REVIEW
Accumulation and the Nebular Hypothesis
The standard disclaimer...
"As usual, what you are about to hear is only our best guess."
The earth is not the same from the surface to the center
Appears to be layered by density
The earth is a lot like an egg (demo?)
Describe how egg is similar to earth's internal structure (Transparency #26)
Core: iron and nickel
Mantle: silicates (probably ultramafic in composition)
Crust: silicates (primarily basalt and granite)
This layering is the result of differentiation
Very important so pay attention!
Density differences - the driving force
Examples:
Oil and water
Water and mercury
Pepsi cans
Candle in the wind
ORBITZ soft drink
Anyway, density differences have resulted in the current layering of the earth
Heaviest stuff in the center, lighter as we move to the surface
The crust is the important part - it's where we live!
The lightest "scum" is accumulating at the surface
Cools where exposed to space
Results in igneous rocks
Form the vast majority of the crust
Primarily composed of one of two fundamentally different rock types (mafic vs. felsic)
Basalt - dark and heavy (specific gravity 3.2)
Forms the relatively low-elevation crust beneath the ocean basins
Refer to Physiographic map
Granite - much lighter rock (s.g. 2.7)
Because granite is less dense, it tends to ride higher in the crust
Forms the major continental land masses.
Essentially floating in a 'sea of basalt'
Both basalt and granite are 'igneous' rocks
Crystallized from magma: the molten (liquid) phase of solid rock
We will discuss how these crustal materials are evolving for the next couple weeks
Definitely an on-going process