1 Locations at risk from tectonic hazards Flashcards
What percentage of earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean?
70%
What are the most powerful earthquakes associated with?
Convergent or conservative boundaries
What are the names of the two zones where earthquakes are found?
- The oceanic fracture zone (OFZ) - a belt of activity through the oceans along the mid-ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and California.
- The continental fracture zone (CFZ) - a belt of activity following the mountain ranges from Spain via the Alps, to the Middle East, the Himalayas to the East Indies and then circumscribing the Pacific.
Where can a small minority of earthquakes also occur?
A small minority of earthquakes can also occur along old fault lines and the hazard is associated with the reactivation of this weakness, for example, the Church Stretton Fault in Shropshire.
What is the violence of a volcanic eruption determined by?
The amount of dissolved gases in the magma and how easily the gases can escape
How many active volcanoes are there throughout the world?
There are about 500 active volcanoes throughout the world and, on average, around 50 of them erupt each year.
What are the three types of plate boundary?
- Divergent (constructive) margins, most clearly displayed at mid-ocean ridges. At these locations, there are large numbers of shallow focus and generally low magnitude earthquake events. Most are submarine.
- Convergent (where plates move together): these are actively deforming collision locations with plate material melting in the mantle, causing frequent earthquakes and volcanoes.
- Conservative (oblique-slip, sliding or transform) margins, where one plate slides against another. Here the relative movement is horizontal and classified as either sinistral (to the left) or dextral (to the right). The lithosphere is neither created nor subducted.
Where are divergent (constructive) margins, most clearly displayed?
At mid-ocean ridges
What type of earthquake occurs at divergent (constructive) margins?
At these locations, there are large numbers of shallow focus and generally low magnitude earthquake events. Most are submarine.
What type of earthquake occurs at conservative (oblique-slip, sliding or transform) margins?
They are sites of extensive shallow focus earthquakes, occasionally of considerable magnitude
What is the name of the plate margin where plates move away from each other?
Divergent ‘spreading ridges’
Why are earthquakes at divergent ‘spreading ridges’ in the oceans typically a low hazard risk?
The earthquakes seen at these boundaries tend to be frequent, small and typically a low hazard risk because of their geographical position (that is, the ocean) and they do not typically trigger tsunamis.
What is happening along the San Andreas Fault in California?
The Pacific Plate (moving north) creates a zone of friction against the North American Plate (moving north at a different speed)
What happens as two plates are moving towards each other (convergent)?
Typically when this happens, one plate starts sliding under the other. As the strain builds over time in the subduction zone, the friction between the two masses of rock is overcome, releasing energy. This will produce both earthquakes - such as the tsunami-generating ones off Japan in 2011 and Aceh which are fed by the melting of the subducting plate.
What are active subduction zones characterised by?
Magmatic activity, a mountain belt with thick continental crust, a narrow continental shelf and active seismicity
Where are passive continental margins found?
Along the remaining coastlines
What are the different tectonic settings where volcanoes are found in?
1 Destructive plate boundaries. These occur at locations where two plates are moving together. Here they form either a subduction zone or a continental collision, depending on the type of plates. When a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. These volcanic eruptions tend to be more infrequent but more destructive.
2 Divergent boundaries create rift volcanoes where plates diverge from one another at the site of a thermally buoyant mid-ocean ridge. These are generally less explosive and more effusive. Here there is basaltic magma, which has low viscosity.
3 Hotspot volcanoes are found in the middle of tectonic plates and are thought to be fed by underlying mantle plumes.
What happens when a less dense oceanic plate collides with a less-dense continental plate at a destructive plate boundary?
The oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone
Why is the oceanic plate typically thrust underneath the continental plate at a destructive plate boundary?
Because of the greater buoyancy of the continental lithosphere, forming a subduction zone
Where do plates diverge from one another at a divergent boundary?
At the site of a thermally buoyant mid-ocean ridge
What type of eruption occurs at divergent boundaries?
These are generally less explosive and more effusive. Here there is basaltic magma, which has low viscosity.
Where are hotspot volcanoes found?
In the middle of tectonic plates
What is a volcanic hotspot?
A volcanic hotspot is an area in the mantle from which heat rises as a hot thermal plume from deep in the Earth. High heat and lower pressure at the base of the lithosphere enable melting of the rock. This molten material, magma, rises through cracks and erupts to form active volcanoes on the Earth’s surface.
What does high heat and lower pressure at the base of the lithosphere enable?
Melting of the rock
What happens as the tectonic plate moves over the stationary hotspot?
As the tectonic plate moves over the stationary hotspot, the volcanoes are rafted away and new ones form in their place.
What happens as oceanic volcanoes move away from the hotspot?
They cool and subside, producing older islands, atolls and seamounts
What are the names of the two types of crust, which are made up of different types of rock?
- Thin oceanic crust, which underlies the ocean basins, is composed primarily of basalt
- Thicker continental crust, which underlies the continents, is composed primarily of granite
What does the geothermal gradient of the Earth’s mantle mean in terms of changes in temperature?
The highest temperatures occur where the mantle material is in contact with the heat-producing core so there is a steady increase of temperature with depth
