Plate Tectonics The Grand Unifying Theory of Geology Earth s lithosphere is divided into a number of pieces, called plates These plates of rigid lithosphere ride over the softer, ductile asthenosphere Motion of these plates cause nearly all geologic features we observe on the earth s surface
Plate Tectonics Plate tectonic theory is powerful. It provides a unified mechanism explaining: Igneous, sedimentary, and metamorphic rocks. The distribution of earthquakes and volcanoes. The origin of continents and ocean basins. The distribution of fossil plants and animals. The genesis and destruction of mountain chains. Continental drift.
Elevations and Lithospheric Loading The weight of the lithosphere causes depression of the asthenosphere and bending in the lithosphere
Buoyancy First described by Archimedes more than 2.2 ka. Floating solids displace water equal to their mass. An iceberg sinks until the mass of water it displaces is equal to the total mass of the iceberg. This concept applies to lithospheric plates. Continental Floats higher. Oceanic Sinks lower.
Why Are Continents Higher? Continental Crust - granite density ~2.6 g/cm 3 Oceanic Crust basalt density ~3.0 g/cm 3 :: Review Hypsograph Exercise ::
The Major Plates of Earth s Lithosphere - Most plates contain both oceanic and continental crust. A few contain only oceanic crust. Thus a plate a continent - Not all boundaries between continents and oceans are plate boundaries
Plate boundaries are zones of many earthquakes - Earthquakes clearly outline major plate boundaries. - Thus active continental margins ARE plate margins. Passive continental margins are NOT plate margins. - So what happens at passive margins?
Active & Passive Margins Margin: boundary between a continent and an ocean Active Margin: A continental shoreline that IS a plate boundary Passive Margin: A continental shoreline that IS NOT a plate boundary Continental Shelf: Thick sediment cover, gentle slope, shallow to moderate depth Abyssal Plain: nearly flat in slope, deep depth, overlies oceanic crust Cross-section of a typical passive margin
The Major Plates of Earth s Lithosphere - Geologists define only three types of plate boundaries 1- Divergent 2- Convergent 3- Transform
Type 1 - Divergent Plate Boundary Divergent - Two plates that pull away or separate from each other. Produce new crust Examples: Mid Atlantic Ridge, East Pacific Rise Effect of movement Sea Floor Spreading :: Divergent Animation ::
Type 2 - Convergent Plate Boundary Convergent - Two plates that move towards or collide with each other. Consume old crust Example, India into Asia, NW Coast of U.S. SW coast of South America Effect of movement Subduction or Collision :: Convergent Animation ::
Type 3 - Transform Plate Boundary Transform - Two plates that slide horizontally past each other. Do NOT consume or create crust Example, North Anatolian Fault (Turkey), Dead Sea Fault (Israel, Jordan), San Andreas Fault Effect of movement Transform Faulting :: Transform Animation ::
Divergent Boundaries Sea-floor spreading causes plates to move apart. Magma wells up to fill the gap. Magma cools, adding material to each plate.
Divergent Plate Boundaries: Sea Floor Spreading
New Oceanic Crust at Mid-Ocean Ridges Intrudes from Dikes Dike: A steep (~vertical) cracks filled with magma
Mid-Ocean Ridges Linear mountain ranges in Earth s ocean basins. Example: The Mid-Atlantic Ridge Snakes N-S through the entire Atlantic Ocean. Elevated ridge 1,500 km wide and 2 km above abyssal plains.
Why Are Mid-Ocean Ridges Elevated? Mid-ocean ridges are high above most of the seafloor This is because new oceanic crust is warmer and less dense, and very little lithosphere has been added, close to the ridges. Further from the ridges, the crust is cool and more dense, and the thicker lithosphere adds even more weight, pulling the whole lithosphere down into the asthenosphere
Mid-Ocean Ridge Activity: Black Smokers & Pillow Basalts Magma heats water and it jets out of these chimney-like vents. The water is black because of dark colored minerals. Strange critters live here! When lava erupts underwater, instead of making coherent flows, it forms rounded bloblike pillows. Commonly called pillow basalts
Pillows In The Field Pillow Pillow Pillow Pillow Pillow www.erictwelker.com/greenscreek.htm
Age of the Sea Floor & Divergent Plate Boundaries
Convergent Plate Boundaries: Subduction When two plates collide (if one is oceanic), one oceanic plate bends and sinks downward into the asthenosphere. This is called subduction. Why does the oceanic plate sink? Because oceanic lithosphere is more dense than the asthenosphere it wants to sink. Why is it more dense? Its colder! Slab sinking rates range 10-15 cm/yr.
Features of Convergent Plate Boundaries: Subduction (Subduction animation)
Convergent Boundaries Accretionary prisms Deformed sediment wedges. Sediments scraped off subducting plates are smeared and welded onto the overriding plates. These contorted sediments can be pushed above sea. Washington s Olympic Peninsula. Taiwan.
Convergent Boundaries Volcanic arc Volcanic belt on an overriding plate. The descending plate contains fluids, squeezed out at ~ 150 km depth. Causes the overlying plate to partially melt. Magmas burn upward, fueling volcanic eruptions. A curved Earth dictates that volcanic belts are curved. Arc type depends on overriding plate. Continental crust Continental arc. Oceanic Island arc.
