MadSci Network: Earth Sciences

Re: How much does the Earth weigh? What does the Lithosphere weigh?

Date: Thu Dec 3 13:44:05 1998
Posted By: David Smith, Faculty Geology, Environmental Science
Area of science: Earth Sciences
ID: 911934466.Es


You have asked two questions that are both easy and difficult to answer. The difficult part is the word "weigh". Scientists make a distinction between an object's weight and its mass. As you may know, the earth pulls down on any object that is near the earth (you and me, for example) We usually call this pull "gravity", though if we were really being careful, we should call it the gravitational pull of the earth on this object, since every object exerts gravitational pull on every other object (that's another whole question). Anyway, this gravitational pull that pulls us down when we fall gives us our weight. Weight, as scientists use it is another word for the force of earth's pull. The mass of an object, on the other hand, is a way to talk about how much stuff is in the object, without worrying about whether the earth is pulling on it or not. You can think of mass as resistance to getting pushed or pulled. For example, it's a lot harder to push a car than it is to push a bicycle, isn't it? That's because a car has more mass. Near the surface of the earth, objects with mass are pulled toward the earth, giving them weight, and it becomes very easy to use the two words interchangeably You can even find cans in the store with the "weight" marked in ounces (which is a unit of weight) and grams (which is a unit of mass, not weight). So what's the problem?

Weight only makes sense near the surface of the earth, and since the earth is not sitting on itself, we can't really talk about the weight of the earth. However, we can use our own weights to figure out how much mass is in the earth because the pull of the earth depends on its mass. So, after all that, I'll answer your questions about weight with answers about mass.

The mass of the earth is given in my Physics book (Physics, by Paul Tippler, published in 1976) as 5.98x10^24 kilograms (10^24 means ten to the 24th power or 10 times itself twenty four times). That's a mass of about 6 million billion billion kilograms. You, on the other hand probably have a mass of less than 50 kilograms. The lithosphere, which is only the thin outer shell of the earth, made up of the lightest parts of the earth has a mass of approximately 1.5x10^23 kilograms, or about 1/40 of the whole earth. I couldn't find this one in a book, so I calculated it, assuming the lithosphere is everywhere 100 kilometers thick and that it has a uniform density of 3 g/cc. Neither assumption is perfect, but each is within about 10% of the real value, so the answer is also within about 10-15% of the true value.

Since the lithosphere is close to the earth's surface we could actually talk about its weight. The lithosphere would weigh about 1.5x10^24 Newtons (that's the metric unit of weight or force) or about 3.3x10^23 pounds (1 Pound = 4.448 Newtons, according to Tippler's book, so 1 Newton is a little less than what a stick of butter weighs).

I hope this was helpful. I'd be delighted to answer more plate tectonics questions if you have them.

David Smith
Associate Professor, Geology and Environmental Science
La Salle University, Philadelphia, PA

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