• Igneous rocks form when molten rock cools and solidifies.

• Molten rock is called magma when it is below the Earth’s surface and lava when it is above.

Igneous Rocks

MagmaMagma•• molten rock below Earth's molten rock below Earth'ssurface.surface.

LLava ava •• magma on the Earth's surface. magma on the Earth's surface.

Pyroclastic materialPyroclastic material•• ( (pyro pyro = fire, = fire, clastic clastic = debris)= debris)•• Airborne lava Airborne lava

—— cools as it falls cools as it falls

Igneous formed from Magma and LavaIgneous formed from Magma and Lava

Igneous Rock classification

• Igneous rocks are classified two different ways:– Where they were formed– What they are made from

(mineral composition)

Igneous Rocks—“Fire Igneous Rocks—“Fire Rocks”Rocks”

Formation FactsFormation Facts1.Texture1.Texture

2. Chemical 2. Chemical CompositionComposition

3. Porphyry3. Porphyry

Dependent on time (cooling rate) and size of Dependent on time (cooling rate) and size of graingrain

Coarse-grainedCoarse-grained-- Slow cooling rate, large -- Slow cooling rate, large crystals and takes place beneath the earth crystals and takes place beneath the earth (intrusive) [example: Granite(most common)](intrusive) [example: Granite(most common)]

Fine-grainedFine-grained—Fast cooling rate, small crystals. —Fast cooling rate, small crystals. Cool quickly at the surface (extrusive) Cool quickly at the surface (extrusive) [Example: Basalt (most common)] [Example: Basalt (most common)]

Most igneous rocks are made of various Most igneous rocks are made of various combinations of six minerals: quartz, combinations of six minerals: quartz, feldspar, pyroxene, amphibole, olivine, and feldspar, pyroxene, amphibole, olivine, and mica (some light and some dark).mica (some light and some dark).

An igneous rock whose cooling rate has An igneous rock whose cooling rate has changed and has two or more different sized changed and has two or more different sized crystals.crystals.

Igneous Rocks—“Fire Rocks”Igneous Rocks—“Fire Rocks”2 types2 types

IntrusiveIntrusive

ExtrusiveExtrusive

Igneous rocks formed deep Igneous rocks formed deep withinwithin the the earth. Intrusive rocks reach the surface earth. Intrusive rocks reach the surface when the soil covering them is removed when the soil covering them is removed by erosion or when the forces of plate by erosion or when the forces of plate movement or earth quakes push them movement or earth quakes push them to the surfaceto the surface..

Igneous rocks formed from lava at the Igneous rocks formed from lava at the earth’s surface. Exterior of the Earth. earth’s surface. Exterior of the Earth. Holes in rocks are formed when gases Holes in rocks are formed when gases are trapped in lava as it cools [example: are trapped in lava as it cools [example: pumice]. No crystals are formed when it pumice]. No crystals are formed when it cools very quickly [example: obsidian].cools very quickly [example: obsidian].

• Igneous rocks that form below the Earth’s surface are called intrusive igneous rocks (or plutonic).

• The word “plutonic” comes from Pluto, the name for the Greek god of the underworld.

• They form when magma enters a pocket or chamber underground and solidifies into crystals as it cools very slowly.

Intrusive Igneous Rocks

Cooling HistoriesCooling HistoriesMinerals need time and space to growMinerals need time and space to grow

•• More time = Bigger crystals More time = Bigger crystals —— visible mineral grains visible mineral grains

P & T control cooling rates of magmaP & T control cooling rates of magma•• Temp Temp

—— Earth is a good insulator Earth is a good insulator»» holds in heat holds in heat»» keeps out cool keeps out cool

—— Atmosphere is a relatively bad insulator Atmosphere is a relatively bad insulator»» transfers heat easily transfers heat easily

•• Earth has pressure Earth has pressure—— Weight of overlying rocks Weight of overlying rocks—— Magma trying to push up (density) Magma trying to push up (density)—— water vapor (steam), wants to expand water vapor (steam), wants to expand

Intrusive Igneous Rock

• Most intrusive rocks have large, well formed crystals. The mineral crystals within them are large enough to see without a microscope.

