Encyclopædia Britannica states that many pieces of evidence support the sea floor spreading theory of plate tectonics, including increased heat flow along mid-ocean ridges, geomagnetic anomalies near ocean ridges, thickness of marine sediments and age of sediments. Sea floor spreading was first proposed by geophysicist Harry Hess in 1960 after reviewing submarine data. The theory is the basis for modern plate tectonics that states plates in the Earth's crust move.
Increased heat along the mid-ocean ridge means that there is molten material near the crests of ridges. Some sites showed heat flow up to four times that of normal along the ocean floor at mid-ocean ridges. Thermal expansion and upwelling of magma also indicate abnormally low seismic wave activity along mid-ocean ridges.
Geomagnetic bands alternate polarity along the crust in the Atlantic mid-ocean ridge. One band of crust is polarized in one direction, and the layer right next to it is the exact opposite polarity. PBS states that this polarity indicates the new sea floor crust magnetized differently when it formed.
The age of rocks along mid-ocean ridges in the Atlantic Ocean are between 150 million and 200 million years old. These rocks are younger than those near the coasts of Europe and America. Material gets progressively older the further away it is from the ridge. Newer rock indicates magma pushing up from the ridge, according to CliffsNotes.