Rocks

• Definition of a rock:– a group of minerals bound together

– makes up a layer of the Earth’s crust

• Lithosphere = thin layer of solid, brittle rock

• Rocks are classified as:– Igneous

– Sedimentary

– Metamorphic

The Rock Cycle

Cooling & Crystallization

Igneous Rocks

• form from the cooling and hardening (crystallization) of molten rock– magma: molten rock

existing beneath the ground = Plutonic

– lava: molten rock existing on the Earth’s surface (from volcanoes) = Volcanic

A. Intrusive (Plutonic) Rocks• igneous rocks that form from magma that cools

slowly below the Earth’s surface

A. Intrusive (Plutonic) Igneous Rocks...

• Slow cooling at depth allows time for crystals to grow.

• Visible interlocking crystals

• Results in a coarse-grained texture

B. Extrusive (Volcanic) Igneous Rocks• igneous rocks that form at the Earth’s surface after lava has

cooled and hardened : Lava cools quickly!

Lava flow in Hawaii…cools and hardens quickly into extrusive igneous rock

Extrusive (Volcanic) Rocks…• Rapid cooling of lava at or near Earth’s surface

does NOT allow time for crystals to grow (exposure to air and water cools lava quickly)

• Rocks will be Fine-Grained texture, with no visible crystals

• If cooling is instant – a Glassy texture will form• No visible crystals• Gases in lava may leave holes (vesicles) in

extrusive rocks (Scoria or Pumice) resulting in a Vesicular texture

Extrusive igneous rocks found around this composite volcano will have a fine-grained texture due to rapid cooling of lava.

Notice the fine-grained appearance of the hardening and cooling igneous rock around this lava flow. No visible crystals are apparent, and this material is very dark, suggesting a mafic composition.

Gases bubbling out of this lava pond will cause air pockets (vesicles) to form in the cooling igneous rocks. These rocks will then have a vesicular texture.

II. Texture of Igneous Rocks• Texture = size, shape,

and arrangement of the mineral crystals that make up a rock

Textures:Intrusive Igneous Rock

Textures (slow-cooling from magma)

Extrusive Igneous Rock Textures (fast-cooling from

lava)

Coarse-GrainedFine-Grained

Very Coarse-Grained Vesicular

PorphyriticGlassy

1. Extrusive (volcanic) Textures

• Extrusive textures have NO visible crystals.

A. Fine-Grained• microscopic crystals

that cannot be seen with the unaided eye.

• this indicates rapid cooling at or near Earth’s surface, which does not allow crystals to grow

Fine Grained Texture…• This sample of

basalt has a fine-grained texture since it cooled quickly from lava.

B. Glassy• Texture with no crystals

at all due to extremely rapid cooling

• “Pillow lavas” on ocean floor sometimes have a glassy texture due to cold water instantly quenching the lava

• Obsidian is the most common glassy textured Igneous rock

Obsidian results from extremely rapid cooling at Earth’s surface; it has a

glassy texture and conchoidal fracture

C. Vesicular

• A texture that is porous with the holes caused by the expanding gases escaping from a froth-like lava

Vesicular Texture…• Scoria is a dark

colored igneous rock that forms from gases bubbling through the lava as it cooled quickly

• This results in a vesicular texture (full of holes)

• Scoria is MAFIC

Vesicular Texture…• Pumice is a light-

colored igneous rock that forms from gases bubbling through the lava as it cooled quickly

• Pumice is FELSIC

These two samples show a vesicular texture, where gas bubbles (vesicles) remain in the

sample after cooling

Pumice is a light colored igneous rock and contains felsic minerals

Scoria is a dark colored igneous rock and contains mafic minerals

2. Intrusive (plutonic) Textures

• Intrusive (plutonic) Textures display visible mineral crystals due to slow cooling

D. Coarse-Grained• large interlocking crystals that can be seen easily with

the unaided eye, from magma, 1 – 10 mm in size! Indicates slow cooling which allows time for crystals to grow

This sample of granite has a coarse-grained texture, with large visible crystals

E. Very Coarse-Grained• Texture with very

large crystals due to very slow cooling of magma at depth!

E. Very Coarse-Grained• extremely large

crystals at least 10mm (1 centimeter) in size

• Some very coarse grained igneous rocks have mineral crystals up to several feet large!!!!

• Indicates very slow cooling of a mineral-rich “hydro-thermal” fluid.

F. Porphyritic

• texture in which large crystals (phenocrysts) are surrounded by a matrix of finer material. Indicates 2-stage cooling, where large crystals form first, and the rest cools rapidly

This sample shows a porphyritic texture, with very large crystals

• The geometric shapes are called “phenocrysts”, and are embedded in a brown colored matrix or groundmass.

Reading the Reference Tables...Notice that igneous rocks are first classified by whether they formed at earth’s surface (Extrusive) or deep underground (Intrusive)

Remember, the resulting texture is glassy, fine grained, or vesicular for extrusive rocks and coarse grained or very coarse-grained for intrusive rocks

The mineral composition of each rock can be found by looking underneath the rock names in the corresponding section below.

These rocks

Contain these minerals

Another Example:

These rocks

Contain these minerals

Notice that three other characteristics (color, density, and elemental composition) appear in the center of the chart.

Light colored rocks appear on the left, dark on the right

Low density rocks appear on the left, high density on the right

Igneous Rock Quiz...• All igneous rocks form deep underground from

the hardening of magma.

• False! Some extrusive igneous rocks form at or near the surface from the cooling and hardening of lava.

Igneous Rock Quiz...• Igneous rocks that do form underground are

referred to as intrusive or plutonic igneous rocks.

• TRUE.

Igneous Rock Quiz...• An igneous rock that forms near Earth’s surface

will display a texture where plenty of mineral crystals can be seen.

• FALSE! Due to rapid cooling, there is no time for mineral crystals to grow large enough to be seen.

Igneous Rock Quiz...• Porphyritic is the type of texture of igneous

rocks characterized by several holes caused by cooling gaseous lava.

• FALSE! If gas bubbles are found, the sample is referred to as vesicular.

Igneous Rock Quiz...

• What is the texture of this igneous rock?

• Aphanitic or fine grained. You cannot see mineral crystals.

Igneous Rock Quiz...

• Did this rock form above ground or under ground?

• UNDERGROUND. You can see many mineral crystals.

Igneous Rock Quiz...

• What is the texture of this igneous rock?

• Coarse-grained

• The rock below is granite. What minerals make up a granite?

Igneous Rock Quiz...

Potassium feldspar, Quartz, Plag. Feldspar, Biotite, some Amphibole.

Igneous Rock Quiz...• Once a rock becomes an igneous rock, it may

never change into a sedimentary or metamorphic rock.

• FALSE! Rocks constantly change into other forms of rock as part of the rock cycle.

Igneous Rock Quiz...• The texture of igneous rocks is the size, shape

and arrangement of the minerals that make up the rock.

• TRUE!!!