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Name: Jillian
Status: Educator
Grade: K-3
Location: LA
Country: United States
Date: March 2009


Question:
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).



Replies:
Planetesimals (young planets) formed by the accretion of nearby, homogeneous, clumps of interstellar dust grains (regarded as homogeneous cold accretion) to form protoplanets.

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


Simplified
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 surface.

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.

Bob Avakian
Oklahoma State University Institute of Technology.
Okmulgee, OK



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