An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30
Inside the Earth
Interior Structure of the Earth:
- 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:
It got this way through the process of Differentiation:
- Weight of the upper layers presses on the interior.
- Extreme compression leads to extreme heating.
An important process seen in rocky bodies throughout
the Solar System.
- 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 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)
The thin solid Crust floats on the Mantle. Only
~100 km thick.
- Layer of soft, mushy silicate rock
- ~2/3 the mass of the Earth
Earth's Magnetic Field
Convection currents get setup in the molten outer core because
of the bottom-to-top difference in temperature:
This setups the Geo-Dynamo:
- Hot base at the Solid Iron Inner Core
- Cooler at the top of the Outer Core/base of the Mantle
The Earth's Magnetic Field extends beyond the surface into
a Magnetosphere that interacts with the solar wind.
- Flowing electrically conducting iron fluid sets up an electric dynamo
- This generates a strong magnetic field
How do we know this is what the Earth looks like inside?
Different kinds of seismic waves are caused by Earthquakes in
In addition, there are surface waves that roll along the surface.
- 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
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:
This allows the plates to slide around.
- Region where basaltic lavas form.
- Lubricates the bottoms of the crustal plates
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.
- Slide Laterally at Transform Boundaries
- Collide Together at Convergent Boundaries
- Move Apart at Divergent Boundaries
- Consequence of plate motions working together to cause large-scale
changes in the continents and oceans over millions of years.
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)
Where two plates are colliding together.
The collision results in two processes:
[Note: the powerful earthquake in Indonesia that triggered the
Indian Ocean tsunami in December 2004 was in one of these deep
- One plate plows beneath the other.
- Sites of deep, powerful Earthquakes and volcanos.
- 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)
Where two plates are moving apart.
- Magma wells up from below and fills the gap, building new
- Older crust is dragged away from the boundary.
- 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.
Locations in the middle of plates:
As the plate slides over the fixed hot-spot, get long chains
of shield volcanoes.
- 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 Earth is a dynamic, actively evolving planet.
It is active today because the Earth's interior is still hot and molten.
- 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.
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Updated: 2007 October 27
Copyright © Richard W. Pogge,
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