Earth Formation and Structure

Early Earth: Age and Formation

• Condensed from solar nebula 4.6 billion years ago along with the rest of the planets in solar system.

• Formed from accretion (collision of planetesimals)

• Very hot!!– Hot enough to melt iron– Gravity pulled iron toward center:

denser materials pulled inward, lighter materials forced outward.

• 3 sources for Earth’s early heat:– Retained heat produced from collisions– Increasing weight of outer layers compressed inner

layers (more stuff crammed into smaller space = more heat)

– High abundance of radioactive materials (energy from these is converted to heat energy)

• Radioactive materials in core produce most of Earth’s internal heat!

Early Earth Structure

• Early Earth began to separate into three

distinct layers:– Core: at center composed

of nickel and iron– Mantle: around core– Crust: outermost layer

of less-dense solids• cooled slowly to form solid

rock about 4 billion years ago

• First crust has since been destroyed/recycled

Crust• Outermost layer of the earth; 10-70km thick

• Thin, rocky layer with two main types: continental and oceanic

• Continental crust is thicker; oceanic crust is thinner

• Crust is divided up into large sections called plates, which move in different directions– Seven major plates:

Pacific, South American, North American, Eurasian, African, Australian, Antarctic

– Many smaller minor plates (35 total) like Juan de Fuca, Nazca, Cocos, Arabian, Indian, Philippine plates

• Lithosphere: Crust plus very upper part of mantle.– Cool and brittle

• Asthenosphere: below lithosphere; upper portion of mantle.– Solid rock begins to melt,

begins to flow

(has a higher plasticity)• Plasticity: ability of a solid to

flow.

Two Portions of Crust:

Mantle

• Zone of rock nearly 2,900 km thick.

• Makes up nearly two-thirds of the Earth’s mass.

• Dense, semi-molten rock (sort of liquid, sort of solid)

• Convection currents here drive movement of crust

Outer Core• Dense liquid layer of iron and

nickel

• Spins/moves with Earth

• Generates Earth’s magnetic field (movement of hot charged particles creates the magnetic field)– Magnetic field concentrated at

North and South poles– Protects Earth from solar winds– Magnetic field switches every

500,000 years or so

Inner core

• Solid iron due to extreme pressure.

• Combined with outer core, makes up nearly one-third the Earth’s mass.

Dating Our Earth

• Estimated age of Earth and Solar System: 4.6 billion years old.

• Evidences:– Earth rocks: oldest Earth rocks are 3.8-4 billion years

old; oldest Earth minerals are 4.3 billion years old• Issues using Earth rocks: rocks constantly weathered,

eroded, and recycled back into the mantle through plate tectonics

– Moon rocks: oldest Moon rocks are 4.4 billion years old.• Subject to less weathering than Earth rocks; no tectonic

activity on moon.– Asteroids: oldest asteroids are 4.6 billion years old.

• Not subject to erosion and weathering.

Dating the Earth, continued

• Date rocks through radioactive dating.– Radioactive dating: using the amount of radioactive

material an object contains to calculate its age.• Use radioactive isotopes (unstable elements with uneven

number of protons and neutrons).

– Half-life: the amount of time it takes for half the starting material (parent isotope) to decay into the ending material (daughter isotope).

• Half-life of Uranium-238: 4.5 billion years (decays into Lead)• Half-life of Potassium-40: 1.3 billion years (decays into

Argon-40)