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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi
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Lecture 27:
The Interior of the Earth
Key Ideas:
Interior Structure of the Earth:
- 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
The Earth's surface is
- 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
The Earth's interior is hot and dense:
- Weight of the upper layers presses on the interior.
- Extreme compression leads to extreme heating.
It got this way through the process of Differentiation:
- Start with a molten mix of metals and minerals
- Heavier metals (Iron & Nickel) sink to the center
- Lighter minerals (Silicates) float to the surface
An important process seen in rocky bodies throughout
the Solar System.
Journey to the Center of the Earth
Solid Inner Core: (5100-6370 km deep)
- 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
The thin solid Crust floats on the Mantle. Only
~100 km thick.
Earth's Magnetic Field
Convection currents get setup in the molten outer core because
of the bottom-to-top difference in temperature:
- Hot base at the Solid Iron Inner Core
- Cooler at the top of the Outer Core/base of the Mantle
This setups the Geo-Dynamo:
- Flowing electrically conducting iron fluid sets up an electric dynamo
- This generates a strong magnetic field
The Earth's Magnetic Field extends beyond the surface into
a Magnetosphere that interacts with the solar wind.
Seismology
How do we know this is what the Earth looks like inside?
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.
In addition, there are surface waves that roll along the surface.
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
The Earth's crust is broken into 16 rigid plates
- 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
This allows the plates to slide around.
Plate Tectonics
The crustal plates slide around on top of the Mantle:
- 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
Where two plates are sliding past each other
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
Where two plates are colliding together.
The collision results in two processes:
Subduction:
- One plate plows beneath the other.
- Sites of deep, powerful Earthquakes and volcanos.
[Note: the powerful earthquake in Indonesia that triggered the
Indian Ocean tsunami in December 2004 was in one of these deep
subduction zones]
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
Where two plates are moving apart.
- 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
Locations in the middle of plates:
- Plume of magma from the crust/mantle transition region
wells up towards the surface.
- Builds up Shield Volcanos in the middles of the plates
As the plate slides over the fixed hot-spot, get long chains
of shield volcanoes.
- 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 Earth is a dynamic, actively evolving planet.
- 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.
It is active today because the Earth's interior is still hot and molten.
- 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.
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