Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 26: Inside the Earth

Key Ideas:

Interior Structure of the Earth: Differentiation; Solid iron inner core and molten iron outer core; Motion of the molten, outer core produces the Earth's magnetic field; Thick, rocky mantle and thin, rocky crust
Crust is broken into tectonic plates: Plate tectonics and continental drift; Types of plate boundaries

Interior of the Earth

The Earth's interior is hot and dense: Upper layers press down on the interior; Compression leads to extreme heating
Differentiation:: Start with molten mix of metals and minerals; Heavy metals (Iron and Nickel) sink to the center; Light minerals (Silicates) float to the top
Thicknesses of layers today:: Crust is about 100km thick; Mantle extends to 2900km, composed of 'mushy silicates'; Outer core extends to 5100km, composed of molten Iron; Inner core is solid Iron

Seismology

Earthquakes create Seismic Waves: P-Waves: compression (pressure) waves that pass through solid and molten regions alike; S-Waves: shearing waves that pass through solids, but are reflected/absorbed by molten regions
Detection of s-waves and p-waves at different locations on the Earth's surface after an earthquake is a powerful probe of the Earth's interior

Earth's Magnetic Field

Convection currents in the molten outer core:: Hot base at the Solid Iron Inner Core; Cooler at the top of the outer core
The Geo-Dynamo:: Flowing electrically conducting iron fluid sets up an "electric dynamo"; Generates a strong magnetic field that extends out past the surface into interplanetary space

The Changing Magnetic Pole

The location of the poles correspond to the rotation axis of the inner core
Pole is currently near Resolute Bay, Canada, but moving about 40km/year (toward Siberia)
Occasionally the magnetic field completely flips (about every 300,000 years)

Continental Drift

Alfred Wegener (1880-1930) first proposed the theory of continental drift
Supporting Evidence:: Shapes match; Animals and plants match; Rocks match; Temperature mismatch to current locations
Future implications:: Himalayas continue to grow; Atlantic ocean grows, Pacific shrinks

The Crust of the Earth

The Crust of broken into 16 rigid plates: Thin Oceanic Plates ~10 km thick; Thick Continental Plates (~50 km thick max)
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

Crustal plates slide around over the Mantle:: Driven by convection currents in the mantle; Speed of a few cm/year
Plate motions:: Slide Laterally; Collide Together; Move Apart

Plate Boundaries

Transform Boundary: Where 2 plates slide past each other
Convergent Boundary: Where 2 plates collide; Causes Subduction and Crustal Buckling
Divergent Boundary: Where 2 plates move apart; Get ocean ridges with new crust at the gap and older crust as you move outwards

The Dynamic Earth

The Earth is a dynamic, evolving planet: Surface has been reshaped by tectonic and weather forces acting over billions of years; Most of the surface is relatively young (few 10s to 100s of Millions of years old)
Active today because its interior is still hot: Started out in a hot, molten state; 80 percent of crustal heating is from radioactive decay

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.