Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 2:30

Lecture 28:
Inside the Earth

Key Ideas:

• Differentiation
• Solid iron inner core & molten iron outer core
• Thick, rocky mantle & thin, rocky crust

Earth's Magnetic Field

• Geo-Dynamo: currents in the molten outer core

Crust is broken into tectonic plates:

• Plate Tectonics & Continental Drift
• Plate Boundaries & Hot Spots

Surface of the Earth

• 71% oceans
• 29% continents

Surface rocks are primarily silicates.

Surface layers have been subjected to

• Water and Wind Erosion
• Volcanic Repaving
• Subduction of the crust into the mantle
• Uplift of mountains

Interior of the Earth

• Weight of the upper layers presses on the interior.
• Extreme compression leads to extreme heating.

• Start with a molten mix of metals and minerals
• Heavier metals (Iron & Nickel) sink to the center
• Lighter minerals (Silicates) float to the surface

Journey to the Center of the Earth

• Solid iron & nickel at 7000 K
• Kept solid by high pressure (pressure freezing)
• 2% of the mass of the Earth
• Floats in the middle of the molten outer core.

Molten Outer Core: (2900-5100 km deep)

• Molten Iron & Nickel, dissolved S & O
• 30% of the mass of the Earth

Mantle: (100 to 2900 km deep)

• Layer of soft, mushy silicate rock
• ~2/3 the mass of the Earth

Earth's Magnetic Field

• Hot base at the Solid Iron Inner Core
• Cooler at the top of the Outer Core/base of the Mantle

• Flowing electrically conducting iron fluid sets up an electric dynamo
• This generates a strong magnetic field

Seismology

Different kinds of seismic waves are caused by Earthquakes in the crust:

• P-waves: compression (pressure) waves that pass through both solid and molten parts
• S-waves: shearing waves that can pass through solid parts, but are reflected/absorbed by molten parts.

[Adapted from figure 2 in Don L. Anderson's article Planet Earth in The New Solar System, 4th ed., Beatty, Petersen, & Chaikin, eds (1999, Cambridge University Press). This is a "still" of the animated PowerPoint graphic I used in lecture. Click on image for full-size version (11Kb GIF)]

Seismologists use P- and S-waves from earthquakes to map the interior of the Earth like a doctor uses MRI or ultrasound to map the inside of a person.

The Crust of the Earth

• Thin Oceanic Plates about 10 km thick
• Thick Continental Plates up to 50 km thick

These plates float on the Mantle above a complex transition zone:

• Region where basaltic lavas form.
• Lubricates the bottoms of the crustal plates

Plate Tectonics

• Plate motion is a few cm/year
• Motion is driven by convection currents in the Mantle.

Plate Motions:

• Slide Laterally at Transform Boundaries
• Collide Together at Convergent Boundaries
• Move Apart at Divergent Boundaries

Continental Drift:

• Consequence of plate motions working together to cause large-scale changes in the continents and oceans over millions of years.

Transform Boundary

• Speeds of a few cm/year.

These boundaries form Transverse Faults:

• Example: San Andreas Fault between the North American & Pacific Plates
• Plates can stick at the boundary, building up strain.
• The strain breaks, and the crust jumps many meters
• Source of strong near-surface Earthquakes (1906 quake that leveled San Francisco)

Convergent Boundary

The collision results in two processes:

Subduction:

• One plate plows beneath the other.
• Sites of deep, powerful Earthquakes and volcanos.

Crust Buckling:

• Form high mountains & plateaus where continental plates collide with oceanic or continental plates (e.g., Andes, Himalayas, Sierra Nevada)
• Form volcanic island arcs and deep ocean trenches where 2 oceanic plates collide (e.g., Aleutians, Japan, Indonesia)

Divergent Boundary

• Magma wells up from below and fills the gap, building new crust.
• Older crust is dragged away from the boundary.

Mid-Atlantic Ridge:

• Boundary of North American & Eurasian plates
• Rocks get older the farther you move away from the ridge.
• Splitting Iceland into two parts.

Sea-floor spreading was the first hard evidence of tectonic motions.

Hot Spots

• Plume of magma from the crust/mantle transition region wells up towards the surface.
• Builds up Shield Volcanos in the middles of the plates

• Example: Hawaiian Island Chain
• Big Island is the youngest & most active island.
• The further along the chain you go, the older the island.

The Dynamic Earth

• The surface of the Earth has been shaped and reshaped over billions of years by the forces of plate tectonics and weathering.
• Most of the surface is relatively young, a few 10s to 100s of Millions of years old for the most part.

• Started out in a hot, molten state ot formation.
• About 80% of crustal heat comes from radioactive decay.

We will use the Earth as the basis of comparison when we look at other rocky bodies in the Solar System.

My geologist colleagues will likely be most upset with me for having compressed their life's work into one brief lecture. The subject is rich and fascinating, but what I want to emphasize here are the basic processes at work in the interior of our planet. When we consider the other terrestrial planets in the Solar System, we will return to these ideas, and see how they are similar and different from the Earth.

If you would like to learn more about this fascinating subject, you can start with this excellent hypertext summary of plate tectonics, This Dynamic Earth by W. Jacquelyne Kious and Robert Tilling of the US Geologic Survey.