Igneous Rocks

The Rock Cycle

Formation All igneous rocks form from the cooling of melted or molten rock. Lava is molten rock that has exited the Earth through a volcano Lava

Mt. Etna

Lava

Magma is molten rock that exists underground in large chambers. Formation Magma

Extrusive Rocks Volcanic rock is rock that erupted on the surface and cooled quickly. This could take few minutes to a few hours. Volcanic rocks or extrusive rocks are Fine Grained or have small mineral crystals

Intrusive Rocks Plutonic Rock is rock that solidifies within the Earth from magma that cools very slowly. This cooling could take thousands of years. Plutonic rocks have a large grain size because the crystals have a long time to grow Plutonic Rocks -- Coarse Grained

Porphyritic Texture Large Crystals in a fine-grained Setting. This indicates there were two different cooling rates. One slow and one fast.

Composition Composition refers to the chemical make-up of the rock. This is based on the dominant mineral content.

Mafic Rocks Mafic igneous rocks contain large amounts of iron(Fe) and Magnesium(Mg). They have high percentages of minerals like amphibole, pyroxene, magnetite and olivine. This makes them dark in color.

Felsic Felsic igneous rocks have high amounts of Aluminum a Silicon. As a result these rocks are light in color. Common minerals are quartz, Potassium feldspar, and muscovite mica.

The Scheme for Igneous Rock Identification

A Brief Tour

Crystal size Description Grain Size

Igneous rocks have “Intergrown Crystals”

Intergrown

Not Intergrown

Vesicular

Where it was formed Outside the volcano: Extrusive Inside the Earth: Intrusive

Bubbles? Yes= Vesicular

Bubbles? No=Non-vesicular

Color Very Light Light NotVery LightNot Very Dark Dark Very Dark Neither Light nor Dark Lightest Darkest

Density Very Light LightDense Very Dense Medium Density

Practice Name a light-colored, fine-grained rock with no bubbles.

Name a coarse-grained, dense rock.

Name a very light-colored, glassy, extrusive rock with bubbles.

Finding The Minerals Identify the rock. Unless you have other information, work in the middle of the rock’s box.

This is the amount of Potassium Feldspar in the rock.

This is the amount of Quartz in the rock.

This is the amount of Plagioclase Feldspar in the rock.

Use tick marks on a scrap paper to measure the percentage. Potassium Feldspar 25% Quartz 40%