Igneous RocksIgneous Rocks

StandardsStandards

Classify matter in a variety of waysDescribe the composition and structure of

Earth’s materials, including:the major rock types (i.e., sedimentary,

igneous, metamorphic) and their formation

IgneousIgneous Rocks Rocks

• Rocks formed from magma

• Make up 95% of Earth’s crust, much of which is covered by thin layer of sedimentary rocks

Formation of Igneous RocksFormation of Igneous Rocks

• Form from the solidification of magma (molten rock)

Hot temperatures in the aesthenosphere (600 - 1400 C) cause rock to melt

As rock heats up, it becomes less dense and rises

When it cools and solidifies, either underground or above ground, it becomes igneous rock

Classification of Igneous RocksClassification of Igneous Rocks

• Classification is based on:

1. Where rock is formed

2. Texture (which is directly affected by where rock is formed)

3. Mineral content (what minerals are in rock)

Environment where Rock is FormedEnvironment where Rock is Formed

• Two main groups of igneous rocks:

1. Intrusive (plutonic)

2. Extrusive (volcanic)

IntrusiveIntrusive ( (PlutonicPlutonic))

• Named after Pluto, god of the underworld

• Forms from solidification of magma underground

• Forms 1 kilometer or deeper below surface of Earth

• Ex: granite

Photo: http://hsc.unm.edu/medicine/integrative_med/brian_shelley_biosketch.shtml

Granite

ExtrusiveExtrusive ( (VolcanicVolcanic))

• Named after Vulcan, god of fire

• Forms from eruption and solidification of magma (lava) on surface of Earth

• Forms less than 1 kilometer deep

• Ex: basalt

TexturesTextures

• Rock texture refers to the size, shape and arrangement of mineral grains

IntrusiveIntrusive Texture Texture

• Intrusive magmas cool slowly, over hundreds of thousands of years, due to insulating properties of surrounding rocks.

• This allows time for crystals to grow = larger crystals.

• These rocks are medium to large grained, called Phaneritic

• Ex: granite – most abundant rock in Earth’s crust

Intrusive TextureIntrusive Texture

ExtrusiveExtrusive Texture Texture

• Extrusive magmas (lavas) cool rapidly.• There is little time for crystal growth = very

small or micro crystals.• These rocks are fine grained, called

aphanitic• Ex: basalt

Extrusive TextureExtrusive Texture

Extrusive TextureExtrusive Texture

• Some extrusive lavas cool so rapidly there is no time for crystal growth

• In this case they have a glassy texture• A glassy texture can be either massive

(like obsidian [volcanic glass]) or frothy (like pumice)

Obsidian

Pumice

Extrusive TextureExtrusive Texture

• Some extrusive rocks can contain holes that are called vesicles.

• Vesicles form when gases (air bubbles) escape from the cooling rock leaving behind a cavity.

Vesicular Basalt

ExtrusiveTextureExtrusiveTexture

• Some extrusive rocks form a porphyry (or porphyritic texture) – large crystals, called phenocrysts, are present in a fine-grained groundmass.

• Chocolate chip cookie analogy: chocolate chips are like the phenocrysts and cookie dough is like the groundmass.

Porphyritic TexturePorphyritic Texture

ColorColor Classification Classification

• Based on mineralogy • Felsic rocks – igneous rocks that are light

colored. They are rich in minerals that contain lots of silica.

• Mafic rocks – dark colored igneous rocks. They are rich in iron and magnesium

• Intermediate – rocks midway between felsic and mafic.

• Ultramafic – very dark rocks, containing very little silica.

Igneous Rock PairsIgneous Rock Pairs• Igneous rocks are classified in pairs, in

which members of the pairs contain identical minerals, but have different textures (intrusive vs. extrusive)

Color Intrusive ExtrusiveFelsic Granite RhyoliteIntermediate Diorite AndesiteMafic Gabbro BasaltUltramafic Peridotite

FelsicFelsic

RhyoliteGranite

IntermediateIntermediate

AndesiteDiorite

MaficMafic

GabbroBasalt

UltramaficUltramafic

Peridotite