Unit VI. Solid Earth CirculationA. Structure of the Solid Earth

1. Crust :Thin, brittle, variable composition, least dense.

a. Continental Crust: older, thicker, more variable, lots of water/volatilesb. Oceanic Crust: thinner, younger, more uniform composition.

Unit VI. Solid Earth CirculationA. Structure of the Solid Earth

1. Crust: Thin, brittle, variable composition, least dense.2. Mantle: Uniform composition; upper Mantle behaves like a soft plastic (rock that can slowly deform).3. Core: Very hot, very dense. Outer core liquid, inner core solid. Mostly iron (85%) and nickle (6%).Motion of core creates magnetic field.

Unit VI. Solid Earth Circulation A. Structure of the Solid Earth How do we look into the center of the Earth? By tracking seismic waves produced by

EarthquakesVolcanic explosionsLarge human noises (atomic bombs)

P (compression)-waves can pass through solid and liquid media.

S (shear)-waves can only pass through solid media.

P (compression)-waves

S (shear)-waves

Unit VI. Solid Earth Circulation A. Structure of the Solid Earth B. Earth’s Magnetic Field

1. Core is a magnetic dynamo: Transforms fluid motion (convection) into electrical currents that create magnetic field.

Unit VI. Solid Earth Circulation A. Structure of the Solid Earth B. Earth’s Magnetic Field

1. Core is a magnetic dynamo: Magnetic mineral grains (in lava or sediment) align themselves with the Earth’s magnetic field, preserving a record of the field direction when they were deposited. Throughout recorded history, the compass has always pointed North. But, Earth’s magnetic field occasionally collapses and reforms in the opposite direction. These are called magnetic reversals.

Unit VI. Solid Earth Circulation B. Earth’s Magnetic Field

1. Core is a magnetic dynamo:Magnetic field occasionally collapses…. and may reform in the opposite direction.

Magnetic reversals occur every 0.5 to 1 million years. Most recently, 0.7 Myr ago.

Normal and Reversed Polarity

Unit VI. Solid Earth Circulation B. Earth’s Magnetic Field

1. Core is a magnetic dynamo: Transforms fluid motion (convection) into electrical currents that create magnetic field.

2. Why does the core convect? Density differences. Radioactivity: Radioactive decay releases heat energy. Hot core heats the overlying mantle, so the outer edge core is coolest. Causes a fundamental instability.

Unit VI. Solid Earth Circulation A. Structure of the Solid Earth B. Earth’s Magnetic Field C. Plate TectonicsIn the 1920s Alfred Wegener proposed that the continents used to be together based on similar continental outlines, distribution of fossils andmountain chains.

Unit VI. Solid Earth Circulation C. Plate Tectonics

But there was no physical basis for drift, and his ideas were dismissed for half a century.

In the early years after WWII, ocean exploration revealed mountain ranges in the middle of the oceans (mid-ocean ridges) and ships measured the orientation of magnetic minerals on the sea floor.They found magnetic stripes!

Unit VI. Solid Earth Circulation C. Plate Tectonics

1. Sea floor spreading: Magnetic stripes get older away from mid-ocean ridges, proving that oceanic crust is constantly being formed at mid-ocean ridges.

Where does all that ocean crust go?

Oceanic crust is destroyed along subduction zones, where it is recycled back into the mantle.

Unit VI. Solid Earth Circulation C. Plate Tectonics

1. Sea floor spreading2. Earth’s Tectonic Plates3. Plate margins determine earthquakes and

volcanism. a. Divergent Margins : where Earth’s

lithosphere is being pulled apart. Examples: Mid-Ocean Ridges

African Rift Valleys

Unit VI. Solid Earth Circulation C. Plate Tectonics

3. Plate margins a. Divergent Margins: where Earth’s

lithosphere is being pulled apart.

Commonly associated with mild volcanism, shallow earthquakes.

Fluid volcanism

Why?

Unit VI. Solid Earth Circulation C. Plate Tectonics

3. Plate margins a. Divergent Margins: where Earth’s

lithosphere is being pulled apart. b. Convergent Margins: where two

plates are forced together (subduction/collision). Examples?Pacific NorthwestAleutian Island Chain off AlaskaJapan, Italy, IndiaWest coast of South America

b. Convergent Margins: where two plates are forced together (subduction/collision).

Subduction of oceanic plate is commonly associated with volcanism and deep earthquakes.

What kind of volcanism?

Explosive volcanism, because heated rocks have lots of volatile materials in them (water + water tied to mineral (hydrated)).

When do we get collision, and when subduction?

Ocean Crust/Continental Crust = subductionbecause oceanic crust is more dense

Continental Crust/Continetal Crust = collisionequal densities…neither can easily sink.

Still get subduction, but not of continental crust.

Example: India …. Himalaya

Unit VI. Solid Earth Circulation C. Plate Tectonics

3. Plate margins a. Divergent Margins: where Earth’s

lithosphere is being pulled apart. b. Convergent Margins: where two

plates are forced together (subduction/collision).

c. Transform Margins: Where two plates slide past each other.

Examples? Southern California and the San Andreas fault

4. What Drives Plate Tectonics?In order to get rock to move we need:

• Density differences: Created by an inherent instability…. Earth is hottest in its interior due to…radioactive decay.

Liquid core circulates relatively quickly, heats the lower mantle, which becomes less dense than the overlying mantle, and SLOWLY starts to rise. Buoyant.

Rigid lithosphere just along for the ride.

• Plasticity: heat+pressure make rock deformable

Unit VI. Solid Earth Circulation C. Plate Tectonics D. Earthquakes: sudden release of stored energy as a result of rapid movement between two blocks of rock.

Can only happen where rocks are brittle…..in the lithosphere.

Unit VI. Solid Earth Circulation C. Plate Tectonics D. Earthquakes E. Volcanos: Explosive vs Fluid

Where they occur and why it matters.

Most occur at Plate Boundaries

Most explosive volcanism occurs along subduction zones, in continental crust.

Unit VI. Solid Earth Circulation C. Plate Tectonics D. Earthquakes E. Volcanos: Explosive vs Fluid

Hot Spots: Intra-Plate volcanism• Hawaii: Mid-ocean hot spot • Iceland: Hot spot on mid-ocean ridge• Yellowstone: Mid-continent Hot Spot

Iceland: a Hot Spot on a Mid-Ocean Ridge

Yellowstone

The last three Yellowstone eruptions

Caldera forming ash flow tuff

Age (millions of

years)

Volume erupted (km3)

Lava Creek Tuff 0.6 1,000

Mesa Falls Tuff 

1.3 280

Huckleberry Ridge Tuff  

2.1 2,450

Difference

0.8 Myr

0.7 Myr

0.6 Myr?

For our purposes, volcanic eruptions are most important if they are explosive and place lots of material in the stratosphere.

But they also pose a very real threat to nearby, and sometimes very distant, communities.

Predictive tools are improving, and rarely do volcanos erupt without warning.