Transcript Ch. 2
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 02_01a.jpg 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 02_08.jpg 02_09.jpg 02_14.jpg 02_16.jpg