The Changing Earth

Chapter Eleven: Plate Tectonics 11.1 Pangaea 11.2 Sea Floor Spreading 11.3 Plate Boundaries

11.3 Plate boundaries Divergent boundaries (the trailing edge of a plate) = plates moving away from each other (this type of boundary is found over the rising part of a convection cell) Convergent boundaries (the leading edge of a plate) = plates that collide with other plates

11.3 Plate boundaries An edge of a lithospheric plate that slides by another plate is called a transform fault boundary.

11.3 Divergent boundaries Divergent boundaries are found in the ocean as mid-ocean ridges. A divergent boundary is the line between two plates where they are moving apart. This type of boundary is found over the rising plume of a mantle convection cell.

11.3 Divergent boundaries Divergent boundaries can also be found on continents as rift valleys. When a rift valley forms on land, it may eventually split the landmass.

11.3 Convergent boundaries When oceanic plates collide, the “older” plate subducts under the “younger” plate because it is more dense. This forms a valley in the ocean floor called a deep-ocean trench.

11.3 Convergent boundaries What happens if an oceanic plate and a continental plate collide? –The oceanic plate must subduct under the continental plate. A continental plate is simply too buoyant and too thick to subduct under an oceanic plate.

11.3 Mountains and convergent boundaries When an oceanic plate with a continent on it collides with a continental plate; –The oceanic part of the plate subducts under the continental plate, but the continental part of the plate cannot be sucked into the trench because the granite rock continents are made of is less dense. The two continents collide and form a mountain range.

11.3 Mountains and convergent boundaries Mountain ranges are formed when continents collide.

11.3 Transform fault boundaries A good clue for locating transform faults is offsetting. When seen from above, the feature will appear to make a zigzag.

11.3 Slickenslides, evidence of plate boundaries The effect of rock moving against rock is evidence of plate boundaries. (slickensides) The rock surface moving to the right is called slickensides because it is smooth and polished.

11.3 Earth’s lithospheric plates Can you identify the three types of plate boundaries labeled A, B, and C?.

Geology Connection There are many volcanoes on the mainland of the United States. In the recorded history of Earth, more than 500 volcanoes have erupted. Where to go for Volcanoes

Activity Your book will recount part of the plate tectonics story. Use the graphic on this page and others in Chapter 11 to help you outline your story. Make a Plate Tectonics Book