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Physics LibreTexts

9.11 Differentiation

Without differentiation, all rocks on the Earth, Moon, and other rocky worlds would be one uniform material. Instead, differentiation produces the characteristic triple layering inside terrestrial planets: a very dense metallic core of typically iron and nickel at the center surrounded by a mantle of dense rock and a thin surface crust of lighter rock. Gravity is the force that separated these materials.

Differentiation began when the worlds were forming and the Solar System was still a violent place. The Solar System was full of rocky debris. When these chunks slammed into the Earth their kinetic energy was converted into thermal energy. This energy combined with internal heating from radioactive elements was enough to melt the young planet, or at least turn it into a very soft solid that could flow like putty. Dense materials such as metals tended to sink to the center of this non-solid material, while less dense rock-forming minerals floated at the surface. This process of separating materials by their density is called differentiation.

If you want an everyday example of differentiation, observe a well-mixed jar of oil and water. This experiment works best if you use syrupy oil with some color to it like olive oil. As the oil separates out from the denser water it floats to the surface of the jar. This works with solids too. Fill a jar with a mixture of rice and ball bearings. When you stir the contents of the jar, you’ll see the ball bearings congregate at the bottom, and the less dense rice will “float” to the top.

The smelting of metal ores produces a similar layering. If you examined a metal workers cauldran you’d observe that as fresh rock melts the metals sink to the bottom while lower density materials form the bulk of the intermediate material. On the surface a thin layer of slag or low-density rock forms a crust. In our own Earth, when rock containing iron, for example, melts in the correct circumstances the metal sinks to the core of the Earth. Lower density rock moves into the space left behind by the sinking metals, forming the mantle. On the surface floats slag- like crust. You could picture planets as giant smelting vats. (There is one caveat: certain metals chemically bond to low-density minerals in the crust, so they tended to stay with those minerals as they formed. This explains why we find some iron and other metal ores in the crust, even though most of the Earth’s iron is in the core.)

Evidence for this type of differentiation exists in the surface minerals of many of the rocky worlds. One of the lowest-density and most common minerals to solidify as magma cools is called feldspar. Feldspar is a light-colored silicate mineral containing aluminum and some mixture of sodium, potassium, and calcium. Feldspar solidifies from molten lava and then floats toward the surface. Certain surface features of both the Earth and the Moon are rich in feldspar, concentrated in a rock type called basalt. Basalt is a dark-colored rock, most common in lava that erupts from volcanoes that draw their lava from the crust and upper mantle. The Earth’s ocean floors are made of basalt. We also find basalt on the Moon’s surface. The dark patches that make up the features of the “man in the Moon” are actually huge plains of dark, basaltic lava that erupted about 3.5 billion years ago. The brighter highlands of the Moon are a rock called anorthosite, which is almost completely made of feldspar crystals.

When you hold a hunk of rock in your hand, you don’t usually think of it as a light material. To convince yourself that Earth rocks are mostly light, try holding iron meteorite. Most common surface rocks on Earth have an even lower density than a pure feldspar rock. Granite is a pale quartz-rich silicate rock commonly formed from molten materials in continents. Subduction and melting of tectonic plates concentrate low-density silica-rich quartz minerals near the Earth’s surface. In the presence of water, these minerals melt and re-solidify to form granite. Continents are like low-density granite “scum” floating on the denser rocks of the basaltic lower crust and upper mantle like icebergs floating in the ocean. The Moon has no granites because it lacks water and the plate tectonics process that concentrates silica-rich materials.

For a body to differentiate, it must have a certain amount of internal heat to melt, and a certain amount of gravity to separate the materials. So we can infer the size of a body based on whether or not it has experienced differentiation. The major planets were all large enough to differentiate, as were many of the larger moons. But most meteorites are undifferentiated – they have proportions of the elements approximately equal to the solar system’s bulk composition (except for the gaseous elements which were lost). These meteorites must have originated in smaller bodies that never completely melted. A few asteroids did grow large enough to differentiate. One of these is the asteroid Vesta, which has basaltic lava on its surface.