Topic 1: EQ1 Flashcards
What are tectonic hazards?
Tectonic hazards include earthquakes and volcanoes eruptions, but they also include secondary hazards such as tsunamis. They represent significant risk in some parts of world in terms of loss of life, livelihood and economic impact. This is especially the case where active tectonic plate boundaries interact with areas of high population density, and medium and high levels of development. Tectonic hazards can be classified as either seismic or volcanic.
How are earthquakes distributed globally?
The main earthquake zones (often in clusters) are found along plate boundaries. About 70% of all earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean. The most powerful earthquakes are associated with convergent or conservative plate boundaries, although intra-plate earthquakes can also occur.
95% of earthquakes occur at tectonic plate boundaries.
What patterns of tectonic activity does the distribution of earthquakes represent?
-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.
-Scattered earthquakes in continental interiors. 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 Stratton Fault in Shropshire.
What type of hazard are earthquakes?
They’re primary hazards, but can also cause secondary hazards such as landslides and tsunamis.
How are volcanoes distributed throughout the world?
The violence of a volcanic eruption is determined by the amount of dissolved gases in the magma and how easily the gases can escape. There are about 500 active volcanoes throughout the world and, on average, around 50 of the, erupt each year.
Roughly 85-90% of volcanoes occur at plate boundaries.
What are seismic hazards?
These are generated when rocks within 700km of the Earth’s surface come under stress that they break and become displaced.
What are volcanic hazards?
These are associated with eruption events.
What are intraplate earthquakes?
These occur in the middle or interior of tectonic plates and are much rarer than boundary earthquakes.
What is a volcano?
This is a landform which 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?
-These are most clearly displayed at mid-ocean ridges.
-Large number of shallow focus and generally low magnitude (5-6) earthquake events.
-Effusive eruptions (Low VEI of 1-3)
-Ocean ridge with central rift valley. Form volcanic islands.
-Example is Iceland forming at the Atlantic mid ocean ridge.
-Continental examples involve the Rhine Rift valley in between France and Germany forming the Rhine Graben (as a result of intraplate movement).
What are convergent plate boundaries?
These are actively deforming collision locations with plate material melting in the mantle, causing frequent earthquakes and volcanoes.
What are conservative (transform) plate boundaries?
-This is 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 plate margins do not result in volcanic activity, but occasionally fissure eruptions.
-they are the sites of extensive shallow focus earthquakes (6-8 magnitude).
-They form ridges and scars on the surface.
What happens when plates move away from each other?
These are the ‘spreading’ ridges in the oceans. New oceanic crust, which is both thinner and denser than 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 (that is, the ocean) and they do not typically trigger tsunamis.
What occurs in locations where plates slide past each other?
This can present more risk than divergent plates. In simple terms, this is what is happening along 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).
What occurs when two plates move towards each other (convergent)?
These generate some of the most damaging earthquakes. Typically, 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 in Japan 2011, 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 difference between active and passive subduction zones?
Active subduction zones are characterised by magmatic activity, a mountain belt with thick continental crust, a narrow continental shelf and active seismicity. Passive continental margins are found along the remaining coastlines. Because there is no collision or subduction taking place, tectonic activity is minimal here.
What is the distribution of volcanoes controlled by?
The global geometry of plate tectonics.
How do volcanoes occur at 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. At an oceanic/continental plate collision, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. Surface volcanism typically appears above the magma that forms directly above down-thrust plates. During collisions between two continual plates, however, large mountain ranges such as the Himalayas are formed.
What are the characteristics of destructive boundary volcanoes?
They comprise a large proportion or the world’s active volcanoes and create the most explosive type, characterised by a composite cone associated with a number of hazards. These volcanic eruptions tend to be more infrequent but more destructive.
How do divergent plate boundaries cause rift volcanoes?
They 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.
How do hotspot volcanoes form?
They’re found in the middle of tectonic plates and are thought to be fed by underlying mantle plumes that are unusually hot compared with the surrounding mantle.
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 the melting of rock. This molten material, magma, rises through cracks and erupts to form active volcanoes on the Earth’s surface. As the plate moves over the hotspot, the volcanoes are rafted away and new ones form in their place.
What happens as oceanic volcanoes move away from a hotspot?
They cool and subside, producing older islands, atolls and seamounts. Over long periods of time this can also create chains of volcanoes, such as the Hawaiian Islands.
What are plate tectonics?
They’re a theory developed more than 60 years ago to explain the large-scale movements of the lithosphere (the outermost layer of the Earth). It was based around the evidence from sea floor spreading and ocean topography, marine magnetic anomalies, paleo magnetism and geomagnetic field reversals. It is essential to understand the earths internal structure to understand plate tectonics.
What is the lithosphere?
The surface layer of the Earth is a rigid outer shell composed of the crust and upper mantle. It is on average 100km deep. The lithosphere is always moving, but very slowly, fuelled by the rising heat from the mantle which creates convection currents. The distinction between the lithosphere and asthenosphere is one of physical strength rather than a difference in physical composition.
