Along the center of the mid-ocean ridge is a rift valley that forms when the plates separate.

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2 Newly formed rock from rising magma rises above sea floor and forms mountain ranges known as midocean ridges. Along the center of the mid-ocean ridge is a rift valley that forms when the plates separate. Most found on ocean floor May form where continents are separating (ex. African plate and arabian plate which formed the Arabian Sea

3 As plates pull apart at one plate boundary, they push into neighboring plates at other boundaries Convergent boundaries are boundaries that form where two plates collide. 3 Types Oceanic Continental continental continental oceanic oceanic Oceanic lithosphere collides with continental lithosphere The oceanic lithosphere is subducted under the continental lithosphere due to density and forms deep-ocean trenches. Released fluids cause overlying magma to melt and form magma. The magma rises to the surface and forms volcanic mountains.

4 Two plates of continental lithosphere collide. Neither plate subducts and the colliding edges crumple and thicken, which causes uplift froming large mountains. The Himalaya Mountains fromed in this type of collision.

5 A third type of collision happens between two plates that are made of oceanic lithosphere. One plates subducts under the other plate, and a deep-ocean trench forms. Fluids released from the subducted plate cause mantle rock to melt and form magma. The magma rises to the surface and forms an island arc (a chain of volcanic islands). Japan is an example of an island arc.

6 The boundary at which two plates slide past each other horizontally. They do not slide along smoothly; they scrape and cause sudden spurts of motion felt as earthquakes. They do not produce magma. Example-San Andreas Fault in California is a major transform boundary betwen the North American Plate and the Pacific

7 Mantle Convection Heat generated from the core and radioactivity within the mantle heat the mantle. the less-dense heated material rises through the cooler, denser material around it. As the mantle material moves, it drags the overlying plates along with it.

8 Ridge Push Because mid-ocean ridges lie at a higher elevation than the rest of the ocean floor, gravity causes the ridge to push on the lithosphere that lies farther from the ridge. Slab Pull The Slab pull force is a tectonic plate force due to subduction. Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at trenches

9 The final pieces of evidence for sea floor spreading came from magnetic stripes on the seafloor, together with ocean deep drilling: mapping magnetic anomalies (stripes) on the sea floor Earth's magnetic field reversed throughout its history (see above) ferromagnetic domains within partially molten rock can align with the ambient magnetic field ; when rock cools below Curie temperature (about 500 deg C), the ferromagnetic domains retain direction of magnetization (i.e. they can no longer change) a rock magnetized in the direction of current field enhances the local magnetic field; a rock magnetized in the opposite direction (e.g. magnetized during a reversal) weakens the local magnetic field; the magnetization in the rock is called remnant magnetization magnetometer towed by a ship over the ocean floor records changes in magnetization from which remnant magnetization can be deduced alternating remnant magnetization over time period of several reversal of Earth's magnetic field causes "magnetic stripes" in record stripes are symmetric with respect to mid-ocean ridge axis get age of seafloor from dredging or ocean drilling (see below) conclusion: new crust is formed at MOR that then moves away from MOR as it cools and ages ocean deep drilling drill into oceanic crust and determine age of basement rock oceanic crust away from MOR are older than near MOR no oceanic rock older than 200 Mio yrs (vs. oldest continental rocks are almost 4 Billion years old) determine ages of rocks in drill core and relate ages to magnetic reversals conclusion: oceanic rocks must be consumed somewhere -> subduction zones

10 New lithosphere is formed along mid-ocean ridges the new material pushes the two plates on either side apart -> seafloor spreading near the ridge, the new lithosphere is warm, has low density and is very buoyant then lithosphere cools and density increases; also material "grows" to bottom, so plate is thickening as oceanic plate becomes older, it eventually becomes so dense that it can't float on the asthenospere any longer the plate starts to sink/subduct no oceanic lithosphere is older than 200 Mio yrs a plate usually consists of both continental and oceanic lithosphere only oceanic lithosphere subducts continental lithosphere too buyonant for subduction when two plates with oceanic lithosphere come together, the denser one is forced to sink the subducting part of a plate is also called a slab when all old oceanic lithosphere of a plate is consumed, subduction stops and plate tectonics rearranges

11 lithosphere is broken into several plates (12 major plates) that move about (a few cm per year) new crust is formed at MORs old crust is consumed at subduction zones both MORs and subduction zones are locations of EQs and volcanism in general: EQs and volcanoes mark plate boundaries; i.e. plates interact at boundaries globally, new crust is formed and old crust is consumed at same rate i.e. Earth does not expand "the grand unifying theory of geosciences that explains" movement of continents earthquakes volcanism most major features on Earth's surface, incl. mountain building formation of new lithosphere consumption of old lithosphere