Metal Inside Earth
Country: United States
Date: March 2009
Hi! One of my kindergarten students wants to know why
metal in found inside the earth. We have researched this a bit
already and are having difficulty coming up with a strong answer
(that we can understand).
Planetesimals (young planets) formed by the accretion of nearby, homogeneous,
clumps of interstellar dust grains (regarded as homogeneous cold accretion) to
The protoplanets formed through the process of homogeneous cold accretion, which
was accompanied by bombardment, gravitational compression, and radiogenic decay -
all of which resulted in a temperature increase of the early Earth. After one billion
years of heating, early Earth heated to the melting point of iron, which resulted in
the iron catastrophe. The iron catastrophe is represented by denser material sinking
toward the center and lighter material moving toward the surface of Earth. This
initiated convection and differentiation (layering) of Earth. Some recent work has
suggested that the theory of the iron catastrophe might not be entirely correct, that
is, we are not sure of the temperatures associated with early Earth.
Convection is the driving force behind plate movements and is thus responsible for most
of the plate movements, earthquakes, and volcanoes that occur on Earth.
The common minerals (silicates) are made of the common elements in Earth's crust (O, Si,
Al, Fe, Ca, Na, K, Mg). These elements are common in the crust because of the
differentiation associated with the iron catastrophe.
Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences
Metals are dense and dense items sink. Try to float a metal ball, or a marble, on water
and it will sink. A ping-pong ball, however, will not sink because it is not dense.
When Earth first formed it got hot, really hot, so hot, that some believe it was hot
enough to melt iron. The dense iron sank toward the center of Earth because gravity
pulls things toward the center. When you fall down, you are really falling toward the
center of Earth.
Most of the dense metals sank toward the center of Earth when the early Earth was really
hot, and the lower density elements rose toward the surface of Earth. Earth is much
cooler now than it was during the Iron Catastrophe when the iron sank; that occurred
about 4,000,000,000 years ago. That is a really long time ago.
The process of some material sinking and other things rising is known as convection. The
process of moving heat and material is similar to a convection oven that is commonly
found in many household kitchens. Convection ovens move heat by means of a fan. Earth’s
mantle convects and moves rock! Dense materials sink, and hot, low density materials
rise. Convection is the driving force behind tectonic plate movements and is also
responsible for earthquakes and volcanoes. Earth’s inner core is iron-rich and is also
responsible for Earth’s magnetic field and the northern lights.
Convection is one way to move heat; can you think of two other ways to move heat?
Thanks for the question,
Dr. Leslie Kanat
Sounds like you have a very bright student, there.
Early on, when Earth was forming it was so hot that everything was melted and liquid.
Even when melted, metals are heavier than sand. The metals sank down and the sand
floated to the top. When the Earth cooled and hardened, the metals ended up under the
Hope this helps. It is very simplified and I have said nothing about how ore deposits
form nor where the metals came from in the beginning. If your student asks more
questions, which I hope they do, please do not hesitate to ask.
Oklahoma State University Institute of Technology.
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