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Name: Mason
Status: Student
Age: 8
Location: CA 
Country: United States
Date: November 7, 2005


Question:
I am 8 years old, and in the second grade. My question is this: Is the planet Earth a rock? My theory is that the Earth has an outer rock crust and magma inside, that sand and water can rest within the divots of large rocks (oceans and soil/sand on Earth), that Earth may have many living things ON it, but the planet Earth is still a rock.

My mom thinks that classifying a planet as a rock, especially Earth, is too simple. Earth has volcanic, seismic, and atmospheric activity that she does not consider a rock as having.

I have not figured out a way to scientifically answer the question. Nor could we find it addressed on the Internet ­ other than mention of "3rd rock from the sun".



Replies:
I did some research of my own on your question. Two 'Google' searches on the terms: "Earth rocky planet" and "Earth geological structure". There were many 'hits' for both search terms from which I copied two each. My conclusion is that both you and your mother are both right! It is a matter of how planets are classified overall versus how planets are put together.

The planets Mercury, Venus, Earth, and Mars are classified as "rocky planets" to distinguish them from Jupiter, Saturn, etc. which are classified as "giant gaseous planets" because they consist mainly of condensed gases. The "rocky planets" have largely silicate crusts with various layers of solid or molten layers underneath. That is the reason for that distinction. It is not meant to suggest that the Earth (or other rocky planets) are just lumps of stone. The "gaseous planets" tend to be large because there needs to be sufficient gravitational attraction to keep them from evaporating. They also tend to be more distant from their parent stars too, because the heat from the parent star would cause the volatile gases to evaporate. The "rocky planets", composed of less volatile substances tend to be closer to their parent star. From some recent observations there is some overlap here for reasons that are not well understood.

Your description of the Earth's various layers as being complex structures interacting with one another (for example, Tectonic plates, volcanoes, magma, metal core, and so on) is also correct. But that describes how the Earth (or other "rocky planets") are put together. They are not all put together the same way for a variety of reasons, such as the distance from the Sun, the composition of the atmosphere, larger or smaller magnetic fields, and so on.

So both descriptions apply, but they distinguish the composition/structure of the planets from different points of view. Backing away from your question a bit I caution you not to get caught up too tightly in trying to put planets, stars, molecules, animals,..., whatever into "name boxes". Putting something into a "name box" is useful, but it really does not tell us anything new about what we are studying. Too often, even scientists who should know better, get caught up arguing about which "name box" to put something in, when it really does not add anything to our understanding.

"Earth rocky planet"
http://www.space.com/scienceastronomy/050613_super_earth.html
http://www.solstation.com/stars/4planets.htm

"Earth geological structure"
http://pubs.usgs.gov/publications/text/inside.html
http://www.washington.edu/burkemuseum/geo_history_wa/The%20Restless%20Earth %20v.2.0.htm

Vince Calder



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