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).