Plate Tectonics
Dynamic
Surface continually shaped by erosion and mountain building
Erosion takes place when precipitation falls on land, wearing away at rocks and landforms
Mountain building takes place mostly underground, driven by forces deep within the Earth
Three basic layers: core, mantle, crust
Each layer has further subdivisions within it
Core
Inner core is solid (metallic)
Outer core is liquid (molten)
Very hot (contains some heat left from when the earth first formed and some heat generated by ongoing radioactive decay)
This heat escapes outward toward the surface through the mantle
Mantle
Consists of rock that would feel solid if touched but rock that can move and deform
This is behavior that a physicist would describe as elastic – a deformable solid
Under heat/pressure from core, the rock of the mantle can move slowly
Two parts: lower and upper mantle
Upper mantle is the more malleable part (aka asthenosphere)
Crust
Solid brittle rock that comprises Earth’s surface
Very thin relative to the rest of the planet; thin shell of rigid rock around a deformable interior
Two types: continental and ocean crust
Continental crust: relatively thick and made dominantly of rock with high silica silicate minerals (rock often referred to as felsic – high silica rock); comprises continents
Ocean crust: relatively thin and made dominantly of rock with low silica silicate minerals
(rock often referred to as mafic – low silica rock); comprises oceans
Essentially, the continental crust is thick and light; the ocean crust is thin and heavy
Under the crust is a layer of solidified mantle rock
Combined with the crust, this layer comprises the lithosphere
Under the lithosphere is the upper mantle
Not one cohesive shell; broken into pieces called plates
Crustal plates or lithospheric plates
Most larger plates contain both continental and ocean crust
Crustal plates move (mobile crust)
Plate tectonics: the movement of the crustal plates
Many major geological events take place on plate boundaries where they interact with each other
A lot of plate tectonics theory centers around the dynamics of plate boundaries
Three basic kind of plate boundaries
Divergent boundaries: boundaries which occur where plates move away from each other
Convergent boundaries: boundaries which occur where plates move toward each other
Transform boundaries: boundaries which occur where plates move laterally past each other
Plates spread apart (“rift”)
Divergence is driven from below by hot rising rock in the mantle which concentrates in certain areas of the mantle rather than being evenly distributed
In ares of high heat flow, the hot mantle rock rises; the rising rock pushes up on the overlying crust and pushes it apart
When the hot rising rock pushes up under a continent, the continent begins to crack and stretch and thins to create a rift valley – a long, narrow valley
As the spreading continues, the rift valley floods with water and creates a narrow sea
Eventually, the spreading sea opens into a full ocean basin
Most divergent boundaries occur in ocean basins and are marked by mid-ocean ridges
Mid-ocean ridge: a large linear mountain range on the ocean floor
The divergent boundaries form ridges because the ocean crust is being pushed up from underneath and pushed apart
In the center of the ridge is a rift where the two diverging plates spread away from each other at a rate of 4-5cm/year
In the gap between the two spreading plates hot rock from the underlying mantle
rises up and solidifies to form new crust – new ocean crust
As the plates spread, new crust is continuously added to the back side of the plate
All major ocean basins have mid-ocean ridges
Eg. mid-Atlantic ridge that runs north to south for the length of the Atlantic ocean
Plates collide
Subduction: when one plate is pushed downward in a convergence
This is why convergent boundaries are also called subduction zones
New ocean crust is created by divergence and old crust is destroyed by convergence → essentially recycling
Three subtypes of convergent boundaries:
Ocean crust / ocean crust
One plate is subducted under the other plate
Marked by a deep ocean trench
Deep ocean trench: a linear trench significantly deeper than the rest of the ocean floor
As one plate subducts under the other, the subducting plate experiences friction as well as higher temperatures and pressures, so its crust begins to melt
The melting crust produces magma that rises up toward the surface, some of which erupts to form volcanic islands parallel to the deep ocean trench
Volcanic island arc: the chain of volcanic islands parallel to the trench
Eg. volcanic Japanese islands
Ocean crust / continental crust
Also marked by deep ocean trenches
Ocean crust is almost always subducted since it is heavier
Same process for volcano formation may occur, but volcanoes are located on the continent rather than separate islands
The force of the plate collisions also causes the formation of mountains
Continental crust / continental crust
Results in intense mountain building, crustal deformation, and uplift
One plate slides laterally past another
Eg. San Andreas fault of California (Pacific plate/North American plate)
All plate boundaries are sites of crustal movement and geologic activity
One exception to this idea is Hawaii, a mid-plate hot spot
Under the Pacific plate lies a small area of concentrated heat flow in the mantle
When heat rises within the mantle, the mantle’s upper layer begins to rise
Rising hot rock generates magma that rises through the crust and erupts to create a volcano on top of the plate and, as the volcano grows, a volcanic Hawaiian island
The Pacific plate keeps moving, so the island moves with it
Then, a new volcano and a new island form on top of the hot spot and the process continues
The current volcanic activity is happening on the southern end of the big island of Hawaii
Each island gets progressively older northwestward through the island chain, having moved farther from the hotspot over time
