IGNEOUS ROCKS. I.Igneous rocks are formed from molten material. - The term igneous is derived from...

IGNEOUS ROCKS

I. Igneous rocks are formed from molten material.

- The term igneous is derived from Latin term meaning “from FIRE”!!

A. Extrusive igneous rocks are formed when moltenrock cools above ground. Usually they are formedafter the material has been erupted by a volcano.

1. This molten materialcools quickly.

2. No crystals are visibleto the eye.

ENVIRONMENT

B. Intrusive igneous rocks are formed when molten rock cools below ground.

1. These rocks cool very slowly.

2. There is lots of time for visible crystals to grow

Cools quickly Cools slowlyCools above ground Cools below groundNo visible crystals Visible crystals

EXTRUSIVE INTRUSIVE

TEXTURE

• Coarse:

– INTRUSIVE igneous rock composed of large mineral grains.

• GRANITE

TEXTURE

• Fine:

– EXTRUSIVE igneous rock composed of fine-grained minerals.

• BASALT

TEXTURE

• Glassy:

– HIGHLY VISCOUS, silica-rich magma cools rapidly, no crystals.

• OBSIDIAN

TEXTURE

• Vesicular:

– HIGHLY VISCOUS, large amount of dissolved gases, rock full of holes.

• PUMICE

COMPOSITION• Mafic:

– Low silica– Rich in Iron and Magnesium

• DARK COLOR.

• BASALT

COMPOSITION

• Felsic:

– High silica• LIGHT COLOR.

• RHYOLITE

Igneous Rock Structures

• Intrusions vs. Extrusions:

- Form Underground

- Form on the surface of the Earth

INTRUSIONS• Batholiths:

– At least 100 km2 of surface exposure.

• Means “Deep Rock”

• Sierra Nevada Mountains

Exposed Batholiths

INTRUSIONS• Laccoliths:

– Flat-bottom intrusion that pushes over-lying rock layers into an arc.

• Means “Lake of Rock”

• Black Hills–South Dakota

INTRUSIONS• Stocks:

– Like a Batholith, but covers less than 100 km2.

• Devils Tower–Wyoming

INTRUSIONS• Dikes:

– Igneous intrusion that cuts across rock layers.

• Black Canyon–Colorado

INTRUSIONS• Sills:

– Hardened magma formed between parallel layers of rock.

• Salisbary Crags–Scotland

EXTRUSIONS• Volcanic Necks:

– Solidified central neck of a volcano.

• Shiprock–New Mexico