The lithosphere is broken into huge sections, which are the plate tectonics.
What are the two different types of crust?
-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.
The low density of the thick continental crust allows it to ‘float’ high on the much higher density mantle below.
What is the mantle?
This is the layer below the Earth’s crust. The mantle has a temperature gradient, with the highest temps occurring where the mantle material is in contact with the heat-producing core so that there is a steady increase of temp with depth. Rocks in the upper mantle are cool and brittle, while in the lower mantle they’re hot and plastic (but not molten). In the upper mantle they’re brittle enough to break under stress and produce earthquakes. However, in the lower mantle they’re plastic and flow when subjected to forces instead of breaking. The lower limit of brittle behaviour is the boundary between the upper and lower mantle.
What are convection currents?
Heat which is derived from the Earth’s core (radioactive decay) rises within the mantle to drive convection currents, which in turn move the tectonic plates. These convection currents operate as cells. We already know that plates can move in a number of directions when in contact with each other, and that the type of movement can be translated into a particular hazard risk.
What is sea floor spreading?
This occurs at divergent boundaries under the oceans. This is a continuous input of magma forming a mid-ocean ridge, for example the Mid-Atlantic Ridge. On land a rift valley forms. A technique involving the reconstruction of paleo magnetic reversals (called paleomagnetism) can be used to date the age of new tectonic crust.
How does gravity effect plate movement?
There is likely to be a combined force of convection and gravity driving tectonic plate movement. Gravity in particular causes the denser oceanic crust to be pulled down at the site of subduction. At constructive margins, magma is simply ‘gap filling’, rather than the main driver pushing the plates in opposite directions away from each other.
What is Paleomagnetism?
This results from the zone of magma ‘locking in’ or ‘striking’ the Earth’s magnetic polarity when it cools. Scientists can use this tool to determine historic periods of large-scale tectonic activity through the reconstruction of relative plate motions. They create a geo-timeline. The Earth’s magnetic poles change direction roughly every 400,000 years, and therefore so does the lava’s orientation as it cools to form new crust under the oceans. This can be used as evidence for sea floor spreading.
What is the Benioff Zone?
This is an area of seismicity corresponding with the slab being thrust downwards in a subduction zone. The different speeds and movements of rock at this point produces numerous earthquakes. It is the site of intermediate/deep-focused earthquakes. This theoretical framework is therefore an important factor in determining earthquake magnitude, since it determines the position and depth of the hypocentre.
What is the hypocentre?
This is the ‘focus’ point within the ground where the strain energy of the earthquake stored in the rock is first released. The distance between this and the epicentre on the surface is called the focal length.
What are subduction zones?
These are broad areas where two plates are moving together, often with the thinner, more dense oceanic plate descending beneath a continental plate. The contact between the plates is sometimes called a thrust or mega thrust fault. When the frictional stress between two locked plates passes a threshold, a sudden failure occurs along the fault plane that can result in a ‘mega thrust’ earthquake. This releases strain energy and seismic waves. The locked fault can hold for hundreds of years, building up enormous stress before releasing. The process of stress, strain and failure is referred to as the ‘elastic-rebound theory’.
What is a locked fault?
This is a fault that is not slipping because the frictional resistance on the fault is greater than the shear stress across the fault, that is, it is stuck. Such faults may store strain for extended periods that is eventually released in a large magnitude earthquake when the frictional resistance is overcome. The 2004 Indian Ocean tsunami was the result of a mega-thrust locked fault (subducting Indian Plate) with strain building up at around 20mm per year. It generated huge seismic waves and the devastating tsunami.
How was the Andes created?
The Andes owe their existence to a subduction zone on the western edge of the South American Plate; in fact, this type of boundary is often called the Andean boundary as it is the primary example.
At what depths below the surface do the Earth’s different layers sit?
0-100km - Crust.
100-2900km - mantle
2900-5100km - outer core
5100-6378km - inner core
What are earthquakes caused by?
The sudden movements comparative,y near to the Earth’s surface along a fault. Faults are zones of pre-existing weakness in the Earth’s crust.
What is the 4 step sequence in the formation of an earthquake?
1) The movements are preceded by a gradual build-up of tectonic strain, which stored elastic energy in crustal rocks.
2) When the pressure exceeds the strength of the fault, the rock fractures.
3) This produces the sudden release of energy, creating seismic waves that radiate away from the point of fracture.
4) The brittle crust then rebounds either side of the fracture, which is the ground shaking, that is, the earthquake felt on the surface.
What is the hypocentre?
This is the ‘focus’ point within the ground where the strain energy of the earthquake stored in the rock is first released. The distance between this and the epicentre on the surface is called the focal length.
What is the usual depth of the hypocentre?
It can occur at any depth between the Earth’s surface and about 700km. The rupture usually propagates along the fault with the earthquake waves coming from both the hypocentre and the fault plain itself. The most damaging events are usually shallow focus, with a hypocentre of less than 40km.
