Igneous Rocks

Goals To understand what your “granite” counter top is really made out of and to use that knowledge to make plate tectonic interpretations.

Igneous rocks Rocks cooled and crystallized from molten rock Made up of an interlocking network of mineral crystals called grains Grains

Igneous rocks Molten rock is called magma—can be either all melt or a “slush” of melt and crystals Magma erupted onto the Earth’s surface called lava

Igneous rock compositions Minerals in igneous rocks tell us a lot about the tectonic environment in which rocks formed Calcium plagioclase feldspar Pyroxene Potassium feldspar Quartz

Ultramafic igneous rocks Lots of iron and magnesium Very low silica (<45% SiO 2 ) Contain olivine and pyroxene—Often green Make up the Earth’s mantle Peridotite Green is olivine Black is pyroxene This is what the mantle looks like

Mafic igneous rocks Lots of iron, magnesium, and calcium Low silica (45–55% SiO 2 ) Contain pyroxene, amphibole, and calcium (black lab) plagioclase—Dark gray to black in color Make up oceanic crust

Intermediate igneous rocks Contain iron, calcium, and sodium Medium silica (55–65% SiO 2 ) Contain amphibole, and calcium-sodium (chocolate lab) plagioclase—Salt-and-pepper or medium-gray color Found in the continents

Felsic igneous rocks a.k.a. silicic Contain sodium, aluminum, and potassium High silica (>65% SiO 2 ) Contain potassium feldspar, quartz, and sodium (yellow lab) plagioclase—Makes them light colored Make up most of the continents

Mafic to Felsic comparison A B C

Mafic Felsic (silicic) Intermediate B A C

Igneous composition and temperature Bowen’s reaction series: Progression of minerals that can crystallize as temperature decreases

Discontinuous: discrete steps between mineral phases. Each one is entirely different Continuous: Plagioclase feldspars can have any amounts of calcium and sodium

Fractionation Mafic minerals crystallize first—use up iron and magnesium and increase concentration of aluminum, potassium, and sodium. Both mafic and felsic rocks crystallized from a single melt containing all the major elements. Rock with concentrated pyroxene produced by fractionation

Fractional Crystallization

Igneous rock textures Texture: Size, shape, and alignment of mineral grains in a rock –Provides information about local environment in which rocks form –Slower cooling = bigger crystal grains

Intrusive igneous rocks Cool and crystallize inside the earth –Earth is a great insulator. Intrusive rocks cool slowly –Grow large crystals.

Extrusive igneous rocks a.k.a. volcanic Cool and crystallize outside the earth –Water and air remove heat quickly. Extrusive rocks cool rapidly –Grow small crystals or form glass.

Intrusive igneous textures Phaneritic texture: coarse-grained –All mineral grains >0.5mm in diameter –Minerals visible to naked eye –Indicates slow cooling

Intrusive igneous textures Pegmatitic texture: very coarse-grained –Largest mineral grains >3cm in diameter –Indicates very slow cooling

Extrusive igneous textures Aphanitic texture: fine-grained –All mineral grains <0.5mm in diameter –Minerals invisible to naked eye –Indicates rapid cooling

Extrusive igneous textures Porphyritic texture: fine and coarse grains –Isolated grains >0.5mm surrounded by grains invisible to the naked eye –Indicates a mixed cooling history

Extrusive igneous textures Glassy texture: no mineral grains –Extremely rapid (almost instantaneous) cooling –Looks like glass Arrow head made from volcanic glass

Extrusive igneous textures Vesicular texture: bubbly –Gas bubbles trapped in volcanic rock –Like frozen foam from a coke or beer –Indicates explosive eruption and very fast cooling

Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Granite: Felsic, phaneritic (intrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Diorite: Intermediate phaneritic (intrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Basalt: Mafic, aphanitic (extrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic or porphyritic

Andesite: Intermediate, aphanitic (extrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic or porphyritic

Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Rhyolite: Felsic, aphanitic (extrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Gabbro: Mafic, phaneritic (intrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

Peridotite: Ultramafic, phaneritic rock Makes up the Earth’s mantle Crystallized from a melt when the Earth formed Technically metamorphic rocks

Felsic Intermediate Mafic Ultramafic Granite Diorite Gabbro Peridotite Rhyolite Andesite Basalt Aphanitic Phaneritic

Intrusive rock bodies Dike: Sheet-like intrusive rock body that cuts across layering in surrounding rocks Mafic dike in the Grand Canyon

Intrusive rock bodies Sill: Sheet-like intrusive rock body that follows layering in surrounding rocks

Intrusive rock bodies Lacolith: Large mushroom- or dome-shaped intrusive body consisting of a dome-shaped sill and a smaller feeder dike

Intrusive rock bodies Batholith: Very large intrusive body with an area of >100 km 2 Sierra Nevada Batholith

Igneous rocks and plate tectonics Continental rifts have bimodal volcanism, or, dominated by felsic and mafic igneous rocks

Igneous rocks and plate tectonics Oceanic divergent boundaries dominated by mafic igneous rocks

Igneous rocks and plate tectonics Oceanic-oceanic convergent boundaries dominated by mafic and intermediate igneous rocks Volcanoes Trench

Igneous rocks and plate tectonics Continental-oceanic convergent boundaries dominated by intermediate igneous rocks Mt. Shasta, CA Mt. Rainier, WA