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Name: Donna 
Status: Educator
Age: 40s
Location: N/A
Country: N/A
Date: February 15, 2004

I have a question regarding the earth....crust or ??
I have discovered the 3-4 layers of the earth, but am onfused about some things..... I thought there were layers such as:

Top soil
Etc..... (not in this order)

Under the topsoil that we this not true?

Hi Donna!

At the present development the zonal structure of the plabet Earth is considered to be divided into 3 principal regions:

crust, mantle and core.

The crust has a variable thickness, being 35-70 km thick in the continents and 5-10 km thick in the ocean basins. The crust is composed mainly of alumino-silicates.

1)The crust is the upper region and is about 40 kms. (25 miles) thick in continental regions, beeing highly heterogeneous, composed mainly of rocks of variable composition between granite and basalt. Covered by the oceans, the crust is about 5-10 km. ( 3-6 miles) thick( in the average) and believed to be composed mainly by alumino-silicates.
2) Below the crust is the mantle, that goes down for 2,900 km (1,800 miles). The mantle is solid. It is believed to be composed mainly by magnesium silicates with lesser amounts of iron, calcium and aluminum silicates and divided into layers characterized by different types of minerals.
3) Further below is the core that is believed to be mainly composed of metallic iron, nickel and some sulfur all in the molten state. At about 5,080 km (3,160 miles) there is the inner core composed mainly also by iron. This iron is under a very high pressure so it is in solidified.

And...thanks for asking NEWTON!

(Dr. Mabel Rodrigues)

The items listed in the question are all relatively near the surface and components of the crust. However, the sequence and composition of layers near the surface is strongly dependent on local geology. For instance, oil is only found in certain places. In other places, the crust is very different. I suggest you contact the geological survey office for your state for information on your local conditions and a basic text on geology and earth science for definitions of the items you have listed.

You can find links to all the state geological surveys in the US at the Association of American State Geologists, which is hosted by the University of Kansas. Their link is:

Andy Johnson

The Earth does have four distinct layers---inner and outer cores, mantle and crust (there are some other sub-layers that geologists talk about, but for our purposes, we'll stick to the big ones). The Earth is huge, and the crust is like the skin of an orange in comparison--not very thick compared to the overall size. All of the activity that you mentioned occurs in and on the crust. There are many layers in the crust resulting from many geologic processes. Some of these layers are rocks (igneous, metamorphic or sedimentary) or some transition stage (for example rocks and sand) in the rock cycle. Sometimes economically important materials like oil are trapped in rocks. You don't say where you are, but I am in the Chicago area, and if I dug a hole, I would go through soil, glacial materials, then different layers of rock (for example, limestone which was deposited long ago when this area was covered by a shallow sea). If I could keep going, I would go into the mantle, the outer core and inner core. It is difficult to grasp the scale and enormity of the Earth, and your struggle with it is common. It is hard to look at things like the Grand Canyon and really understand that it is a fraction of the depth of the Earth itself.

Pat Rowe

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