7. what are the two most abundant elements in the earth? describe the major categories of materials constituting the earth. does the crust have the same composition as the whole earth? 8. what are silicate rocks? give four examples of such rocks, and explain how they differ from one another in terms of their chemical composition 9. how did researchers first obtain a realistic estimate of earth's average density? what observations led to the realization that the earth is largely solid and that a particularly dense core lies at the center?