In addition to paleomagnetism, what other evidence was needed to determine the rate of seafloor spreading?
A the strength of the magnetic field
B the age of each strip of rock
C minerals in the rocks
D the thickness of the rock layers
What causes the earth’s magnetic field?
A currents in the earth’s liquid-metal outer core
B magnetic particles located at the earth’s poles
C molten material in the mantle
D the mineral composition of the earth’s crust
