Theory of Plate Tectonics

Although the evidence for continental drift was strong, there was still one big question: What causes the continents to move? The answer wouldn't come until after World War II. Harry Hess, a geology professor, suggested that the seafloor was actually spreading, which caused the continents to move.

Here's how it works:

Parts of the ocean are thin enough that magma erupts onto the sea floor. When enough magma erupts it pushes the plates away from each other. In other parts of the ocean the plates move down into Earth's asthenosphere, which is a layer of magma below the crust. This is the theory of seafloor spreading.

One piece of evidence for seafloor spreading is alternating magnetic patterns. The Earth's magnetic field switches poles every few thousand years. The rocks on the seafloor align themselves with the poles and that pattern alternates over time. These magnetic patterns match each other on the opposite sides of magma eruptions.