Dynamic
Surface continually shaped by erosion and mountain building
Erosion takes place when precipitation falls on land, wearing away at rocks and landforms
Mountain building takes place mostly underground, driven by forces deep within the Earth
Three basic layers: core, mantle, crust
Each layer has further subdivisions within it
Core
Inner core is solid (metallic)
Outer core is liquid (molten)
Very hot (contains some heat left from when the earth first formed and some heat generated by ongoing radioactive decay)
This heat escapes outward toward the surface through the mantle
Mantle
Consists of rock that would feel solid if touched but rock that can move and deform
This is behavior that a physicist would describe as elastic – a deformable solid
Under heat/pressure from core, the rock of the mantle can move slowly
Two parts: lower and upper mantle
Upper mantle is the more malleable part (aka asthenosphere)
Crust
Solid brittle rock that comprises Earth’s surface
Very thin relative to the rest of the planet; thin shell of rigid rock around a deformable interior
Two types: continental and ocean crust
Continental crust: relatively thick and made dominantly of rock with high silica silicate minerals (rock often referred to as felsic – high silica rock); comprises continents
Ocean crust: relatively thin and made dominantly of rock with low silica silicate minerals
(rock often referred to as mafic – low silica rock); comprises oceans
Essentially, the continental crust is thick and light; the ocean crust is thin and heavy
Under the crust is a layer of solidified mantle rock
Combined with the crust, this layer comprises the lithosphere
Under the lithosphere is the upper mantle
Not one cohesive shell; broken into pieces called plates
Crustal plates or lithospheric plates
Most larger plates contain both continental and ocean crust
Crustal plates move (mobile crust)
Plate tectonics: the movement of the crustal plates
Many major geological events take place on plate boundaries where they interact with each other
A lot of plate tectonics theory centers around the dynamics of plate boundaries
Three basic kind of plate boundaries
Divergent boundaries: boundaries which occur where plates move away from each other
Convergent boundaries: boundaries which occur where plates move toward each other
Transform boundaries: boundaries which occur where plates move laterally past each other
Plates spread apart (“rift”)
Divergence is driven from below by hot rising rock in the mantle which concentrates in certain areas of the mantle rather than being evenly distributed
In ares of high heat flow, the hot mantle rock rises; the rising rock pushes up on the overlying crust and pushes it apart
When the hot rising rock pushes up under a continent, the continent begins to crack and stretch and thins to create a rift valley – a long, narrow valley
As the spreading continues, the rift valley floods with water and creates a narrow sea
Eventually, the spreading sea opens into a full ocean basin
Most divergent boundaries occur in ocean basins and are marked by mid-ocean ridges
Mid-ocean ridge: a large linear mountain range on the ocean floor
The divergent boundaries form ridges because the ocean crust is being pushed up from underneath and pushed apart
In the center of the ridge is a rift where the two diverging plates spread away from each other at a rate of 4-5cm/year
In the gap between the two spreading plates hot rock from the underlying mantle
rises up and solidifies to form new crust – new ocean crust
As the plates spread, new crust is continuously added to the back side of the plate
All major ocean basins have mid-ocean ridges
Eg. mid-Atlantic ridge that runs north to south for the length of the Atlantic ocean
Plates collide
Subduction: when one plate is pushed downward in a convergence
This is why convergent boundaries are also called subduction zones
New ocean crust is created by divergence and old crust is destroyed by convergence → essentially recycling
Three subtypes of convergent boundaries:
Ocean crust / ocean crust
One plate is subducted under the other plate
Marked by a deep ocean trench
Deep ocean trench: a linear trench significantly deeper than the rest of the ocean floor
As one plate subducts under the other, the subducting plate experiences friction as well as higher temperatures and pressures, so its crust begins to melt
The melting crust produces magma that rises up toward the surface, some of which erupts to form volcanic islands parallel to the deep ocean trench
Volcanic island arc: the chain of volcanic islands parallel to the trench
Eg. volcanic Japanese islands
Ocean crust / continental crust
Also marked by deep ocean trenches
Ocean crust is almost always subducted since it is heavier
Same process for volcano formation may occur, but volcanoes are located on the continent rather than separate islands
The force of the plate collisions also causes the formation of mountains
Continental crust / continental crust
Results in intense mountain building, crustal deformation, and uplift
One plate slides laterally past another
Eg. San Andreas fault of California (Pacific plate/North American plate)
All plate boundaries are sites of crustal movement and geologic activity
One exception to this idea is Hawaii, a mid-plate hot spot
Under the Pacific plate lies a small area of concentrated heat flow in the mantle
When heat rises within the mantle, the mantle’s upper layer begins to rise
Rising hot rock generates magma that rises through the crust and erupts to create a volcano on top of the plate and, as the volcano grows, a volcanic Hawaiian island
The Pacific plate keeps moving, so the island moves with it
Then, a new volcano and a new island form on top of the hot spot and the process continues
The current volcanic activity is happening on the southern end of the big island of Hawaii
Each island gets progressively older northwestward through the island chain, having moved farther from the hotspot over time
