Plate Boundaries
When two tectonic plates meet, they form a plate
boundary.
The areas adjacent (next to) these plate boundaries are called plate margins.
types of Plate Boundaries
- Convergent – also known as destructive margins
- Divergent – also known as constructive margins
- Conservative – also known as transform margins
Divergent
At a divergent plate boundary (constructive p.m), two plates are moving apart (diverging) – leading to the formation of new crust.
In oceans, this divergence forms mid-ocean ridges and on continents it forms rift valleys.
Mid-Ocean Ridges (Divergent) example
Mid-ocean ridges of underwater mountains can extend for 60,000km across the ocean’s floor.
The mid-Atlantic ridge is an example of a divergent plate boundary with oceanic crusts- 25,000 miles long
Tectonic Hazards at Mid- Ocean Ridges at volcanoes
Submarine volcanoes can occur, some which grow above sea level to create new islands. E.g. Iceland on the mid- Atlantic ridge. These are generally less explosive and more effusive, especially when they occur underwater.
Tectonic Hazards at Mid- Ocean Ridges at earthquakes
Shallow-focus earthquakes (less than 70km into the crust) occur frequently, but they pose little threat to humans as they are small and underwater.
Effusive:
a type of eruption in which lava steadily flows out of a volcano
Rift Valleys (Divergent)
When continental plates move apart, the crust stretches and breaks into sets of parallel cracks (faults).
The land between these faults then collapses, forming steep-sided valleys called rift valleys.
Rift Valleys (Divergent) example
The east-African rift valley is an example of a divergent plate boundary with continental crusts.
Tectonic Hazards at Rift Valleys
Earthquakes:
Similar to mid-ocean ridges – shallow and low magnitude.
Tectonic Hazards at Rift Valleys
Volcanoes:
Yes! The thinning crust allows magma to rise
The Subduction Zone:
Broad areas where two plates are moving together. Often the thinner, more dense oceanic plate descends beneath the continental plate.
The Locked Fault
In a subduction zone, as plates move together they can get stuck due to frictional resistance. Such faults may store strain for extended periods, that is eventually released in a large magnitude earthquake.
The Benioff Zone
An area of seismicity corresponding with the slab being thrust downwards in a subduction zone.
Convergent Boundaries (Destructive and collision margins)
At a convergent plate boundary, the plates move towards each other.
There are three types of convergent plate boundaries – dependent on the type of plate found there:
1. Oceanic meets continental
2. Oceanic meets oceanic
3. Continental meets continental
Convergent Boundaries - Destructive (Oceanic meets Continental)
- Oceanic crust is more dense than continental, so when they collide, the oceanic crust subducts underneath into the mantle. This is marked by deep ocean trenches.
- As the oceanic plate subducts, the continental plate is folded and slowly pushed up, forming a chain of fold mountains.
Tectonic Hazards (Oceanic meets Continental)
Explosive volcanic eruptions are generated as magma created from the melting oceanic plate pushes up through the faults in the continental crust to reach the surface. These are generally less frequent than other volcanoes, but are more destructive.
The Pacific Ring of Fire
The reason for the amount of tectonic activity around the Pacific Ring of Fire is due to the convergent plate boundary.
Convergent Boundaries - Destructive (Oceanic meets Oceanic)
- When two oceanic plates collide, one plate (the denser or faster) is subducted beneath the other – deep ocean trenches occur at this boundary.
Convergent Boundaries - Collision (Continental meets Continental)
When two continental plates meet, a collision margin occurs.
* As both plates have about the same density, and are less dense than the
asthenosphere beneath them, neither plate is actually subducted.
* Instead, they collide and sediments between them are crumpled and forced up
to form high fold mountains. E.g. The Himalayas
* Inevitably, there may be some subduction caused when the compressed (and therefore denser) sediments result in plate subduction beneath them.
No volcanoes!
(Continental meets continental) example
Nepal sits on the boundary between the Eurasian and Indian tectonic plates. It is a convergent boundary where two continental plates collide (figure 1).
In 2015, Nepal experienced a 7.8 magnitude earthquake.
Explain the reasons why this earthquake occurred.
case study - San Andrea Fault in California
- The San Andreas Fault marks the junction between the North American and Pacific Plates.
- The fault is 1300 km long, extends to at least 25 km in depth, and has a north west-south east trend.
- It is classified as a right lateral (dextral) strike-slip fault.
- Although both plates are moving in a north westerly direction, the Pacific Plate is moving faster than the North American Plate, so the relative movement of the North American Plate is to the south east.
- The Pacific Plate is being moved north west due to sea floor spreading from the East Pacific Rise (divergent margin) in the Gulf of California.
- The North American Plate is being pushed west and north west due to sea floor spreading from the Mid Atlantic Ridge (divergent margin).
- Movement along the fault is not smooth and continual, but sporadic and jerky.
- Frictional forces lock the blocks of lithosphere together for years at a time.
- When the frictional forces are overcome, the plates slip suddenly and shallow focus earthquakes are generated.
- Landscape and manmade features (eg rivers, fences and roads) are displaced across the fault as movement occurs.
- San Francisco has historically suffered significant earthquakes, notably in 1906 and 1989.
- The average rate of movement along the San Andreas Fault is between 30mm and 50mm per year over the last 10 million years.
- If current rates of movement are maintained Los Angeles will be adjacent to San Francisco in approximately 20 million years.
- The last major earthquake on this fault was in 1906 at 8.3 on the Richter scale.
Conservative Plate Margins
At a conservative plate margin, two plates are sliding past one another – resulting in a major break in the crust between them as they move.
The break itself is called a fault, and where it occurs on a large scale it’s known as a transform fault
Tectonic Hazards at Conservative Plate Margins
Earthquakes:
Powerful, but shallow earthquakes can occur due to frictional resistance – the plates stick as they move past one another, causing stress and pressure to build.
Volcanoes: None! Why?
