First, watch the video below to see how a new tool called M.A.D. (Magnetic Anomaly Detector) was used to prove sea floor spreading.
Earth's Magnetic Field Because the outer core of Earth is liquid iron that revolves around the solid inner core, a magnetic field is produced that protects us from harmful things coming from space. This magnetic field is what causes a compass to point north and this also causes magnetic minerals in magma to point north as well. It is important to note that Earth's magnetic field flips every 300,000 years or so and this reversal would cause a compass that was pointing north to point south.
Magnetic Striping of the Sea Floor After mid-ocean ridges and trenches were discovered, more evidence was discovered that helped to support the theory that the continents had moved around. These two things were the magnetic striping on the ocean floor and the age of seafloor rocks.
Magnetic striping - Magnetic minerals are found in rocks. When the rocks are in melted form, these minerals will line up with earth's magnetic field. This similar to how iron filing line up when they are next to a magnet as seen in the pictures below. The magnetic minerals in magma will line up either pointing north or pointing south, depending on the magnetic field at the time. But once the magma solidifies, the minerals are locked into place and as such can record the reversals of Earth's magnetic field. The result of this is stripes of magnetic minerals that parallel the mid-ocean ridges.
Age of the seafloor rocks - when scientists measured the age of seafloor rocks, they found that younger rocks are found closer to the mid-ocean ridges, and older rocks are found closer to trenches. These 2 pieces of evidence support the idea that the sea floor is spreading and that our theory of plate tectonics is valid by showing that the sea floor is not all the same age and is constantly being created at mid ocean ridges and destroyed in subduction zones.