Igneous RocksMr. Ahearn

Earth Science 2010

What are Igneous Rocks?

• Rocks that cooled and crystallized directly from molten rock, either at the surface or deep underground.

• “Ignis” Latin for fire.

• >95% of Earth’s outer 30 miles consist of igneous rocks.

• 1st type to form on Earth

What is Magma?

• Molten rock that flows within the Earth. Could be completely liquid or a mixture of liquid solid crystals and dissolved gasses.

• Called lava when at surface.• Different minerals crystallize at different

temperatures changing the composition of magma.

Types of Magma

• Rhyolitic – 70% SiO2

• Andesitic- 60% SiO2

• Basaltic- 50% SiO2

• Silica (SiO2) is most abundant in magma and has greatest effect on characteristics– Affects melting temp, viscosity

Creation of Magma

• Partial Melting• Heat• Pressure- higher

pressure --> higher M.P.

• Water- lowers M.P.

Classification of Igneous Rocks

• Texture- size and shape of mineral crystals and way in which crystals grow during cooling.– Glassy

– Fine-grained

– Coarse-grained

– Very coarse-grained

– Porphyritic

• Composition- minerals rock contains– Felsic, Intermediate, Mafic, & Ultramafic

Igneous Textures• Extrusive- minerals crystallize slowly

underground for thousands of years --> crystals large, easily seen.(phaneritic– Slow cooling occurs when magma intrudes

preexisting solid rocks – Pegmatites- rocks with exceptionally large crystals

(could be several meters long).

Pegmatite

Igneous Textures

• Extrusive- solidify from lava quickly--> small or no crystals. (aphanitic)– Called volcanic– Basalt

• Porphyritic- large crystals surrounded by regions w/ smaller crystals or invisible grains.– Form by slow cooling followed

by rapid cooling.

Glassy Texture• Instantaneous cooling of lava.

– Obsidian- forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture

– Pumice- bubbly, highly gaseous lava.

Igneous Composition• Ultramafic- (mg, Fe)

– Dark, very dense.– Crystallize slowly deep in Earth– Mostly olivine and pyroxene (very little silica)– peridotite

• Mafic- – Silica content 40-50%– Mostly pyroxene, calcium feldspar, little olivine.– Most abundant in Earth’s crust.– Basalt, Gabbro.

Peridotite a generic name used for coarse-grained, dark-colored, ultramafic igneous rocks. Peridotites usually contain olivine as their primary mineral, frequently with other mafic minerals such as pyroxenes and amphiboles. Their silica content is low compared to other igneous rocks and they contain very little quartz and feldspar.

Newfoundland

Gabbro a coarse-grained, dark-colored, intrusive igneous rock. It is usually black or dark green in color and composed mainly of the minerals plagioclase and augite. It is the most abundant rock in the deep oceanic crust.

Basalt a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill.

Igneous Composition• Intermediate

– 55-65% silica (lighter in color)– Some pyroxene, amphibole, sodium feldspar, mica

and little quartz.– Andesite, Diorite

• Felsic (feldspar and silica)– >65% silica (lighter in color)– K feldspar, muscovite mica, quartz.– Granite, Rhyolite

Andesitea fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite.

DioriteDiorite is a coarse-grained, intrusive igneous rock that is intermediate in composition between granite and gabbro. It’s composition is very similar to the fine-grained extrusive rock known as andesite.

Granite a light-colored igneous rock with grains large enough to be visible with the unaided eye.  It forms from the slow crystallization of magma below Earth’s surface.  Granite is composed mainly of quartz and  feldspar with minor amounts of mica, amphiboles and other minerals.   This mineral composition usually gives granite a red, pink, gray or white color with dark mineral grains visible throughout the rock

Rhyolite a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals.