Igneous Rocks and Processes Overview of rock types The hot Earth Magma Bowen’s Reaction Series Igneous rocks

Rocks are naturally formed solid aggregates usually (but not always) composed of minerals.

Rocks are classified by their mode of formation Rocks are classified by their mode of formation. There are three major rock forming processes on Earth, producing three kinds of rocks. Igneous Rocks Sedimentary Rocks Metamorphic Rocks Formed when magma (molten rocks) solidifies Environment: Hot enough to melt rock, pressure varies All non-igneous rocks formed by processes acting on the surface of the Earth Environment: normal for Earth’s surface Formed by chemically and physically altering rocks under heat and pressure deep within the Earth’s crust. Environment: High pressure, not hot enough to melt rock

The Rock Cycle Sedimentary Rocks Igneous Rocks Metamorphic Rocks

Igneous Rocks Igneous rocks form from magma (a complex mix of molten rock with dissolved gases and other materials) Magmas that cool slowly inside the Earth form plutonic igneous rocks. The mineral crystals in these rocks are usually large because they had lots of time to grow. Magmas that erupt onto Earth’s surface are called lavas and very quickly. As they cool, volcanic igneous rocks form. If mineral crystals form, they tend to be small because they had little time to form. If cooling is fast enough, no minerals form and the lava solidifies to form a solid glass, with no crystalline structure.

Dissolved gasses - mostly H2O, SO2, CO2 When the temperature is high enough (and the pressure* is low enough), some solid Earth materials will begin to melt, forming magma. Magma - complex mixture of high temperature solid, liquid, and gaseous material. Dissolved gasses - mostly H2O, SO2, CO2 *If the pressure is too high, melting will not occur – which is why magma is more commonly formed in the crust than the mantle, even though mantle temperatures are much higher than crust temperatures.

Bowen’s Reaction Series felsic

olivine pyroxene amphibole biotite K-feldspar quartz muscovite 1200oC temperature K-feldspar quartz muscovite 600oC

Mafic igneous rocks contain minerals with high melting points Intermediate igneous rocks contain minerals with intermediate melting points felsic Felsic igneous rocks contain minerals with low melting points Bowen’s Reaction Series

felsic

Crystalline Igneous Rock Texture Texture - refers to the size of mineral crystals. The more slowly a magma cools, the more time crystals have to grow. Magma that cooled slowly produces large crystals, and the resulting rock is coarse-grained (phaneritic). In magmas that cooled quickly only small crystals form, and the resulting rock is fine-grained (aphanitic).

Crystalline Igneous Rock Texture Texture - refers to the size of mineral crystals. The more slowly a magma cools, the more time crystals have to grow. Porphyritic Texture Sometimes a magma will have a complex cooling history, and may stay at a mineral’s crystallization temperature for a long time, allowing those mineral crystals to grow large. If the magma is later cooled more quickly, only smaller crystals of the other minerals will form.

Igneous Rocks Basalt Basalt Gabbro Different magma bodies have different chemical compositions. These chemical compositions are usually described by the total percentage of silica tetrahedra in the magma. Igneous Rocks Mafic Igneous Rocks - Silica poor (<52%) Iron, calcium and magnesium rich Contain minerals formed at relatively high temperatures. They are high density and tend to be dark colored. Common minerals: olivine, augite, calcium-rich plagioclase feldspar Basalt University of Hawaii Basalt Gabbro Devil’s Tower National Monument University of Montpellier

Basalt - fine grained mafic rock T/P color Si % Basalt - fine grained mafic rock low light high Basalt Columnar Jointing University of Hawaii Mafic felsic Devil’s Tower National Monument

Gabbro - coarse-grained mafic rock low light high T/P color Si % University of Montpellier Mafic felsic

Intermediate Igneous Rocks - Silica rich (52% - 69%) Different magma bodies have different chemical compositions. These chemical compositions are usually described by the total percentage of silica tetrahedra in the magma. Igneous Rocks Intermediate Igneous Rocks - Silica rich (52% - 69%) Intermediate elemental composition Contain minerals formed at intermediate temperatures. They are intermediate density and tend to contain an equal combination of dark and light colored minerals. Common minerals: plagioclase feldspar, mica, hornblende Diorite Diorite Andesite http://volcanoes.usgs.gov/Products/Pglossary/andesite.html

low light high T/P color Si % Intermediate felsic Diorite - coarse-grained intermediate rock. Approximately half and half dark and light minerals, no quartz.

Andesite - fine-grained intermediate rock. low light high T/P color Si % http://volcanoes.usgs.gov/Products/Pglossary/andesite.html Intermediate felsic

Felsic Igneous Rocks - Silica rich (>69%) Different magma bodies have different chemical compositions. These chemical compositions are usually described by the total percentage of silica tetrahedra in the magma. Igneous Rocks Felsic Igneous Rocks - Silica rich (>69%) Iron, calcium and magnesium poor Contain minerals formed at relatively low temperatures. They are low density and tend to be light colored. Common minerals: K-feldspar, mica, quartz Granite Granite Rhyolite Rhyolite http://geology.asu.edu/~sreynolds/

Rhyolite - fine grain felsic rock low light high T/P color Si % http://geology.asu.edu/~sreynolds/ Felsic http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/photogallery/

Granite - coarse grained felsic rock low light high T/P color Si % Granite - coarse grained felsic rock Felsic

Felsic

Volcanic Igneous Rocks with no Minerals Some magmas cooled too fast for minerals to form. The rocks are made of glass and contain no minerals, so are NOT described using the felsic-intermediate-mafic system. obsidian scoria glassy – obsidian and other volcanic glass vesicular – full of holes formed by bubbles in the magma. fragmental or pyroclastic – formed from debris spewed out by the volcano. bombs!

Volcanic Igneous Rocks Plutonic Igneous Rocks