• The more slowly molten rock cools within the Earth, the larger the igneous rocks crystals will be.

• Examples of intrusive igneous rocks are granite, gabbro and diorite

Granite

Granite is a very coarse-grained igneous rock.Granite is a very coarse-grained igneous rock.Crystals are >2 cm, often larger.Crystals are >2 cm, often larger.

GraniteGranite

Feldspar Feldspar mineral mineral graingrain

Quartz mineral Quartz mineral graingrain

Biotite mineralgrainBiotite mineralgrain

Extrusive Igneous Rocks

• Extrusive igneous rocks form when magma makes its way to Earth's surface. The molten rock erupts or flows above the surface as lava, and then cools forming rock.

• Most extrusive (volcanic) rocks have small crystals. Examples include basalt, rhyolite, and andesite.

Granite & RhyoliteGranite & Rhyolite

RhyoliteRhyolite

GraniteGranite

What are the textures in these two rocks ?

What are the textures in these two rocks ?

Intrusive rocksIntrusive rocks•• cool beneath Earth's surface cool beneath Earth's surface•• cool very slowly cool very slowly•• higher P & T higher P & T

Extrusive rocksExtrusive rocks•• cool on the Earth's Surface cool on the Earth's Surface•• cool relatively fast cool relatively fast•• lower T & P lower T & P

Categories of Igneous RocksCategories of Igneous RocksGraniteGranite

RhyoliteRhyolite

Volcanic Glass

• Pumice, obsidian, and scoria are examples of volcanic glass.

• These rocks cooled so quickly that few or no mineral grains formed.

• Most of the atoms in these rocks are not arranged in orderly patterns, and few crystals are present.

Glassy Igneous Rocks

• Pumice (left)• Scoria (bottom left)• Obsidian (bottom)• Note gasses in the lava can

cause fine holes called vesicles as seen in the pumice and scoria.

Glassy Igneous Rocks cool so rapidly, that atoms don’t have enough time to get together, bond and form crystals. To cool this quickly the rocks MUST be extrusive.

Classifying Igneous Rock by

mineral composition

Magma types

• A way to further classify these rocks is by the magma from which they form. An igneous rock can form from, granitic, andesitic, or basaltic magma.

• Magma composition determines the physical & chemical properties of an igneous rock

Granitic Rocks

• Granitic igneous rocks are light-colored rocks of lower density than basaltic rocks.

• Granitic rocks are coarse-grained

• Granitic magma is thick and stiff and contains lots of silica but lesser amounts of iron and magnesium. Violent volcanic eruptions

• It is the most common rock type on the continental land masses. Yosemite Valley in the Sierra Nevada and Mt. Rushmore are two notable examples of granitic rocks

Basaltic Igneous Rocks

• Basaltic igneous rocks are dense, dark-colored rocks.

• They form from magma that is rich in iron and magnesium and poor in silica, which is the compound SiO2.

• The presence of iron and magnesium in minerals in basalt gives basalt its dark color.

• Basaltic lava is fluid and flows freely from volcanoes in Hawaii, such as Kilauea.

• Basalt is the most common rock type in the Earth's crust (the outer 10 to 50 km). In fact, most of the ocean floor is made of basalt

Andesitic Rocks

• Andesitic igneous rocks have mineral compositions between those of basaltic and granitic rocks.

• Many volcanoes around the rim of the Pacific Ocean formed from andesitic magmas.

• Like volcanoes that erupt granitic magma, these volcanoes also can erupt violently.

• Rocks made from andesite tend to be fine-grained.

Igneous RocksIgneous Rocks

IntrusiveIntrusive

(coarse grained) (coarse grained) ExtrusiveExtrusive

(fine grained) (fine grained)

Light Light colored colored

GraniteGranite

Porphyry Porphyry RhyoliteRhyolite

AndesiteAndesite

PumicePumice

Dark Dark coloredcolored

GabbroGabbro BasaltBasalt

ObsidianObsidian

Mixed Mixed colorcolor

DioriteDiorite AndesiteAndesite

How do we tell where the igneous rocks formed?

What can we derive from the rocks about the conditions of formation?

How do we tell where the igneous rocks formed?

What can we derive from the rocks about the conditions of formation?