Dynamic Earth Class February 2006

Volcanic Imagination (Chapter 4) Exploring the Earth’s Interior

How do we know about the Earth’s Interior? By studying Meteorites By studying Meteorites Direct observation (rocks originating from depth) Direct observation (rocks originating from depth) Experiments at high pressure Experiments at high pressure By studying earthquake waves (Seismology) By studying earthquake waves (Seismology)

Meteorites have struck the Earth in the past. Many are probably pieces of proto-planets similar in composition to Earth.

Meteorites Accumulate Daily

Meteorites Stony meteorites are rich in olivine and pyroxene Similar to Earth’s lithosphere

Meteorites Iron meteorites are made of iron and nickel Earth’s interior (core) is similar

Lafayette Meteorite

Types of Meteorites - I Stones Stones Primarily silicates (like Earth’s crust and mantle) Primarily silicates (like Earth’s crust and mantle) >90% of all meteorites >90% of all meteorites Irons Irons Iron-nickel alloys Iron-nickel alloys Stony irons Stony irons Combination of stony and iron meteorites Combination of stony and iron meteorites

Types of Meteorites - I

Types of Meteorites - II Falls Falls Meteorites observed falling to the ground Meteorites observed falling to the ground Primarily stones (suggests they are more common) Primarily stones (suggests they are more common) Finds Finds Meteorites discovered on the ground Meteorites discovered on the ground Primarily irons (collected because they are unusual looking) Primarily irons (collected because they are unusual looking)

Composition of Meteorites

Chemical Composition of Earth

How do we know about the Earth’s Interior? By studying Meteorites By studying Meteorites Direct observation (rocks originating from depth) Direct observation (rocks originating from depth) Experiments at high pressure Experiments at high pressure By studying earthquake waves (Seismology) By studying earthquake waves (Seismology)

Large Volcanic Eruptions Voluminous volcanic eruptions Sample significant part of mantle Can infer something about mantle composition

Kimberlites Rapidly injected rock Volatile-rich Often contain diamonds Known to form at high pressure - deep in mantle (>400 km) Hosted by mantle rock

Kimberlites

Kimberlites Sample Mantle Peridotite

How do we know about the Earth’s Interior? By studying Meteorites By studying Meteorites Direct observation (rocks originating from depth) Direct observation (rocks originating from depth) Experiments at high pressure Experiments at high pressure By studying earthquake waves (Seismology) By studying earthquake waves (Seismology)

Diamond-anvil cell

Multi-anvil Press

Seismology Study of the propagation of mechanical energy released by earthquakes. Study of the propagation of mechanical energy released by earthquakes. When energy is released, waves of motion (like the effect of a pebble tossed into a pond) are set up in the Earth. When energy is released, waves of motion (like the effect of a pebble tossed into a pond) are set up in the Earth.

Structure of the Earth Seismic velocity (how fast earthquake waves travel through rocks) depends on the composition of material and pressure. Seismic velocity (how fast earthquake waves travel through rocks) depends on the composition of material and pressure. We can use the behavior of seismic waves to tell us about the interior of the Earth. We can use the behavior of seismic waves to tell us about the interior of the Earth.

Seismic waves Waves are started because of initial tension or compression in the rock. Waves are started because of initial tension or compression in the rock. Path of waves are curved because different rock types at different depths change speed at which waves travel Path of waves are curved because different rock types at different depths change speed at which waves travel

Most common types of earthquake waves: P-waves and S-waves – Body waves P-waves and S-waves – Body waves Primary waves travel the fastest in the crust and usually are the first waves to arrive Primary waves travel the fastest in the crust and usually are the first waves to arrive Secondary (or Shear) waves are slower and therefore take longer to arrive Secondary (or Shear) waves are slower and therefore take longer to arrive

Three Main Types of Seismic Waves P-waves travel faster than S-waves, so they arrive at the recording station sooner

Types of Seismic Waves

Fig Difference in travel- time for P and S waves tells us how far away the earthquake is from the recording station

Seismic Travel-time Curve

Structure of the Earth Seismic velocity (how fast earthquake waves travel through rocks) depends on the composition of material and pressure. Seismic velocity (how fast earthquake waves travel through rocks) depends on the composition of material and pressure. We can use the behavior of seismic waves to tell us about the interior of the Earth. We can use the behavior of seismic waves to tell us about the interior of the Earth.

Changes in P- and S- wave Velocity Reveal Earth’s Internal Layers Velocities generally increase in each layer

Surface waves Rayleigh waves Love waves

Refraction Reflection Refraction and Reflection of a Beam of Light

Refraction and reflection of seismic body waves

P-and S-wave Pathways Through Earth

Travel paths for shallow seismic waves

P-wave Shadow Zone

S-wave Shadow Zone

P wave shadow zone S wave shadow zone

Seismology and Earth structure

Layers of the Earth

Earth’s CORE Outer Core - Liquid Fe, ~2200 km thick, No S-waves transmitted -> S-& P-wave Shadow Zones Outer Core - Liquid Fe, ~2200 km thick, No S-waves transmitted -> S-& P-wave Shadow Zones Inner Core - solid Fe (some Ni, Co, S, C), ~2500 km thick Inner Core - solid Fe (some Ni, Co, S, C), ~2500 km thick How do we know? Meteorites, Seismology, Magnetic field How do we know? Meteorites, Seismology, Magnetic field

Earth’s Geodynamo

Origin of Earth’s magnetic field: the geodynamo The basic idea: an electric motor is a dynamo Motion of the liquid outer core -- a conductor -- in a magnetic field generates current The current generates a stronger magnetic field

Origin of Earth’s magnetic field: the geodynamo

Modeled Geodynamo

Ocean crust records magnetic reversals

Magnetic Reversals in the Ocean

Magnetic Reversals

…the inner core rotates faster than the mantle.

Isostasy: Another key to Earth’s Interior Buoyancy of low-density rock masses “floating on” high-density rocks; accounts for “roots” of mountain belts Buoyancy of low-density rock masses “floating on” high-density rocks; accounts for “roots” of mountain belts First noted during a survey of India First noted during a survey of India Himalayas seemed to affect plumb bob Himalayas seemed to affect plumb bob

The less dense crust “floats” on the less buoyant, denser mantle Mohorovicic Discontinuity (Moho)

Crust as an Elastic Sheet Continental ice loads the mantle Ice causes isostatic subsidence Melting of ice causes isostatic uplift Return to isostatic equilibrium

Uplift Formed by Removal of Ice Sheet

Northern hemi- sphere during the last glacial age

Evidence of isostatic uplift after melting of ice sheet Uplifted beach ridges

Earth’s internal heat Original heat Original heat Subsequent radioactive decay Subsequent radioactive decay Conduction Conduction Convection Convection

Earth Formation

Temperature vs. Depth