6.3 The Plates Move Apart Mrs. Avant

Boundary Types O Divergent: occurs where plates move apart. Most divergent boundaries are found in the ocean. O Convergent: occurs where plates push together O Transform: occurs where plates scrape past each other

Rift Valleys and Mid-Ocean Ridges O Divergent boundaries = spreading centers O Mid-ocean ridges show where the ocean floor is spreading. As ridges widen, they create gaps called rift valleys. O Most mid-ocean ridges contain a rift valley in the center.

Sea-Floor Rock and Magnetic Reversal O The seafloor is younger near a mid-ocean ridge and older farther away. O Scientist made a discovery about the Earth’s magnetic field, while studying the seafloor. Scientist think the Earth’s magnetic field has to do with convection currents in the mantle. O The Earth’s magnetic poles switch places every so often. This switch is called magnetic reversal.

Magnetic Reversal O Scientist found that magnetic reversal is recorded in seafloor rock. O As molten rock rises and cools, some magnetic minerals line up with the Earth’s magnetic field. O When the magnetic field reverses, the cooling records the change. O Bill Nye “Magnetic Reversal” Bill Nye “Magnetic Reversal”

Continents split apart at divergent boundaries O Continents and the sea floor spread apart at divergent boundaries. O The boundary forms when hot material rises from deep in the mantle. This heat causes the crust to bulge upward. O The crust begins to crack and rift valleys form. O Magma rises through the cracked, thin crust, forming volcanoes.

Continents can be used to track plate movements O Places called hot spots, heated rock rises in plumes, or thin columns, from the mantle. O This can cause a volcano to rise and form at the surface of a plate.

Ch. 6 Section 4: Convergent and Transform O At convergent boundaries, where plates push together, crust is destroyed. O When one plate sinks beneath another, it is called subduction. O There are three types of convergent boundaries: continental-continental collision, oceanic-oceanic subduction, and oceanic-continental subduction

Convergent Boundaries O Continental- Continental Collision: Two plates carrying continental crust push together. O Oceanic- Oceanic subduction: one plate with oceanic crust sinks, under another plate with oceanic crust. O Oceanic-Continental subduction: ocean crust sinks under continental crust.

Transform Boundaries O At these boundaries, land is neither created nor destroyed. O Transform boundaries occur mostly on the sea floor near mid- ocean ridges, but they can also occur on land.