University of Colorado GEOLOGY 1010

Class Note 18

Earth's Interior

The polar radius of the Earth is about 6357km.

The difference is due to centrifugal flattening.

The Earth is divided into crust, mantle and core.

• The Crust is subdivided into oceanic and continental.
  • The oceanic crust is thin (5-8 km), basaltic (<50% SiO₂), dense (density > 3) and young (<250My).
  • The continental crust is thick (30-60km), granitic (>60% SiO₂), light (density < 2.85), and old (250-3700 Myrs).
  • The lower boundary of the crust is a seismic reflector called the Moho.

• The mantle is solid silicate material and is subdivided into upper mantle, transition zone, and lower mantle. The boundaries are believed to correspond to changes in the crystal structure of olivine.
  • The upper mantle is made of peridotite and eclogite rocks and extends from the Moho to a depth of 400km.
  • The transition zone extends from 400 to 670km depth and is bounded by prominenet seismic reflectors at these depths.
  • The lower mantle extends from 670 to 2900km. The 670km discontinuity is believed to be due to a change in coordination of Si from 4 to 6. The lower mantle is about 50% of the total mass of the planet.

• The core is metallic iron-nickel and is subdivided into outer and inner core.
  • The outer core is liquid iron-nickel and extends from 2900 to 5150km depth.
  • The inner core is solid iron-nickel and extends from 5150 to 6378km depth.

Much of what we know about the Earth's interior is gained from geophysical evidence, mostly seismic. The different parts of the Earth's interior transmit seismis waves very differently, and the boundaries reflect waves and also partially convert P to S wave energy.

• We measure very precidsely the arrival of different seismic waves from a single earthquake source at different points on the surface.

• We can see P-wave seismic signals reflected from each of the boundaries.

• We see P to S conversions coming from some of the boundaries.

• We see a large S-wave shadow on the opposite side of the Earth from an earthquake so we know that the innermost part of the Earth does not transmit S-waves and is therefore liquid.

We can also measure the Earth's rotation and orbital velocity and gravity field. to give us an estimate of total mass, and, knowing the radius, we obtain a bulk density. Seismic velocities are related to density.

The different types of crust "float" in the dense silicates of the mantle, much like icebergs in the ocean.

The continents, being light and thick, float well above the denser, thinner ocean crust. This elevation equilibrium is called isostacy. It means that continents, and mountain ranges within them, have deep roots of lighter material.

Isostacy also menas that gravity should be equal over the surface of the globe. Gravity is measured by means of a gravimeter. Gravity anomalies may be observed over areas that are out of equilibrium.

In addition to geophysical constraints, we also have geochemical constraints on the compositions of the Earth's interior.

• The Earth is presumed to have formed from particles that were similar in composition to primitive meteorites. These meteorites are called chondrites.

• The Earth's upper mantle must be of a composition that can produce basalt by partial melting. We can measure the major and trace element compositions of basalts and try to reproduce the melting process in the lab.

• Many basalts and other volcanic rocks bring to the surface unmelted inclusions called xenoliths, from their source regions.

• We can reproduces temperature and pressures of the Earth's interior in the laboratory. We can see what minerals occur at the various depths and measure their physical properties such as density and seismic velocity.

Class Note 19

Class Note 17

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