Seafloor spreading is part of a larger theory called plate tectonics. This theory holds that the lithosphere is composed of rocky, crustal plates that slowly move over the liquid asthenosphere. The theory began life as an idea called continental drift in the early 20th century. The original thought was that continental plates pushed their way through the ocean crust like very slow moving ships. Eventually, we came to discover when taking magnetic readings of the ocean floor that magnetic stripes in the ocean crust due to magnetic polar shifts were mirror images across oceanic ridges where new crust is born. Another advance is that ocean crust was eventually able to be dated, and the ocean crust nearer to oceanic ridges was newer while ocean crust further away was older. This led to the idea of seafloor spreading where land mass and ocean crust were all part of the same series of plates, and new ocean crust was constantly being formed by volcanic activity at ocean ridges and spreading outward.