Igneous RocksBy Sarah Barron

Basic Facts About Igneous Rocks

0 Igneous rocks- crystalline solids which form directly from the cooling of magma.

0 This exothermic process involves a phase change from a liquid state of matter to a solid state of matter.

0The surface of the earth, where it exposed to the coldness of space, is made of igneous rock.

Naming Igneous Rocks

0 Igneous rocks are given names based upon two things: what they are made of (composition) and how big the crystals are (texture).

Texture

0How large are the mineral grains in the final solid rock?

0The slower the magma cools, the larger the mineral grains will be.

0Classifications by texture name igneous rocks to be either intrusive or extrusive.

Comparisons of Texture

Intrusive Extrusive

0Coarsely-grained 0Mineral grains are

large enough to see without a magnifying glass

0Also referred to as phaneritic

0Finely-grained0Mineral grains are

only visible with aid of a magnifying glass

0Also referred to as aphanitic

Texture Classifications

Phaneritic Aphanitic

0Granite0Gabbro

0Obsidian0Basalt

Composition

0The composition of igneous magmas is directly related to where the magma is formed

0Magmas are generally mafic if they are associated with crustal spreading

0Magmas are felsic if they are associated with crustal compression and subduction

Mafic

0Basalt is produced if the magma erupts at the surface

0 Gabbro is produced if the magma never makes it out of the magma chamber

0Basalt and gabbro are compositionally the same however, they are two different rocks based on texture.

Intermediate

0 Intermediate magmas are associated with crustal compression and subduction like the felsic magmas.

0 Intermediate magmas go through the same differentiation process as felsic magmas as well.

0 Intermediate magmas produce diorite and andesite.

0Diorite is intrusive whereas andesite is extrusive.

Felsic

0Mafic seafloor basalt and continental sediments are subducted back into the crust, where they re-melt, thus continues the differentiation process.

0Magma enriched in the lighter elements is the product.

0The final result of the differentiation process, the felsic magmas, lead to the formation of granite or rhyolite.

0Granite is intrusive whereas rhyolite is extrusive.

Classifications of Composition Chart

Mafic-Basalt Intermediate-Diorite

Felsic- Granite

References

0Google.com/images 0 jersey.uoregon.edu/~mstrick/AskGeoMan/

geoQuerry13.html