The Iron Catastrophe

The "iron catastrophe" – perhaps the most significant single event in Earth history – occurred when the temperature of the planet reached, and passed, the melting point of iron (1538°C). Of course no one was there to observe this amazing event, but based on laboratory experiments, mathematical calculations, and physical evidence of Earth’s internal chemistry scientists hypothesize that it was this event, known informally as the “iron catastrophe,” that organized Earth with the internal layers that characterize the modern planet.

When the melting point was reached, droplets of liquid iron, one of Earth’s most abundant elements (about 35% overall), flowed (slowly at first) toward the planet interior under the pull of gravity. Compounds of lighter elements rich in silicon (Si), oxygen (O), and other light elements that were also molten (i.e., Al, Na, Ca, K) were displaced toward the surface. Ultimately this led to the present internal structure of Earth in a process called chemical differentiation..

Figure 1.“Iron Catastrophe.” (a) Primitive Earth was an undifferentiated mass of solid rock and heating raised the temperature to the melting point of iron. The “iron catastrophe” occurred - liquid iron flowed inward to form the core complex and (b) displaced lighter compounds rich in silicon, oxygen, and other elements outward to form the mantle and crust. As the first great tide of molten iron moved slowly through Earth, friction from the movement created heat that further raised the temperature another 2000C. This had a dramatic effect and caused the planet surface to develop a magma ocean. Later, as Earth cooled, this molten rock solidified to form the first solid crust and mantle. We can date the “molten period” from two observations: 1) the oldest meteorites and lunar rocks are about 4.4 - 4.6 billion years old, and 2) the oldest known Earth rocks are about 3.8 - 4.1 billion years old (although mineral grains from Australia date to 4.4 billion years old). The difference in these two ages, roughly half a billion years, may represent the period when Earth was molten. As Earth cooled and the rate of bombardment waned, the surface changed from liquid rock to solid volcanic crust. This was an important time in Earth history because once the solid crust formed and Earth’s surface cooled, life made its presence known, we call this next period of time the Archean Era (beginning 3.8 billion years ago).

Figure 3. Molten Earth. During the Hadean Era, Earth’s surface became molten. Melting may have extended to a depth of several kilometers. During the “iron catastrophe” about one third of the primitive planet's mass sank to the center, and in the upheaval, heating rates increased and much of the planet turned to magma. During this process, Earth was converted from a homogenous body, with roughly the same kind of material at all depths, to a layered (heterogeneous) body, with a dense iron core, a brittle crust composed of lighter materials possessing relatively lower melting points, and between them the solid mantle, of medium density.

Figure 3. The “iron catastrophe” led to the formation of a dense metallic core made mostly of iron, a low density crust (of easily-melted compounds made of silicon, oxygen, and various metals such as aluminum, potassium, sodium, and calcium), and a medium density mantle. Why would molten iron shape Earth’s entire structure? Because, during the “iron catastrophe” iron flowed toward the center of the planet and displaced lighter elements toward the surface, causing chemical differentiation. Among the lighter elements, most important were oxygen (O) and silicon (Si) that are significant components of rocks composing the continents. The denser elements – mostly iron - accumulated as a massive nucleus in Earth’s center. They formed a core complex with a solid inner layer and a liquid outer layer, containing about one-third of Earth’s mass.

Compare the abundance of elements in the crust with the values for Earth as a whole (Figure 4). Because most of the iron sank to the core, that element drops to fourth place in the crust. Conversely, silicon, aluminum, calcium, potassium, and sodium are far more abundant in the crust than in the whole Earth. The reason for the different make up is that elements favored in the crust form light-weight chemical compounds that are easily melted. Materials such as these melted early during the differentiation, rose to the surface by convective overturning and accumulated.

Figure 4- Earth Chemistry. Compared to the whole Earth, the crust is relatively enriched with the lighter elements oxygen (O) and silicon (Si) and depleted of the heavier elements iron (Fe) and magnesium (Mg).

To learn more, go to The Structure of the Earth, or return to either Origin of the Earth, the Earth-Science Introduction or the Earth Introduction.

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