Download Comparison of the rocky planets

Earth Structure
broadest view:
1) solid Earth and 2) atmosphere
atmosphere primarily composed of nitrogen (78%)
and oxygen (21%)
important gas CO2 = 0.03%--for T regulation
Venus ~1000x more than Earth
Mars ~1/1000 of Earth
atmospheric density decreases away from surface
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Where did atmosphere come
from?
gases that formed the original atmosphere were derived from volcanic
eruptions, which, similar to present-day eruptions, brought water
vapor, hydrogen, carbon dioxide, nitrogen, and sulfur to the surface
from deep in the earth (no free oxygen).
The abundance of oxygen was much lower and did not begin to rise in
abundance until photosynthetic algae began to consume CO2 and give
off O2. There is evidence for this different atmospheric composition in
the rock record because unoxidized iron (Fe2+) could be transported in
river systems and deposited as iron oxide in huge economic deposits
known as Banded Iron Formations.
Bulk Earth composition
mostly oxygen, silicon, iron and magnesium
How do we know?
Composition based on:
1) seismic wave velocities through Earth
2) Mantle samples in volcanic rocks (xenoliths)
3) Compositions of “primitive” meteorites
Solid Earth structure
Earth's radius varies from 6357-6378 km (it bulges at the equator). 6370
km is a good "average" radius of the earth.
Earth is chemically layered:
A cross section of the earth starting at outer edge (= 0 km) and drawing a line
to the very center (6370 km) shows 3 distinct layers based on chemical
composition.
1) Crust (variable but <70 km thick),
2) Mantle (~2850 km thick; from surface down to 2900 km depth)
3) Core (2900-6370 km)
How do we know about these layers?
Very clear jumps in velocities of seismic waves at boundaries between layers
MOHO = crust-mantle boundary
The layers…
What is their composition?
core - heavy metallic elements, mostly Iron, some Nickel (plus minor
amounts of a "light element"). Due to pressure and temperature effects,
the core is segregated into a liquid outer core and a solid inner core.
Dimensions: Inner = 5150-6370 km, Outer = 2891-5150 km.
Mantle-made up of Mg and Fe bearing silicate minerals (peridotite). The
mantle is solid but can convect on geologic timescales.
Crust-two types, oceanic and continental very different in terms of
composition, thickness, age and behavior--this will be important when
we get back to talking about plates…
Oceanic vs Continental crust
Oceanic crust
Continental crust
Location:
Underlies oceans
Underlies land
Composition:
Richer in Mg, Fe
Richer in Si, K, Na
Density:
Heavy (will sink)
Light (floats)
Thickness:
~7 km thick
35-40 km thick
age of oldest crust
present today: 180 million years
4.1 billion years
How did the Earth get layers?
Differentiation: process of separating a material into different chemical
compositions
STEP 1) Need to heat up the mixture to melt it and separate it.
Hypotheses suggest that "early heavy bombardment" by violent collisions
with residual matter (planetesimals and larger bodies) could provide the
heat and energy.
Impact Energy (kinetic energy),
Plus Gravitational Energy (movement of dense material to center)
Plus heat coming from radioactive elements decaying (Uranium, Thorium,
Potassium)
Plus, once cooling begins, Heat of Crystallization
STEP 2) Differentiation and formation of Earth's layers
One hypothesis: heavy bombardment and radioactive decay caused 3065% of the proto-earth to melt into a MAGMA OCEAN, and the rest was
probably soft. Heavy elements moved toward the Earth's center, light
elements moved to the surface.
Based on some recent work, this all happened quite fast, within the first
200 million years of Earth’s existence.
The amount of continental crust we have today probably accumulated
over longer periods of time (2 Gyrs?).
How does this compositional
layering fit with the plate story?
The plates are defined based on rheology (behavior of material ),
not composition.
The tectonic plates reflect the lithosphere (lithos=rock)-rigid, nonflowing -the lithosphere consists of the crust plus uppermost mantle
beneath it lies the asthenosphere (mantle which is soft and flowable
on geologic timescales-it is still almost entirely solid if not all solid)
these differ in their viscosity or resistance to flow and basic reflect
temperature-with depth, T goes up and viscosity goes down-lithosphere = plates; asthenosphere allows the plates to move
General surface topography
70% of surface covered by water
generally, the ocean fills areas above
oceanic plate because of the topographic
differences of oceanic vs continental
lithosphere
What is going on?
different densities-> different buoyancy
continents are like icebergs in water
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