The Wadati-Benioff Zone How do we know where the subducting plate is? Mega Thrust Earthquakes occur along the interface between the upper and lower plate The pattern of earthquakes outlines the location and shape of the lower plate The deepest and largest earthquakes in the world occur at subduction zones What is the fate of the sinking plate?
The current hypothesis about the fate of subducted plates suggests that they may sink all the way to the lower mantle near the core/mantle boundary Geophysical techniques and models may help figure this out Subducted Plate Graveyard
Subduction and Island Arcs Island Arc: Chain of volcanic islands that form behind a subduction zone. Island arcs may eventually get swallowed by the subduction zone. What happens when a continent collides into another continent? What happens when the subducting oceanic plate runs out?
Continent-Continent Collision Eventually, a subduction zone may swallow all of the dense oceanic crust Two plates of continental material (?) cannot subduct because they are not dense enough. (collision animation)
Transform Boundaries Lithosphere slides past; not created or destroyed. Most transforms offset spreading ridge segments. Some transforms cut through continental crust. Characterized by Earthquakes. Absence of volcanism.
Oceanic Transforms The MOR axis is offset by transform faults. Offset of linear MOR is geometric necessity on a sphere. Transforms provide strong evidence of sea-floor spreading. Abundant earthquakes common between ridge segments. Earthquakes vanish past ridge segment overlaps.
Transform Plate Boundaries No Net Loss or Gain of Lithosphere Transforms indicate where plates slide past each other; Most are fairly short Most occur as fracture zones adjacent to mid-ocean ridges Oceanic transforms were first properly described recognized by J.T. Wilson Incorrect! Transform Animation
Continental Transform Plate Boundaries Some transform boundaries occur on the continents San Andreas Fault is a transform The San Andreas Transform moves with a right-lateral sense of motion (paper demo) Accommodates motion between the spreading center in Baja in the south, and in the north the SAF ends at a triple junction (So what is a triple junction?)
SAF Aerial View
Triple Junctions Triple Junction Where three plates meet. Named by the three types of boundaries that meet at the junction, e.g. ridgeridge-ridge, or transform-transformtrench
Hot Spots Not all volcanoes on Earth are related to plate boundaries Some occur at hot spots; locations where hot molten rock rises through tectonic plates Hot spots arise from hot mantle plumes that are stationary through time and buoyantly rise to through the crust, e.g. Hawaii, and Yellowstone (hot spot animation)
Hawaiian Hot Spot The islands of Hawaii exist because of a hot spot The hot spot is currently under the big island of Hawaii The pacific plate is moving to the NW So, Oahu (where Honolulu is) is older not currently active Highest volcanoes are on Hawaii and the islands get more eroded (because they are older) to the NW So, the orientation of the chain of islands can tell us what the plate motion is!
Hawaii Hot Spot Track The orientation of the Hawaiian archipelago changed at 40Ma This tells geologists that the motion of the pacific plate changed at 40Ma.
Other Hot Spot Tracks There are lots of hot spots on Earth Iceland is unique: A hot spot and a divergent plate boundary all above sea level.
The Birth of a Divergent boundary How are divergent boundaries formed?...rifting If continental material (granite) is being rifted, then it is sometimes called continental rifting Stretching causes thinning of the crust (think of taffy or silly putty) (rifting animation)
Continental Rifting Today? Eastern Africa is currently being rifted apart. Called the East African Rift Associated Earthquakes, Volcanism, and Faulting.
Rifting in the U.S.? The western U.S. most of Nevada, part of Utah, California, Arizona, Oregon, and Idaho are being stretched apart Called the Basin and Range province Maybe this will evolve into a rift.
Death of a Subduction Zone Eventually, a subduction zone may swallow all of the dense oceanic crust Two plates of continental material (?) cannot subduct because they are not dense enough. (collision animation)
What Drives Plate Motions? The old hypothesis was that the asthenosphere contains convection cells, which drive plate motions. This is only partly true These cells bring hot molten material up in some regions (divergent) and pulls cold material down in other regions (subduction) Scientists tested this hypothesis and found that they could not replicate the motions of plates with simple convection cells. So, convective cells probably do exist, but they are not the main driver of plate motions
Ridge-Push and Slab-Pull forces Divergent boundaries are driven by a ridge-push force. Ridge-Push is driven by gravity Slab-Pull force drives subduction (also from gravity) Dense lithosphere sinks and once it starts to sink the density difference pulls the rest of the plate with the sinking slab
Velocities of the Plates Thanks to GPS and other geodetic techniques, we can measure the motions of the Earth s plates on a up to the minute basis. A permanent GPS station
GPS Motions in Los Angeles, CA
Relative Plate Velocities Ignore red arrows! They are absolute motions. Black arrows show relative motions between plates.
Plates and Continents Over Geologic Time The evidence for plate tectonics is overwhelming. The theory of plate tectonics illustrates how the Earth s surface changes through time This theory also highlights the fact that our planet is constantly changing its surface by Addition of molten material Recycling of old oceanic crust