Changes Within the Earth

 The earth’s outer shell is not one solid sheet of rock  The lithosphere and mantle are broken into a number of moving plates  These plates are not anchored in place, but slide over a hot and flexible layer of the mantle

 Alfred Wegener proposed that there was once a single continent “Pangaea”  180 million years ago, Pangaea began to break into separate continents  He used fossils from around the world to prove his theory  Many scientists still remain unconvinced of this theory

Pangaea

 The ocean floor is not flat  Scientists found many different landforms on the ocean’s floor  The rocks from the ocean floor were much younger than the rocks on the continents  Theory: Molten rock from the mantle rises beneath the underwater ridge and breaks through a split (rift valley)  The rock then spreads in both directions

 “How do the continents move?”  “What force is powerful enough to send gigantic plates sliding around the earth?”  Today, scientists believe it is convection

 When plates pull away from each other, they form a verging plate boundary, or spreading zone.  Examples: rift valley, earthquakes, volcanic action

 When an oceanic plate meets a continental plate, it slides beneath the lighter plate down toward the mantle  Oceanic plates are denser than continental plates  Examples: volcanoes which will form mountains

 When two continental plates collide, neither will sink  Buckling, folding, and faulting of rocks pressed against each other will produce mountain ranges  Example: Himalayas

 When two plates slip or grind past each other  Example: San Andreas Fault, CA