Igneous Rocks

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Igneous: granite, basalt, crystallize from hot molten rock

There are two main types of igneous rock: intrusive and extrusive

Meteorites; igneous rocks from space Cinder: small chunks of rock about the size

of a grape that solidify in the air; full of holes made by gas bubble

Intro to Info

Igneous rock never contain fossils and are normally hard

Volcanic bombs happen when small pieces of lava blast through air at high speed and cool very rapidly, most turn to volcanic ash and some form pumice which is light enough to float on water

Intro To Info

Phaneritic: size of grains are large enough to see unaided; characterized by slow cooling in plutonic environment

Aphanitic: grain size is too small to see with naked eye; characterized by rapid cooling in volcanic environments

Porphyritic: rock has distinct differences in the size of crystals

Igneous Grain Size

Any igneous rock made on the surface of the volcano

Grains are too small to see because magma cooled quickly on earth’s surface and crystals do not have time to grow large and call these rocks fine grained

Sometimes lava cools so rapidly that no crystals grow and instead the lava turns to a glass like rock called obsidian or volcanic glass

Examples: rhyolite, andesite, obsidian

Extrusive Igneous Rocks

Also known as Plutonic Rocks Any igneous rock made underground Characterized by coarse grains in rocks Crystal minerals are large enough to see

because magma cools slowly which gives crystals time to grow and call these rock coarse grained

Examples include: granite, gabbro, diorite

Intrusive Igneous Rocks

Extrusive vs. Intrusive Igneous Rocks

Intrusive Extrusive

Cooled and solidified at a considerable depth

Visual appearance reveals interlocking crystals forming the rock mass

Characterized by large crystal sizes Cooling and hardening is typically

quite slow Surface feels relatively rough

Characterized by fine-grained textures

Formed at or above the surface of the planet

Cooling and hardening is typically quite rapid

Surface feels relatively smooth

Both• Silica is the most abundant component• Named and identified on the basis of composition and texture • Formed from the crystallization of minerals• Contain feldspar minerals• May become transformed into sedimentary rocks when exposed to the Earth’s surface

Identifying Igneous Rocks

Rock Grain Size Color

Granite Coarse Light

Gabbro Coarse Dark

Pegamite Very coarse Light and dark

Basalt Fine Dark

Rhyolite Fine Light

Andesite Fine Medium

The diagrams below show the crystals of four different rocks viewed through the same hand lens. Which crystals most likely formed from molten material that cooled and solidified most rapidly?

ROCK NAME TYPE ( Igneous, Sedimentary, metamorphic) FORMATION Igneous (Extrusive, Intrusive) (volcanic, plutonic) (felsic or mafic) Sedimentary ( Clastic, chemical, biochemical, organic) Metamorphic (Regional, Contact Metamorphism) (low grade-high grade) ENVIRONMENT TEXTURE Igneous (phaneritic, aphanitic, porphyritic) Sedimentary (grain size) Metamorphic Foliated or non-foliated HARDNESS RANGE MINERALS PHYSICAL DESCRIPTION (Color) USES OTHER PROPERTIES

Need To Know on Rocks

A Few Igneous Rocks

Is an extrusive igneous rock Forms from Felsic Found where there are explosive volcanoes

along subduction boundaries Has small crystals

Andesite

Is intrusive igneous rock Sometimes called black granite Could occur as dikes or sills as it hardens

in the ground Normally dark and mottled in appearance Minerals: plagioclase and hornblende It’s extrusive form is andesite Hardness: 7

Diorite

It is the most abundant of Earth’s Volcanic Rocks and major component of ocean’s crust

Fine grained Dark colored Low viscosity (runny) Moon’s dark patches Minerals: augite and

plagioclase

Common extrusive igneous rock

Made from solidified lava

Sometimes made from volcanic bombs

Hardness: 6-7

Basalt

Extrusive igneous rock Degrades over time Black and sharp with

frosted glassy surface Used for surgical

blades but was once important in ceremonial and sacrificial knives

Found in young lava flows

Minerals: quartz and feldspar

Hardness: 5.5 Often found with

pumice

Obsidian

Intrusive Igneous Rock Forms from mafic type magma Is the intrusive form of basalt Normally very dark color Commonly made into countertops

Gabbro

Extrusive igneous rock Made from thick sticky lava

Rhyolite

Igneous rock sometimes containing crystals of beryl and garnet

Pegamite

Igneous Rock Some giant stones of Stonehenge are

dolerite

Dolerite

Intrusive igneous rock Made of mostly quartz, feldspar and mica Pink and white; coarse grained rock Found in India, Italy, US Used in Mt. Rushmore and buildings Appears in huge bubbles of cooling magma

called batholiths Hardness: 6

Granite

Extrusive igneous rock Vesicular because gases escaped from the

lava when cooled

Scoria

Extrusive Igneous Rocks

Has volcanic origin vesicular because

gases escaped from the lava when cooled

Can be related to rhyolite with its composition

Used to make building blocks, concrete, toothpaste, cosmetics, and soap

Can float in water Often found with

obsidian Hardness: 6

Pumice

The following questions and answers are from the New York State Regents Website: http://www.regentsprep.org/Regents/core/questions/topics.cfm?Course=ESCI

Geology.com

Other Resources