Seafloor SpreadingSeafloor Spreading

•Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart.  As the plates move apart, the rocks break and form a crack between the plates.  Earthquakes occur along the plate boundary.  Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano. •As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates.  As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor.  This chain is called an oceanic ridge.  http://library.thinkquest.org/17457/platetectonics/4.php

• The boundaries where the The boundaries where the platesplates move apart are 'constructive' move apart are 'constructive' because new crust is being formed because new crust is being formed and added to the ocean floor.  The and added to the ocean floor.  The ocean floor gradually extends and ocean floor gradually extends and thus the size of these thus the size of these platesplates increases.  increases. 

• As these As these platesplates get bigger, others get bigger, others become smaller as they melt back become smaller as they melt back into the Earth in the process called into the Earth in the process called subductionsubduction..

http://library.thinkquest.org/17457/platetectonics/4.php

Plate BoundariesPlate Boundaries

•ConvergentConvergent

•DivergentDivergent

•Transform-FaultTransform-Fault

Convergent Convergent BoundariesBoundaries•Boundaries between two Boundaries between two

plates that are plates that are collidingcolliding

•There are 3 types…There are 3 types…

Type 1Type 1•Ocean plateOcean plate colliding with a colliding with a

less dense less dense continental platecontinental plate•Subduction ZoneSubduction Zone: where the : where the

more dense plate slides more dense plate slides under the less dense plateunder the less dense plate

•VOLCANOESVOLCANOES occur at occur at subduction zones subduction zones

• Oceanic-Continental Convergence Oceanic-Continental Convergence When When an oceanic plate pushes into and subducts an oceanic plate pushes into and subducts under a continental plate, the overriding under a continental plate, the overriding continental plate is lifted up and a mountain continental plate is lifted up and a mountain range is created. Even though the oceanic range is created. Even though the oceanic plate as a whole sinks smoothly and plate as a whole sinks smoothly and continuously into the subduction trench, the continuously into the subduction trench, the deepest part of the subducting plate breaks deepest part of the subducting plate breaks into smaller pieces. These smaller pieces into smaller pieces. These smaller pieces become locked in place for long periods of become locked in place for long periods of time before moving suddenly and generating time before moving suddenly and generating large earthquakes. Such earthquakes are large earthquakes. Such earthquakes are often accompanied by uplift of the land by as often accompanied by uplift of the land by as much as a few meters.much as a few meters.Oceanic-Oceanic Oceanic-Oceanic ConvergenceConvergence

Andes Mountains, Andes Mountains, South AmericaSouth America

Type 2Type 2

•Ocean plateOcean plate colliding with colliding with another another ocean plateocean plate

•The more dense plate The more dense plate slides under the less dense slides under the less dense plate creating a plate creating a subduction subduction zone zone called a called a TRENCHTRENCH

• Oceanic-Oceanic ConvergenceOceanic-Oceanic Convergence When two When two oceanic plates converge one is usually oceanic plates converge one is usually subducted under the other and in the subducted under the other and in the process a deep oceanic trench is formed. process a deep oceanic trench is formed. The Marianas Trench, for example, is a deep The Marianas Trench, for example, is a deep trench created as the result of the Phillipine trench created as the result of the Phillipine Plate subducting under the Pacific Plate. Plate subducting under the Pacific Plate.

Oceanic-oceanic plate convergence also Oceanic-oceanic plate convergence also results in the formation of undersea results in the formation of undersea volcanoes. Over millions of years, however, volcanoes. Over millions of years, however, the erupted lava and volcanic debris pile up the erupted lava and volcanic debris pile up on the ocean floor until a submarine on the ocean floor until a submarine volcano rises above sea level to form an volcano rises above sea level to form an island volcano. Such volcanoes are typically island volcano. Such volcanoes are typically strung out in chains called island arcs. strung out in chains called island arcs.

Aleutian Islands, AlaskaAleutian Islands, Alaska

Type 3Type 3

•A A continental platecontinental plate colliding colliding with another with another continental continental plateplate

•Have Collision Zones:Have Collision Zones:–a place where a place where foldedfolded and and thrust faulted mountainsthrust faulted mountains form. form.

• Continental-Continental ConvergenceContinental-Continental Convergence When two continents meet head-on, When two continents meet head-on, neither is subducted because the neither is subducted because the continental rocks are relatively light and, continental rocks are relatively light and, like two colliding icebergs, resist like two colliding icebergs, resist downward motion. Instead, the crust tends downward motion. Instead, the crust tends to buckle and be pushed upward or to buckle and be pushed upward or sideways. The collision of India into Asia sideways. The collision of India into Asia 50 million years ago caused the Eurasian 50 million years ago caused the Eurasian Plate to crumple up and override the Plate to crumple up and override the Indian Plate. After the collision, the slow Indian Plate. After the collision, the slow continuous convergence of the two plates continuous convergence of the two plates over millions of years pushed up the over millions of years pushed up the Himalayas and the Tibetan Plateau to their Himalayas and the Tibetan Plateau to their present heights. Most of this growth present heights. Most of this growth occurred during the past 10 million years. occurred during the past 10 million years.

