Differentiation of the Earth

1. Differentiation of the Earth • Differentiation is the process by which random chunks of primordial matter were transformed into a body whose interior is divided into concentric layers that differ from one another both physically and chemically. • This occurred early in Earth’s history, when the planet got hot enough to melt.

2. What was the starting point for differentiation? • Heterogeneous/Hot starting model • Initial layering as Earth solidified from gas • Homogeneous/Cold starting model • Little or no initial layering because Earth formed from the agglutination of cold, uniform particles • Neither model seems to work completely

3. When did differentiation happen? • About 4.5 billion years ago • After beginning of Earth’s accretion at 4.56 billion years ago • Before the formation of the Moon’s oldest known rocks, 4.47 billion years ago

5. Sources of heat to melt Earth • Frequent and violent impacts • There was likely one particularly large impact • Moon aggregated from the ejected debris • Earth’s spin axis was tilted • Decay of radioactive elements • This heat generation was greater in the past than today

6. Basic processes of differentiation • In a liquid or soft solid sphere, denser material sinks to the center and less dense material floats to the top. • When rock is partially melted, the melt and the remaining solid generally have different chemistry and density. The melt is usually less dense than the “residue.” The melt is enriched in “incompatible” elements. The residue is enriched in “compatible” elements.

7. Earth’s Core • Iron, nickel, and other heavy elements were the densest material and formed the core. Core radius is 2900 km. • They are about 1/3 of the planet’s mass • Inner core is solid. Inner core radius=1200 km. Inner core is solid because pressure is too great for iron to melt at Earth’s current temperature. • Outer core is liquid. Some of the iron in the outer core is iron sulfide.

8. The Iron, Oxygen, Sulfur, Magnesium, and Silicon story • There were large amounts of these five elements in the early Earth • The fate of the iron was controlled by its affinity for bonding with oxygen and sulfur. • Iron bonds preferentially with sulfur. All available sulfur is consumed. Iron remains. • Oxygen bonds preferentially with magnesium and silicon. This uses up the magnesium and silicon. Oxygen remains. • Iron then combines with oxygen. Oxygen is now used up. Iron remains as elemental iron. • The iron, magnesium, and silicon oxides are light and form the Earth’s crust and mantle. • The iron sufide is dense, but less dense than iron, so it forms the outer part of the core of Earth. • The elemental iron is densest of all, so it forms the inner core of the Earth. • Note: The amount of oxygen in the starting material plays a key role in determining the size of the core of a planet. What does adding oxygen do to the core radius? What does adding sulfur do to the core radius?

9. Earth’s Crust • Lighter rocks floated to the surface of the magma ocean. • The crust is formed of light materials with low melting temperature and is up to 40 km thick. • These are generally compounds of silicon, aluminum, iron, calcium, magnesium, sodium, and potassium, mixed with oxygen. • Fragments of crustal rocks (zircons) of age 4.3-4.4 billion years were found recently in western Australia. If this is confirmed, we can conclude that Earth cooled enough for a solid crust to form only 100 million years after the large impact.

10. Earth’s Mantle • Lies between the crust and the core. • Depth range is 40 km to 2900 km. • The mantle consists of rocks of intermediate density, mostly compounds of oxygen with magnesium, iron, and silicon • New continental crust may be produced during partial melting of mantle material.