Igneous Rocks

Standards  Classify matter in a variety of ways  Describe the composition and structure of Earth’s materials, including: the major rock types (i.e., sedimentary, igneous, metamorphic) and their formation

Igneous Rocks Rocks formed from magma Make up 95% of Earth’s crust, much of which is covered by thin layer of sedimentary rocks

Formation of Igneous Rocks Form from the solidification of magma (molten rock) Hot temperatures in the aesthenosphere (600   C) cause rock to melt As rock heats up, it becomes less dense and rises When it cools and solidifies, either underground or above ground, it becomes igneous rock

Classification of Igneous Rocks Classification is based on: 1.Where rock is formed 2.Texture (which is directly affected by where rock is formed) 3.Mineral content (what minerals are in rock)

Environment where Rock is Formed Two main groups of igneous rocks: 1.Intrusive (plutonic) 2.Extrusive (volcanic)

Intrusive (Plutonic) Named after Pluto, god of the underworld Forms from solidification of magma underground Forms 1 kilometer or deeper below surface of Earth Ex: granite

Photo: Granite

Extrusive (Volcanic) Named after Vulcan, god of fire Forms from eruption and solidification of magma (lava) on surface of Earth Forms less than 1 kilometer deep Ex: basalt

Textures Rock texture refers to the size, shape and arrangement of mineral grains

Intrusive Texture Intrusive magmas cool slowly, over hundreds of thousands of years, due to insulating properties of surrounding rocks. This allows time for crystals to grow = larger crystals. These rocks are medium to large grained, called Phaneritic Ex: granite – most abundant rock in Earth’s crust

Intrusive Texture

Extrusive Texture Extrusive magmas (lavas) cool rapidly. There is little time for crystal growth = very small or micro crystals. These rocks are fine grained, called aphanitic Ex: basalt

Extrusive Texture

Some extrusive lavas cool so rapidly there is no time for crystal growth In this case they have a glassy texture A glassy texture can be either massive (like obsidian [volcanic glass]) or frothy (like pumice)

Obsidian Pumice

Extrusive Texture Some extrusive rocks can contain holes that are called vesicles. Vesicles form when gases (air bubbles) escape from the cooling rock leaving behind a cavity.

Vesicular Basalt

ExtrusiveTexture Some extrusive rocks form a porphyry (or porphyritic texture) – large crystals, called phenocrysts, are present in a fine-grained groundmass. Chocolate chip cookie analogy: chocolate chips are like the phenocrysts and cookie dough is like the groundmass.

Porphyritic Texture

Color Classification Based on mineralogy Felsic rocks – igneous rocks that are light colored. They are rich in minerals that contain lots of silica. Mafic rocks – dark colored igneous rocks. They are rich in iron and magnesium Intermediate – rocks midway between felsic and mafic. Ultramafic – very dark rocks, containing very little silica.

Igneous Rock Pairs Igneous rocks are classified in pairs, in which members of the pairs contain identical minerals, but have different textures (intrusive vs. extrusive) ColorIntrusive Extrusive FelsicGranite Rhyolite IntermediateDiorite Andesite MaficGabbro Basalt UltramaficPeridotite

Felsic RhyoliteGranite

Intermediate AndesiteDiorite

Mafic Gabbro Basalt

Ultramafic Peridotite