5. Seafloor Spreading - Background

Teacher Background
Seafloor spreading is one of the most critical pieces of evidence in the development and support of the theory of plate tectonics. It is through this process that new oceanic crust is formed along the mid-ocean ridges as oceanic plates diverge and separate. Magma wells up into the gap, hardens, and forms new crust. As the plates continue to separate, these newly formed pieces of oceanic crust separate and make room, gradually moving outward away from the mid-ocean ridges at the rate of a few centimeters per year.

Subduction zone diagram: Image courtesy of NASA.Subduction zone diagram: Image courtesy of NASA.After several million years of this slow journey away from the mid-ocean ridges, the oceanic crust collides with a different tectonic plate in a process known as subduction. If the oceanic crust meets continental crust, the denser oceanic crust is forced under the continental crust. If the oceanic crust meets oceanic crust, then one or the other will be forced below the other. In these subduction zones, earthquakes are common due to the build up and sudden release of energy at the junction. Also, as the oceanic crust dives down below the other plate into the mantle, the old plate melts, forming a pool of magma that is forced back up through the crust above as volcanoes on the continent or a chain of volcanic islands on one side of the subduction zone.

In some instances, some continental crust is riding along on the oceanic plate and collides into another piece of continental crust. This is currently taking place in the Himalayas and was part of the process that created California (see the background section of Making California for more details). At these continent-continent convergent boundaries, the two sections of continental crust ram into one another, causing the crust above to buckle and fold into tall mountain ranges (the Himalayas and the Sierra Nevadas).

Using these patterns, students can infer both the direction and the relative speed each tectonic plate is moving. The following table shows all the plates and their approximate direction and speed.

 Tectonic Plate  Approximate direction  Approximate velocity (cm/year) 
 African  W  1.62
 Antarctic  SW  2.21
 Arabian  NW  2.24
 Australian  N  5.81
 Caribbean  W  3.03
 Cocos  NE  11.57
 Eurasian  W  2.52
 Indian  N  2.59
 Juan de Fuca  NE  10.45
 Nazca  E  3.59
 North American  W  4.11
 Pacific  NW  11.78
 Phillippine  NW  10.57
 Scotia  W  4.89
 South American  W  4.83

Age of the seafloor: Red indicates newly formed crust. Blue indicates oldest oceanic crust. Image courtesy of National Oceanic and Atmospheric Administration.Age of the seafloor: Red indicates newly formed crust. Blue indicates oldest oceanic crust. Image courtesy of National Oceanic and Atmospheric Administration.This table was created using the Rice University Plate Motion Calculator. For further information on the types of plate boundaries, see the background section of the Plate Patterns lesson.

The ultimate cause of sea floor spreading (and the theory of plate tectonics as a whole) is still debated in the scientific community. Some argue that it is driven by convection currents in the mantle. Others argue that the upwelling of magma and creation of new crust at mid-ocean ridges pushes the older crust out of the way (the “Ridge Push” theory). Others argue that the sinking of the old crust at subduction zones drags the oceanic crust along behind it (the “Slab Pull” theory).

Student Prerequisites
Students should have created or studied a map with data about earthquake, volcano and mid-ocean ridge locations. Students should know that the Earth’s crust is divided into large plates the size of continents or oceans. Students should know about convection currents in the Earth’s mantle and should understand how those could affect the motion of the tectonic plates above.