Igneous Rocks: Terminology

Igneous rocks are formed as a result of cooling and crystallization from a magma

Magma is molten rock, rich

in silica (SiO2), which containsdissolved volatiles such as gasses

Lava is magma extruded on orvery near the Earth’s surface

Igneous Rocks• 2 Types

– Extrusive (Volcanic Rock)• Cool above the Earth’s surface (fast cooling)

– Magma which has reached the surface is known as lava

– The heat from the lava goes quickly and the lava solidifies

• Example: basalt, rhyolites, pumice, obsidian (smooth volcanic rock)

– Intrusive (Plutonic Rock)• Cool beneath the Earth’s surface (slow cooling)• Example: granite

Type of Igneous Rock Formed• Depends on three factors:

– Chemical composition of the magma• Example: The amount of silica (SiO 2 ) found in magma can

vary from 75 % to less than 45 %. – Temperature of solidification

• The temperature of cooling determines which types of minerals are found dominating the rock's composition

• Low temperature rocks are rich in minerals composed of silicon, potassium, and aluminum.

• High temperature igneous rocks are dominated by minerals with higher quantities of calcium, sodium, iron, and magnesium

– Rate of cooling which influences the crystallization process• Igneous rocks that form through a gradual cooling process

tend to have large crystals. Relatively fast cooling of magma produces small crystals.

• Volcanic magma that cools very quickly on the Earth's surface can produce obsidian glass which contains no crystalline structures.

Classifying Igneous Rocks• Classified based on:

A. Texture• The size shape and arrangement of constituent

minerals

• Related to the cooling history– Fast cooling: crystals do not grow to any great size– Slow cooling: crystals grow large

B. Composition• The percentage of particular minerals contained in

the rock– Certain minerals are light in color; others dark

• Reflects the source of magma from which the rock was produced

Igneous Rocks: Cooling History• Length of time for cooling determines

the texture and degree of crystallization– Extrusive

• Fast cooling– Fine grained

» Example: Basalt – Glassy texture

» Example: Obsidian (like glass)» Example: Pumice (holes)

Lava that cools very quickly can lock in gases as it solidifies

Result: Rock with many holes

Think: Pumice stone

ie. Basalt

Igneous Rocks: Cooling History

– Intrusive• Slow cooling

– Coarse grained and larger crystals» Example: Granite» Example: Diorite» Example: Gabbro

ie. Granite

Extrusive Rock Bodies: Volcanoes

Mafic (basaltic) eruptions

• Character:– fissures or fractures– mafic magma– fluid– not explosive

Extrusive Rock Bodies: Volcanoes

Mafic (basaltic) eruptions

• Structures

Lava tube

Flood basalt

Tephra Pillow lavaCaldera

Shield volcano

Extrusive Rock Bodies: Volcanoes

Felsic or Silicic (granitic) eruptions

Character:– typical at convergent plate boundaries– extrude cool magma– viscous so does not flow freely– highly explosive

Extrusive Rock Bodies: Volcanoes

Felsic or Silicic (granitic) eruptions

• Structures

Crater

Composite cone

Tephra

Pyroclastic flow

Intrusive Rock Bodies: Plutons

• Character– cools below the surface of the earth

• Structures formed by Igneous Rocks– Pluton– Batholith– Stock– Neck– Dike– Laccolith– Sill– Pipe– Xenoliths

Pluton

• When magma cools below the surface of the earth it forms intrusive igneous rocks.  These rocks produce the following structures:

• Pluton: An intrusive rock body of any size.• Batholith: A pluton greater than 100 km2. • Stock: A pluton less than 100 km2. • Dike: Tabular intrusive rock which cuts discordantly

(vertically) across strata. • Laccolith: A concordant (horizontal) intrusion that

generally has a horizontal floor but which arches up the strata above it; lens or spoon shaped.

• Sill: A concordant intrusion that is injected between layers; no upward arch.

• Pipe: Volcanic neck; conduit through which magma travels from chamber upwards.

• Xenoliths: Inclusions of surrounding rock in magma.

Intrusive Rock Bodies: Plutons

Volcanic Neck and Dike

Aerial Photo of Shiprock, New Mexico (Volcanic Neck)

From: http://www.ngdc.noaa.gov/seg/hazard

Volcanic Neckand Dike

Edinburgh Sill

From: http://www.ngdc.noaa.gov/seg/hazard

Sill - Yellowstone National Park

Sierra Nevada Batholith

Stock – Chisos Mountains Texas

Laccolith – Black Hills, Wyoming

Patagonia Pluton - Arizona

Xenoliths – Petersburg, Virginia

Pipe – Organ Pipes National Park, Australia

Igneous Rock: Metallic Mineral Deposits

• Rare minerals often found in Igneous rocks

• Place found:– Often found in intrusive (plutonic rock)

• Think mining: where is the good stuff found– Copper, gold …