EQ1: Why are some locations more at risk from tectonic hazards? Flashcards
(35 cards)
Where are the main earthquake zones found?
(Often in clusters) along plate boundaries.
Where are about 70% of all earthquakes found?
The ‘Ring of Fire’ in the Pacific Ocean.
What are the most powerful earthquakes associated with?
Convergent or conservative boundaries, although rare intra-plate earthquakes can occur.
What are intra-plate earthquakes?
These occur in the middle or interior of tectonic plates and are much rarer than boundary earthquakes.
What is 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 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 type of hazards are earthquakes and what can they cause?
Primary hazards (ground movement and ground shaking) but also cause secondary hazards such as landslides and tsunamis.
What is the violence of a volcanic eruption dependant on?
The amount of dissolved gases in the magma and how easily the gases can escape.
How many active volcanoes are there in the world and, on average, how many erupt each year?
About 500
Around 50 of them erupt each year
What are seismic hazards?
Generated when rocks within 700km of the Earth’s surface come under such stress that they break and become displaced.
What is a volcano?
A landform that develops around a weakness in the Earth’s crust from which molten magma, volcanic rock, and gases are ejected or extruded.
What is a divergent (constructive) plate boundary?
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 (under the sea).
What is a convergent plate boundary?
Where plates move together. These are actively deforming collision locations with plate material melting in the mantle, causing frequent earthquakes and volcanoes.
What is a conservative plate boundary?
(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). Lithosphere is neither created nor subducted, and while conservative margins do not result in volcanic activity, they are the sites of extensive shallow focus earthquakes, occasionally of considerable magnitude.
What are the three ways plates can move?
Pull away from each other
Slide past each other
Crunch into each other
What happens when plates move away from each other and where does this happen?
The places where they move away from each other are the divergent ‘spreading ridges’ in the oceans. New oceanic crust, which is thinner and denser than the continental crust, is created. The earthquakes seen at these boundaries tend to be frequent, small and typically a low hazard risk because of their geographical position (the ocean) and they don’t usually trigger tsunamis.
Name a location where plates are sliding past each other and what is happening there.
The San Andreas Fault in California, where the Pacific Plate (moving north) creates a zone of friction against the North American Plate (moving north at a different speed). This presents more risk.
Describe what happens when plates move towards each other (convergent).
These are the plate boundaries that generate some of the largest and most damaging earthquakes. Typically when this happens, one plate starts sliding under the other. As the strain builds up over time in the subduction zone, the friction between the two masses of rock is overcome, releasing energy. This will produce both earquakes - such as the tsunami generating ones off Japan in 2011 and Aceh in Indonesia in 2004 - and volcanoes, the magma of which are fed by the melting of the subducting plate. The subduction zones at the edge of the Pacific Plate are the reason for the Ring of Fire that is a feature of this ocean.
What is the distribution of volcanoes controlled by?
The global geometry of plate tectonics.
What is a destructive plate boundary?
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 buoancy of the continental lithosphere, forming a subduction zone. Large mountain ranges are formed, such as the Himalayers. These volcanic eruptions tend to be less frequent but more destructive.
What is a divergent plate boundary?
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, especially when they occur under water deep in the ocean floor, for example the Mid-Atlantic Ridge. Here there is basaltic magma, which has low viscosity.
What are hotspot volcanoes?
Found in the middle of tectonic plates and are thought to be fed by underlying mantle plumes that are unusuakky hot compared with the surrounding mantle. The presence of a hotspot is inferred by anomolous volcanism (not at a plate boundary), such as the Hawaiin volcanoes within the Pacific Plate. 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. As the tectonic plate moves over the stationary hotspot, the volcanoes are rafted away and new ones form in their place. As oceanic volcanoes move away from the hotspot, they cool and subside, producing older islands, atolls and seamounts.
What are the two different types of crust?
Thin oceanic crust, which underlines the ocean basins, is composed primarily of basalt.
Thicker continental crust, which underlines the continents, is composed primarily of granite. Its low density allows it to ‘float’ high on the much higher density mantle below.
What does the Earth’s mantle have?
Temperature gradient (geothermal gradient).