Divergent BoundariesDivergent Boundaries

•Boundary between two plates Boundary between two plates that are moving apart or that are moving apart or riftingrifting

•RIFTINGRIFTING causes causes SEAFLOOR SEAFLOOR

SPREADINGSPREADING

Features of Features of Divergent Divergent BoundariesBoundaries

•Mid-ocean ridgesMid-ocean ridges

•Rift valleysRift valleys

•Fissure volcanoesFissure volcanoes

Divergent BoundariesDivergent Boundaries

Places where plates are coming Places where plates are coming apart are called divergent apart are called divergent boundaries. When Earth's brittle boundaries. When Earth's brittle surface layer (the lithosphere) is surface layer (the lithosphere) is pulled apart, it typically breaks pulled apart, it typically breaks along parallel faults that tilt slightly along parallel faults that tilt slightly outward from each other. As the outward from each other. As the plates separate along the plates separate along the boundary, the block between the boundary, the block between the faults cracks and drops down into faults cracks and drops down into the soft, plastic interior (the the soft, plastic interior (the asthenosphere).asthenosphere).

The sinking of the block forms a central The sinking of the block forms a central valley called a rift. Magma (liquid rock) valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this seeps upward to fill the cracks. In this way, new crust is formed along the way, new crust is formed along the boundary. Earthquakes occur along the boundary. Earthquakes occur along the faults, and volcanoes form where the faults, and volcanoes form where the magma reaches the surface. Plate magma reaches the surface. Plate separation is a slow process. For separation is a slow process. For example, divergence along the Mid example, divergence along the Mid Atlantic ridge causes the Atlantic Ocean Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimeters per to widen at only about 2 centimeters per year.year.

Transform Fault Transform Fault BoundariesBoundaries

•Boundary between two Boundary between two plates that are plates that are slidingsliding past past each othereach other

•EARTHQUAKES EARTHQUAKES along along faultsfaults

• Transform BoundariesTransform BoundariesPlaces where plates slide past each other Places where plates slide past each other are called transform boundaries. Since the are called transform boundaries. Since the plates on either side of a transform plates on either side of a transform boundary are merely sliding past each boundary are merely sliding past each other and not tearing or crunching each other and not tearing or crunching each other, transform boundaries lack the other, transform boundaries lack the spectacular features found at convergent spectacular features found at convergent and divergent boundaries. Instead, and divergent boundaries. Instead, transform boundaries are marked in some transform boundaries are marked in some places by linear valleys along the places by linear valleys along the boundary where rock has been ground up boundary where rock has been ground up by the sliding. In other places, transform by the sliding. In other places, transform boundaries are marked by features like boundaries are marked by features like stream beds that have been split in half stream beds that have been split in half and the two halves have moved in and the two halves have moved in opposite directions.opposite directions.

• Perhaps the most famous transform Perhaps the most famous transform boundary in the world is the San boundary in the world is the San Andreas fault, shown in the drawing Andreas fault, shown in the drawing above. The slice of California to the above. The slice of California to the west of the fault is slowly moving north west of the fault is slowly moving north relative to the rest of California. Since relative to the rest of California. Since motion along the fault is sideways and motion along the fault is sideways and not vertical, Los Angeles will not crack not vertical, Los Angeles will not crack off and fall into the ocean as popularly off and fall into the ocean as popularly thought, but it will simply creep towards thought, but it will simply creep towards San Francisco at about 6 centimeters San Francisco at about 6 centimeters per year. In about ten million years, the per year. In about ten million years, the two cities will be side by side! two cities will be side by side!

• Although transform boundaries are not Although transform boundaries are not marked by spectacular surface features, marked by spectacular surface features, their sliding motion causes lots of their sliding motion causes lots of earthquakes. The strongest and most earthquakes. The strongest and most famous earthquake along the San Andreas famous earthquake along the San Andreas fault hit San Francisco in 1906. Many fault hit San Francisco in 1906. Many buildings were shaken to pieces by the buildings were shaken to pieces by the quake, and much of the rest of the city was quake, and much of the rest of the city was destroyed by the fires that followed. More destroyed by the fires that followed. More than 600 people died as a result of the than 600 people died as a result of the quake and fires. Recent large quakes along quake and fires. Recent large quakes along the San Andreas include the Imperial Valley the San Andreas include the Imperial Valley quake in 1940 and the Loma Prieta quake quake in 1940 and the Loma Prieta quake in 1989. in 1989.

San Andreas Fault, CASan Andreas Fault, CA

AnimationsAnimations

• How Ocean Crust FormsHow Ocean Crust Forms

• Plate BoundariesPlate Boundaries

• Seafloor SpreadingSeafloor Spreading

• Seafloor SpreadingSeafloor Spreading

AnimationsAnimations

• Seafloor SpreadingSeafloor Spreading

• Seafloor Spreading VideoSeafloor Spreading Video