Chapter 3 – Igneous Rocks, the Origin Chapter 3 – Igneous Rocks, the Origin and Evolution of Magmaand Evolution of Magma

Rocks are composed of an aggregate of one or more minerals.

One exception: obsidian (rock) is composed of volcanic glass with no orderly internal arrangement of atoms.

Three rock types.

Igneous RocksIgneous Rocks

Igneous rocks: rocks “born of fire”; all igneous rocks were once molten.

Intrusive igneous rocks cool and solidify deep inside earth.

Form from solidified magma.

Extrusive igneous rocks cool and solidify on earth’s surface.

Form from solidified lava.

Igneous Rock PicturesIgneous Rock Pictures

Intrusive (Plutonic) Igneous Rock

Extrusive (Volcanic) Igneous Rock

Sedimentary RocksSedimentary Rocks

Sedimentary rocks are composed of sediment.

Sediment can be either clastic (pieces) or chemical.

Clastic sediment is pieces of pre- existing rock like gravel, sand, silt and clay.

Chemical sediment is chemicals dissolved in water.

Sedimentary Rock PicturesSedimentary Rock Pictures

Gravel clasts.Chemical

sedimentary rock.

Metamorphic RocksMetamorphic Rocks

Metamorphic rocks: Changed by heat and pressure. Change is accomplished without melting.

Rock CycleRock Cycle

One of the three rock types can become any other rock type through various geologic processes. This is known as the rock cycle.

Rock Cycle DiagramRock Cycle Diagram

One of the three rock types can become any other rock type through various geologic processes. This is known as the rock cycle.

Rock Cycle DiagramRock Cycle Diagram

Igneous RocksIgneous Rocks

90% of all crustal rocks. Most covered by sedimentary rocks. Hard to see.

Magma: molten rock underground.Cools slowly. Large crystals. 1 MY cooling historyPlutonic: Pluto-god of underworld.

Lava: molten rock on earth’s surface.Cools quickly.Small or no crystals. Hours-weeks cooling.Volcanic: Vulcan-god of fire

Eight Major Igneous RocksEight Major Igneous Rocks

Light color High silica Low Fe, Mg Felsic

Intermediate color Intermediate silica Intermediate Fe, Mg

Dark color Low silica High Fe, Mg Mafic

Very dark color Very low silica Very high Fe, Mg Ultramafic

Plutonic Large Xls Slow cooling Intrusive

Granite Diorite Gabbro Peridotite

Volcanic Small Xls Fast cooling Extrusive

Rhyolite Andesite Basalt Komatiite/ Kimberlite

Start Chemistry of Igneous Rock

felsic - high in silica - ≈70%+

granite/rhyolite

intermediate - in-between felsic and mafic - ≈60%

diorite/andesite

mafic - low in silica - ≈ 50%

gabbro/basalt

ultramafic - really low in silica - below 44%

peridotite/xxx

Granite Picture

Rhyolite Picture

Diorite Picture

Andesite Picture

Gabbro Picture

Basalt Picture

Bowen’s Reaction Series

Could you create every igneous rock by melting the upper mantle?

assumption - chemistry of the magma is the same as the upper mantle

BRS - yes, order in which minerals crystallize

fractional crystallization

crystal separationfilter pressingcrystal settling

crystal flotation

Bowen’s Reaction

Page 199 Figure 1

Fig. 7.21.a

What if the Magma Stays Underground?

magmatic stoping - moving through the solid rock (country rock)wedgingbreaking (xenolith)melting

plutonsbatholith - larger than 100 km2

most batholiths contain multiple intrusionsstock - smaller 10km2 or lessdike – discordant intrusionsill – concordant intrusion

Fig. 7.18