Chapter 5 : Rocks from molten liquidsRocks from molten liquids

Including adaptions from Dupre and Copeland (2004)

Andesite - granite at the surface

Hand sample-Santiago de Chile

Microscope slide

Genetic Classification of Igneous Rocks

• IntrusiveIntrusive: crystallized from slowly cooling magma intruded within the Earth’s crust; e.g. granite, gabbro

Genetic Classification of Igneous Rocks

• ExtrusiveExtrusive: crystallized from rapidly cooling magma extruded on the surface of the Earth as lava or erupted as pyroclastic material.

Extrusive Igneous Rocks Include:

• rocks formed from the cooling of lavaslavas

• rocks formed by the cooling of pyroclasticpyroclastic material, i.e. fragmented pieces of magma and material erupted into the air

Last Days of Pompeii-Karl Briullov--Copyright © 1999. George Mitrevski.

Auburn University

Mt St Helens crater 08-7-80, 60 mph, 800 deg F-pyroclastic flow- USGS

Composition and Classification of Igneous Rocks

• Chemistry: e.g. % SiO2

• Mineralogy: e.g.– Felsic (Feldspar and Silica)– Intermediate– Mafic (Magnesium and Ferric)– Ultramafic

Why Figure 5.4 I so important

• The color and mineral distribution indicate an increasing density and melting temperature. Darker igneous rocks generally weigh more and are formed at higher temperatures and pressures. This reflects the density-stratification of the whole Earth!

Felsic Igneous Rocks: -Igneous rocks rich in minerals

high in silica. They include:

Granite Granite

RhyoliteRhyolite

Intermediate Igneous Rocks:

-Igneous rocks in between in composition between felsic and

mafic igneous rocks. They include:

Granodiorite Granodiorite DaciteDaciteDioriteDioriteAndesiteAndesite

Mafic Igneous rocks -very low silica content, and consist

primarily of mafic minerals. The most common ultramafic rock is:

PeridotitePeridotite

What controls the melting temperatures of minerals?

*External pressurepressure*and Water content

*Internal compositioncomposition (including internal water content)

Factors Affecting Melting of Minerals (and Rocks)

• PressurePressure: Increased Pressures raises melting points

• Water Content (internal and Water Content (internal and external to the mineral)external to the mineral): Increased Water Content lowers melting points

• CompositionComposition: Felsic minerals melt at lower temperatures than mafic minerals

How do magmas form?

When rocks melt (or partially melt).

Why do rocks melt?Why do rocks melt?

When the temperature exceeds the melting pointmelting point of the rock or some

minerals within the rock.

Partial Melting

--Occurs when some of the minerals forming a rock melt at lower temperatures than other minerals within the same rock

If different minerals melt at different minerals melt at

different pressuresdifferent pressures that means that different minerals become

solid at different temperatures too. What does this imply about the internal composition of a internal composition of a

magma body as it coolsmagma body as it cools??????

Fractional Fractional CrystallizatioCrystallization!n!

What is Magmatic Differentiation ?

If, during fractional crystallization, the remaining magma were to erupt it would be

(a) more felsic or

(b) more mafic than the original magma????

What do we know about the shapeshape of magmatic

bodies? Are they sheets? Blobs? Spheres?

“Magmatic Pipes -101”

• Dykes are near-vertical• Sills are horizontal and squeeze in

between other layers of rock• Plutons are deep (km) bodies of

solidified magma. An example of a plutonic rock is granite or gabbro.

www.jemmoore.com/flying/ shiprock.html

Shiprock, NM. It's hard to tell, mainly because I was flying at about 12,000ft when I tookthis picture, but this massive piece of rock sticks up over 2,000ft from the surrounding plain, andis as big as a small city. It is on Navajo land, and is a significant spiritual site for the Navajo.The hard, volcanic spines radiating out from the main spire are really amazing. From this high up, you couldn't even see an eighteen-wheeler on the ground next to Shiprock, it would be just a speck.

Summary of Volcanoes

• Subduction volcanoes in continental crustal and oceanic crust at convergent plate margins

• Hot spot volcanoes (anywhere)

• decompression volcanoes at divergent plate margins

END of CHAPTER 5

IGNEOUS ROCKS-rocks from liquid melts