What are Igneous Rocks???? Form when magma cools and minerals crystallize

Composition of magma Type of Igneous rock depends on the composition of the magma – Silica is the most abundant compound and has the greatest effect on magma characteristics 3 classifications of magma based on the amount of silica it contains – Basaltic – Andesitic – Rhyolitic

Chemical composition of lava is a little bit different than the magma it came from – Dissolved gases escape into the atmosphere

Magma Formation Two ways of formation – Melting earth’s crust – Melting within the mantle Four factors – Temperature Increases with depth within earth’s crust Geothermal gradient – Pressure Also increases with depth Result of the weight of overlying rock Pressure increases, melting point increases

– Water content Changes the melting point of the rocks Water content increases, melting point decreases – Mineral content Different minerals have different melting points Basalt – high melting point Granite – lower melting point – Contains more water Rocks high in iron and magnesium – high melting point Rocks high in silicon – low melting point

Partial Melting Process whereby some minerals melt at relatively low temperatures while other minerals remain solid – New elements are added during the process Changing the chemical composition One way different types of igneous rocks are formed

Bowen’s Reaction Series: minerals form in predicatable patterns

Bowen’s Reaction Series – Iron- rich minerals (left side) Go through abrupt changes as magma cools – Feldspars (right side) Lighter minerals Continually change chemical composition as magma cools

Classification of Igneous Rocks Intrusive rocks – Magma cools and crystallizes below the earth’s crust Magma injected into surrounding rock

Extrusive rocks – Cools and crystallizes on the Earth’s surface – Lava flow of flood basalts – Classified by: Mineral composition Physical properties – Grain size – texture

Classification of Igneous rocks Based on mineral compositions – Basaltic Dark colored Lower silica content – Granitic rocks Light colored High silica content – Intermediate Falls between basaltic and granitic Feldspar and hornblende – Ultrabasic Contain only iron rich minerals Always dark

Basaltic Gabbro

Granitic granite

Intermediate Feldspar Hornblende

Ultra basic Pyroxene

Crystal size and cooling rates When lava cools on the earth’s surface – Cools quickly – Not enough time for large crystals to form Rhyolite – hard to see crystals without magnification – Sometimes no crystals form When magma cools beneath earth’s surface – Cools slowly – Large crystals form

Porphyritic Rocks Porphyritic texture Large crystals surrounded by finer crystals of same or different minerals – Complex cooling First – slow cooling – big crystals Then quicker as the magma moves closer to the earth’s surface (or erupted onto the surface) – small crystals porphyritic granite

Vesicular rocks Vesicular texture – Big holes making the rock look spongy Dissolved gases escape when pressure lessens Leaving vesicles behind

Thin sections Easier to observe igneous rock composition when viewed in a thin strip under a microscope hornblende olivine

Igneous rocks in resources Veins – Fluid left during magma crystallization contain high levels of water, silica, and leftover minerals – Fluid fills in the cracks of surrounding rocks – Solidifies into mineral-rich quartz veins

Pegmatites Veins of very large-grained minerals Ores of rare elements form Can produce beautiful crystals

Kimberlites Veins in ultrabasic rock Diamonds Most likely form deep in the crust or mantle – Intrude quickly upward – Forming long, narrow, pipe like structures

Obsidian Normally extrusive Forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. The result is a volcanic glass with a smooth uniform texture that breaks with fracture

Rhyolite Light-colored Fine-grained Extrusive igneous rock Typically contains quartz and feldspar minerals.

Biotite Granite intrusive, most commonly occurring in batholiths

Hornblende greenish-black to black extrusive

Basalt Basalt is dark-colored and fine-grained extrusive

Gabbro Gabbro is dark, medium- to coarse-grained Intrusive igneous rock