic rocks are most common in the ocean basins. The Gabbro family of rocks is mafic.

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

MINERAL COMPOSITION: WHAT IS MINERAL

COMPOSITION?Mineral composition

refers to the different minerals that a rock is

made up of and the percentages of these

minerals.

Granite is mostly made up of POTASSIUM

FELDSPAR (pink/gray), BIOTITE MICA (black),

and QUARTZ (clear/white).

MINERAL COMPOSITION:

SCHEME FOR IGNEOUS ROCK IDENTIFICATIONIN YOUR ESRT:

FINDING THE MINERAL COMPOSITION USING YOUR ESRT:

You can determine the possible mineral

composition of specific igneous

rocks by looking at your ESRT. Simply

find a rock name on the chart, then look

at the minerals found in the column

under that rock name. Your rock

name can contain varying amounts of

those materials.

FINDING THE PERCENTAGE OF MINERALS IN AN IGNEOUS ROCK USING YOUR ESRT:

You can determine the possible mineral

composition of specific igneous

rocks by looking at your ESRT. Simply

find a rock name on the chart, then look

at the minerals found in the column

under that rock name. Your rock

name can contain varying amounts of

those materials.

FINDING THE PERCENTAGE OF MINERALS IN AN IGNEOUS ROCK USING YOUR ESRT:

You can determine the possible mineral

composition of specific igneous

rocks by looking at your ESRT. Simply

find a rock name on the chart, then look

at the minerals found in the column

under that rock name. Your rock

name can contain varying amounts of

those materials.

REVIEW OF IGNEOUS ROCKS:INTRUSIVE

( PLUTONIC)EXTRUSIVE(VOLCANIC)

OBSIDIAN, SCORIA, PUMICE, RHYOLITE, ANDESITE, BASALT

FINE, GLASSY

SMALL

FAST

ON/NEAR EARTH’S SURFACE

GRANITE, DIORITE, GABBRO, PERIDOITE, DUNITE, PEGMATITE

COARSE, VERY COARSE

LARGE

SLOW

INSIDE EARTH ENVIRONMENT OF

FORMATION

SPEED OF COOLING

SIZE OF CRYSTAL

NAME OF TEXTURE

ROCK EXAMPLES

REVIEW OF IGNEOUS ROCKS:

FELSIC MAFIC

Scoria, Basalt, Gabbro, Peridoite,

Dunite

High

High

Dark

Iron (Fe)Magnesium (Mg)

Granite, Pegmatite, Rhyolite, Pumice,

Obsidian

Low

Low

Light

Aluminum (Al)Silicon (Si)

MINERAL COMPOSITION

COLOR

DENSITY

TEMPERATURE AT WHICH MINERALS

IN ROCK CRYSTALLIZE

ROCK EXAMPLES