Chapter 5 : Rocks from molten liquidsRocks from molten liquids

Including adaptions from Dupre and Copeland (2004)

Concepts you should know for the exam

• Texture – grain size of intrusive (e.g., granite) and extrusive rocks (e.g., basalt)

• Relative cooling rates of intrusive and extrusive igneous rocks• Rocks from lavas vs. pyroclastic rocks• Mafic, intermediate and felsic rocks• Know table! (Temperature, Silica, Na, K, Fe, Mg, Ca content)• Melting point as a function of pressure, water content and

composition• Partial melting of magma-generating rocks• Fractional crystallization• Shapes of magmatic bodies – sills, dykes, batholiths• Hot spots and volcanoes (mafic composition)• Volcanoes at convergent margins ( mafic to felsic composition)• Volcanoes at divergent margins (mafic composition)• Lavas, welded tuff, vesicular basalt, volcanic bombs, pyroclastic

flows, volcanic cloud• Relation between the shape of volcano, chemistry, viscosity of

lavas (shield volcano, cinder cone volcano,composite volcano)• Dangerous pyroclastic flows cause almost 30% fatalities natural

disasters vs. tsunamis (~20%)

Lecture OutlineLecture Outline

1.1. Where do magmas form?Where do magmas form?

2.2.How do igneous rocks differ from one How do igneous rocks differ from one

another?another?

3. How do magmas form?3. How do magmas form?

4. Magmatic differentiation4. Magmatic differentiation

5. Forms of magmatic intrusion5. Forms of magmatic intrusion

6. Igneous activity and plate tectonics6. Igneous activity and plate tectonics

Lecture OutlineLecture Outline

1.1. Where do magmas form?Where do magmas form?

2.2.How do igneous rocks differ from one How do igneous rocks differ from one

another?another?

3. How do magmas form?3. How do magmas form?

4. Magmatic differentiation4. Magmatic differentiation

5. Forms of magmatic intrusion5. Forms of magmatic intrusion

6. Igneous activity and plate tectonics6. Igneous activity and plate tectonics

Divergent Plate BoundaryUsually start within continents—grows to become ocean basin

2. How do igneous rocks differ How do igneous rocks differ

from one another?from one another?

Texture – size of crystalsTexture – size of crystals

Coarse-grained rocksCoarse-grained rocks

Fine-grained rocksFine-grained rocks

Mixed texture rocksMixed texture rocks

1. How do igneous rocks differ How do igneous rocks differ

from one another?from one another?

Texture is related to rate of cooling.Texture is related to rate of cooling.

Intrusive igneous rocksIntrusive igneous rocks

Extrusive igneous rocksExtrusive igneous rocks

Andesite - granite near the surface

Hand sample-Santiago de Chile

Microscope slide

Giant’s Causeway, Giant’s Causeway, IrelandIreland

At which plate boundary does water-aided melting help

generate magma?

1. Divergent?2. Transform3. Convergent?

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

Two basic compositional groups:Two basic compositional groups:

Felsic igneous rocksFelsic igneous rocks

Mafic igneous rocksMafic igneous rocks

Why last figure is 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:

DioriteDioriteAndesiteAndesite

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

Lecture OutlineLecture Outline

1.1. Where do magmas form?Where do magmas form?

2.2.How do igneous rocks differ from one How do igneous rocks differ from one

another?another?

3. How do magmas form?3. How do magmas form?

4. Magmatic differentiation4. Magmatic differentiation

5. Forms of magmatic intrusion5. Forms of magmatic intrusion

6. Igneous activity and plate tectonics6. Igneous activity and plate tectonics

4. 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??????

Which rocks are hardest to melt?

1. Granite countertop

2. Hawaiian beach sands?

3. marble

Fractional Fractional CrystallizatioCrystallization!n!

Lecture OutlineLecture Outline

1.1. Where do magmas form?Where do magmas form?

2.2.How do igneous rocks differ from one How do igneous rocks differ from one

another?another?

3. How do magmas form?3. How do magmas form?

4. Magmatic differentiation4. Magmatic differentiation

5. Forms of magmatic intrusion5. Forms of magmatic intrusion

6. Igneous activity and plate tectonics6. Igneous activity and plate tectonics

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????

Lecture OutlineLecture Outline

1.1. Where do magmas form?Where do magmas form?

2.2.How do igneous rocks differ from one How do igneous rocks differ from one

another?another?

3. How do magmas form?3. How do magmas form?

4. Magmatic differentiation4. Magmatic differentiation

5. Forms of magmatic intrusion5. Forms of magmatic intrusion

6. Igneous activity and plate tectonics6. Igneous activity and plate tectonics

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 took this picture, but this massive piece of rock sticks up over 2,000ft from the surrounding plain, and is 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.

Shiprock, NM• www.jemmoore.com/flying/ shiprock.html• “Shiprock, NM. It's hard to tell, mainly because I

was flying at about 12,000ft when I took this picture, but this massive piece of rock sticks up over 2,000ft from the surrounding plain, and is 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.”

6. Igneous activity and plate tectonics

Magmatic geosystems:Magmatic geosystems:

Island arc plate subductionIsland arc plate subduction

Plate divergencePlate divergence

Hot-spot volcanismHot-spot volcanism

Continental plate subductionContinental plate subduction

Geomagmatic systems of EarthGeomagmatic systems of Earth

Geomagmatic systems of EarthGeomagmatic systems of Earth

ISLAND ARCPLATE SUBDUCTION

Mafic to intermediateintrusives (plutonism)Mafic to intermediateextrusives (volcanism)Island arc

volcano

Subduction zone

OceaniclithosphereOceaniclithosphere

Island arc plate subductionIsland arc plate subduction

PLATEDIVERGENCE

Basaltic extrusivesBasaltic intrusives

Mid-ocean ridge

Partial meltingOf upper mantlePartial melting

Of upper mantle

Rising magmaRising magma

Plate divergent boundaryPlate divergent boundary

HOT-SPOTVOLCANISM

Basaltic extrusivesBasaltic intrusives

Hot-spot volcano

Mantle plume(hot spot)Mantle plume(hot spot)

MantleMantle

Hot-spot Hot-spot volcanismvolcanism

CONTINENTALPLATE SUBDUCTION

Mafic to felsic intrusivesMafic to felsic extrusives

Subductionzone

Continentalmargin volcano

Oceanic

lithosphere

Oceanic

lithosphere

Continental mantle

lithosphere

Continental mantle

lithosphere

Continental crust

Continental crustOceaniccrustOceaniccrust

Continental Continental plate subductionplate subduction

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