Tectonic Processes and Hazards Flashcards
Define “seismic hazard”.
Seismic hazards are risks associated with the displacement of large quantities of rock (known as plates) during an earthquake. Seismic hazards are only to do with earthquakes!
Define “volcanic hazard”.
A risk associated with the eruption of volcanoes.
Define “intraplate earthquake”
An earthquake that occurs far from major plate boundaries. These are usually caused by smaller fractures in the Earth’s crust known as “faults”.
Define “volcano”.
A land form that forms from a weak-point in the Earth’s crust from which molten magma, volcanic rock, and gases are ejected.
What are the four main types of plate boundary?
Divergent (Constructive - plates moving apart)
Convergent (Collisional - Plates moving towards each other)
Subduction (Collisional - Plates subducting beneath one another)
Conservative (Plates gliding past each other, or moving in the same direction at varying speeds).
What factors lead to the formation of Plate tectonic theory?
Natural jigsaw fit of the continents (West Africa and Brazil, Madagascar and India)
Sea floor spreading and palaeomagnetism
Ocean topography
What is the “lithosphere”?
The surface layer of crust and upper mantle. On average around 100km thick.
What causes plates to move?
Magma gets heated at the core of the planet, which then becomes less dense and rises, as these columns of magma rise they push and drag the plates above them in different directions. As the magma rises it cools and becomes more dense again, causing it to sink back to the core and the process repeats as such.
Explain why we see some volcanoes far from plate boundaries and give examples of these types of volcanoes.
Volcanoes can form far from plate boundaries at sites called ‘hotspots’. This is an area of the crust they sits above a plume of heat from the mantle. The formation of this plume is not fully understood, but they remain stationary as the plate glides above them over millions of years - this creates island archipelagos such as the Canary islands of Spain and Hawai’i of the US. As an island moves off of the mantle plume, it looses its volcanic characteristics and the volcano that formed the island becomes dormant. The island will gradually sink back into the sea due to weathering and erosion, and new islands will be formed as the crust moves above the plume.
Define “palaeomagnetism”.
Palaeomagnetism results from the zone of magma “locking in” the Earth’s magnetic polarity when it cools, like a giant photograph imprinted onto the volcanic rocks. Scientists use this to determine historic periods of greater seismic activity by reconstructing plate motions, directions and speeds. They create a geo-timeline. This was a great factor in proving the theory of continental drift.
What is the Benioff zone?
The Benioff zone is an area of great seismic activity sitting beneath a subduction zone. The different speeds and movements of rocks at this point produce deep, frequent and powerful earthquakes. A good example of this is the Kermadec Arc, where the Australian and Pacific plate subduct.
What is a subduction zone and how do they form?
A subduction zone are large areas along convergent (collisional) boundaries. They are characterised by the meeting of an oceanic and a continental plate. The younger, less dense oceanic plate ‘sinks’ beneath the older and more dense continental plate and slides beneath it. As it does this, great friction is produced slowly pulling the edge of the continental plate down until it slips and the plate bounces up with great force, usually producing extremely violent “megathrust” earthquakes - this is known as “elastic-rebound theory” (Chile 9,5 1960 and Japan 9,1 2011).
What is a “locked fault”?
A locked fault is a where two plates have met and become stuck, usually on each other. The segment may be locked for many thousands of years, with pressure gradually building, until the segment “slips” (called a rupture) releasing large amounts of energy in the form of an earthquake. The 2004 Indian ocean earthquake and tsunami (Mw9,2) is an example of a locked subduction fault rupture.
What is the difference between “hypocentre” and “epicentre”?
The hypocentre of an earthquake is the ‘focus’ point in the ground from which the seismic waves of an earthquake propagate. They are subterranean. They differ from the epicentre as the epicentre is the point of crust that sits directly above the hypocentre - and the distance between the two is known as the ‘focal length’. A low focal length and high magnitude are the two main causes for destructive earthquakes,
What are the three types of seismic wave and what do they look like?
P-wave (Compressional waves) Least destructive.
S-wave (Amplitude waves) Second most destructive.
L-waves (Side-to-side waves), L-waves (Love) are the most destructive of all seismic waves.
