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Earth as a Rock
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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Update: June 2012
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