Origin and Structureof the Earth

Marshak – Chapter 1(plus an introduction to Chapter 2)

The Earth is part of the solar system and thus most likely formed at the same time…

So, what do we know about the solar system and it’s structure?

These are the observations which are needed to come up with an idea (hypothesis) for how the solar system (and Earth) formed.

Formation of the solar system and differentiation of Earth

• Hypotheses must satisfy observations: planets orbit sun in one direction, axes of rotation nearly perpendicular to orbit, most planets rotate in same direction as orbit about sun, >99% solar system mass in sun, ~99% solar system angular momentum in planets

• Inner Terrestrial - Mercury, Venus, Earth, Mars• Outer Jovian - Jupiter, Saturn, Uranus, Neptune, Pluto?• Terrestrial – dense, rocky, >3 g cm3, Mg, Fe, Si, K, Ca,

metals combined with O• Jovian - “gassy” <~1.5 g cm3, ice, H, He, CH4 methane CO2

• Asteroid belt between Mars and Jupiter, source of meteorites

Origin of our Solar System: The Nebular Hypothesis

The Sun is ~99% of the mass of the solar system

~99% of the angular momentum is in the planets

Inner planets are rocky and dense – terrestrial planets

Outer planets are gassy – gas giant planets

We know the Earth is composed of layers – Why?

Planetary Differentiation• Why?

– There is a motive• Layers of different chemical composition can have

different density, and gravity provides a driving force whereby planets can lower their potential energy by sorting the denser material towards the center.

– There is a means• Solids are hard to sort mechanically, but liquids are

easily separated gravitationally. Partial or complete melting allows large-scale differentiation.

– There was an opportunity• Heating beyond the melting point of most components of

undifferentiated solar material during planet formation is inevitable for bodies above a certain size (> approx. 1,000 km radius) that formed early enough or fast enough.

Chemical Differentiation of the Earth

Early Earth Earth Today

Early Earth likely entirely molten – gravitational segregationof dense metals (mostly Fe) to the center is the result.

Origin of the moon by planetary impact on Earth

This occurred ~4.5 billion years ago (4.5 Ga) (very early in Earth history as age is only ~4.6 Ga)

Whole Earth has significant Fe - due to the core

However, outer layers of Earth are much different

Earth’s crust (thin outer layer) mostly Si and O

Earth’s mantle (between core and crust) issimilar to the crust, but with lower Si, and higher Fe and Mg

Introduction to Plate Tectonics

• Plate Tectonics: Structure of Earth’s surface is largely caused by the formation, movement, and destruction of large rigid plates…

• Major conclusions of Plate Tectonics:– The lithosphere (outermost shell of Earth) is composed of 13 or

more large rigid plates and numerous smaller ones

– The plates move with respect to one another and thus continents are mobile (imbedded in plates)

– Continents are relatively old, ocean basins relatively young

– Geologic activity (earthquakes, volcanoes) is concentrated along the boundaries between plates

January 20, 2011 – Earthquakes in the past 5 yearsfrom www.iris.edu

Earthquakes mark outline of Earth’s tectonic plates.

Known volcanoes of the world – do the locations look familiar?from the Smithsonian Global Volcanism Project

Note that earthquakesand volcanoes generallyoccur in the same locations.

Where are Earths largemountains found?

Are all of these generallyfound in the same places?

Earth’s outermost layer comprises plates which move relative to each other.These movements are now measured by GPS and VLBA techniques.

The Theory of Plate TectonicsThe Theory of Plate Tectonics

Earth’s outer layer broken up into 13 major tectonic plates which are made of the crust and uppermost mantle beneath.

Plates may contain oceanic or continental crust or both

contain both continental

and oceanic crust

Some plates

Others are mainly oceanic crust

Earth is Composed of Multiple Layers from Core to Crust.Crust and Upper Mantle (Lithosphere) = Locked Together as Rigid Plate.

In terms of overall radius of Earth the plates are only 1-2%.

The lithosphere is cold, rigid and solid.What about the asthenospheric mantle beneath?

The rigid lithosphere slides on the ductile asthenosphere, which is partially molten.

LithosphereLithosphere

AsthenosphereAsthenosphere

Cold, rigid

Hot, ductile

Continental crust(mostly granite)

Oceanic crust (mostly basalt)M

an

tle(m

ostly

oliv

ine

)

Lithosphere and asthenosphere refer to the strength (Is it hot, or is it cold? Is it rigid, or does it flow like toothpaste?)

Lithosphere : Strong, rigid, cold outer shell of rock which includes the crust and part of the upper mantle.

Asthenosphere: The hotter, weak, ductile layer of solid rock below the lithosphere that flows plastically. Analogy – cold toothpaste.

Crust, mantle, and core refer to composition (what is it made of?).

Crust: mostly granite on continents mostly basalt on oceans (we will talk

aboutMantle: made mostly of the mineral olivine these later…)Core: mostly iron and some nickel

3 Types of Plate Boundaries• divergent• convergent• transform (strike-slip)

transform divergent convergent

Divergent plate boundary• plates move apart• new lithosphere created (oceanic)• volcanism and earthquakes

convergent plate boundary• plates move towards one another• lithosphere destroyed (oceanic)• volcanism, earthquakes,

mountain belts

Convergent Plate Margins

Ocean-Ocean

Ocean-Continent

Continent-Continent

Lithosphere created at divergent plate boundariesis destroyed at convergent plate boundaries.

Motion at Plate Boundaries

Hotspot volcanoes are created where a plume of bouyant, hot mantle rises.

Hot Spot Volcano Tracks

What Forces Drive Plate Tectonics?

Early Earth wasmostly moltendue to:

1) Impact events2) Gravity3) Radioactivity

Earth’s internal heat is still escaping today and is most obviously expressed in volcanic eruptions.

What role does Earth’s internal heat play in the operation of plate tectonics?

Three modes of heat transfer.

Only convection causes motion.

How does convection work?

• Within Earth’s interior: - Cold dense rock

sinks in subduction zones.

- Hot, ductile mantle inside rises and convection occurs.

Fig 1.15c

Plate tectonics is caused by convection in the mantle.

In detail there are other driving forces, we willdiscuss these later in the semester…..

Plate Tectonics provides a comprehensive explanation for all of

the major features of the Earth that we can observe.

earthquakes

volcanoes

deep ocean trenches

Island chain from hot spots

Fig 1.10

Plate velocities measured with GPS

Confirms plate tectonic motions beyond reasonable doubt!

Mantle tomography – provides images similar to ultrasound.

Mantle tomography – hot material in red (slower seismic wave velocity), cold material in blue (faster seismic wave velocity).

Earthquake locations shown by white dots.

Clearly shows the subducting oceanic lithosphere (cold) beneath the Japan volcanic arc system (hot).

More detailed image of subduction zone beneath Japan.