# Homework Assignment #5

Name:

Section number

Date Due: Oct. 13

1. Give at least one argument that the Earth's core must be denser than its crust, and is made mostly of iron.

2. The half-life of radioactive potassium (40K) is 1.28 billion years. Suppose you measure the ratio of 40K to its daughter product, 40Ar, in a rock sample, and find the ratio to be 1:3. How old is the rock?

3. The Atlantic Ocean is about 200 million years old. The distance between the east coast of the U.S. and the west coast of Africa is now about 4,550 km. What is the average yearly rate at which the Atlantic Ocean has been widening, in cm/yr?

4. Suppose that a P-wave has a constant velocity of 8 km/sec. Suppose further that about 700 seconds after an earthquake occurs near the surface, a seismometer detects a reflected P-wave. How far below the surface is the core-mantle boundary?

5. What evidence do we have that the lunar maria must have formed after the lunar highlands?

6. The Moon's mass is about 7.3 · 1025 g, and its radius is about 1.7 · 108 cm. Calculate the moon's average density. On this basis, should the moon have a substantial metallic core? Why/why not?