Sea-floor Spreading and Changing Earth’s Surface

Sea Floor Spreading  The Mid-Ocean Ridge –Longest chain of mountains in the world –Curves along the ocean floor and extends into all oceans –Split in the middle by a steep valley

Mid-Ocean Ridge

Sea-floor Spreading  American geologist, Harry Hess studied the mid-ocean ridge –Suggested that the ocean floors move like conveyer belts, carrying the continents with them –Movement begins at the mid-ocean ridge

Process of Sea-floor Spreading 1. At the mid-ocean ridge, molten material rises from the mantle and erupts 2. The molten material spreads out, pushing the older rock out on both sides of the ridge 3. The molten material cools and forms a strip of solid rock at the center of the ridge

Sea-floor Spreading  By definition: –The process that continually adds new material to the ocean floor; the sea floor spreads as new crust is added

Evidence for Sea-floor Spreading  Molten Material- rocks found near ridge that can only form when molten material hardens quickly  Magnetic Stripes – rocks on the sea floor demonstrated patterns that represent magnetic reversals  Drilling Samples – showed the youngest rocks were in the center of the ridges and the oldest were farther away

Subduction  However, the ocean floor does not keep getting wider and wider…..  The ocean floor plunges into deep- ocean trenches and back into the mantle in the process called SUBDUCTION.

Subduction  Convection currents push the new crust away from the ridge and toward a trench  The crust cools and becomes more dense the farther it gets from the ridge  Gravity pulls this denser rock down beneath a trench and back into the mantle

Sea-floor Spreading and Subduction

 These processes can change the size and shapes of the oceans  The ocean floor is renewed about every 200 million years  The Pacific Ocean is shrinking because more crust is being subducted than what is being produced  The Atlantic Ocean is expanding because there are fewer trenches

Changing Earth’s Surface  Plate movements can produce changes in Earth’s surface, including…….. –Deformation –Faults –Mountain building –Land subsidence –Volcanoes

Earthquakes  A fault forms where the lithosphere breaks (caused by stress)  Stress builds along a fault storing potential energy  Eventually, the rock suddenly breaks and slides, causing an earthquake  The potential energy changes to kinetic energy as the rock moves

Mountain Building  Plate movement can cause folding or faulting…  Folding- when plates collide they are squeezed together and the layers slowly fold and push upward  Fault-block mountains – blocks of crust slide along normal faults caused by tension

Land Subsidence  Occurs when the land surface sinks  Can be caused by plate movements along divergent boundaries –Leads to rift valleys and ocean basins  Can also occur when uplift raises one part of the crust

Volcanoes  Weak spot in the Earth’s crust where magma comes to the surface  Most occur in belts near plate boundaries  Can form along the mid- ocean ridge, along rift valleys, in subduction zones, or where oceanic plates collide  Hot Spots – an area where magma melts through the crust