Topic 2: Tectonic Processes and Hazards Flashcards
(187 cards)
Distribution of earthquakes?
- most EQ zones are found at or close to tectonic plate boundaries, often in clusters.
- 70% of all EQs are found in the ‘Ring of Fire’ in the Pacific Ocean.
- most powerful EQs are associated with convergent or conservative boundaries.
- Some EQs can occur along old fault lines and the weakness is reactivated, e.g. Church Stretton Fault in Shropshire.
- OFZ and CFZ
What is 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.
What is the continental fracture zone (CFZ)?
- a belt of activity following the mountain ranges from Spain through the Alps to the Middle East and the Himalayas.
Causes of earthquakes?
- sudden release of energy which has been built up over time by stress as two plates move past each other.
Distribution of volcanic eruptions?
- occur at or close to plate boundaries. This is due to the movement of plates at different speeds and directions causing collisions.
- also occur at volcanic hotspots, e.g. Hawaii as magma rises as plume.
- there are 500 active volcanoes in the world.
Distribution of tsunami?
- 90% of tsunami occur within the Pacific Basin.
- most are generated at subduction zones (convergent boundaries) particularly the Japan-Taiwan island arc and South America.
Causes of tsunami?
- occur or generated when a sub-marine earthquakes displaces the sea bed vertically as a result of movement along a fault line at the subduction zone.
- they can generate intense ground shaking damage followed by the damage of the tsunami.
What are the different types of plate tectonics?
- convergent
- divergent
- conservative
Types of conservative plage margins?
- two oceanic plates
- two continental plates
- one continental and one oceanic plate
Continental and oceanic plates meeting - diagram?
- see flashcard
Continental and oceanic plates colliding - description?
- when the plates collide, the oceanic plate slides under (subducts) the continental plate into the mantle and melts.
- deep ocean trenches mark the oceanic plate starts to sink beneath the continental plate.
- the subduction also leads to the formation of fold mountains. Since the plates are constantly moving, most fold mountains will continue to grow.
- the friction created between the plates causes intermediate and deep earthquake in the Benioff zone.
- volcanic eruptions are also generated - magma created by melting oceanic plate pushes up through faults in the continental crust to reach the surface.
Continental and oceanic plates colliding - example?
- Mount Fuji (Japan)
Two oceanic plates colliding diagram?
- see flashcard
Two oceanic plates collision - description?
- when two oceanic plates collide, one plate (denser or faster) is subducted beneath the other.
- deep ocean trenches form when subduction occurs, and the subducted plate then melts - creating magma, which rises up from the Benioff zone to form underwater volcanoes.
- over millions of years the volcanoes rise above sea level to form separate island volcanoes, usually found in curved lines called island arcs.
- the subduction also produced shallow-to-deep focus earthquake.
Example of two oceanic plates colliding?
- 2004 Indian Ocean tsunami - Indian plate sinking underneath the Burma plate (a part of the Eurasian plate)
Two continental plates colliding - diagram?
- see flashcard
- two continental plates colliding - description?
- when two continental plates meet, a collision margin occurs. As both plates have about the same density, and are less dense than the athenosphere beneath them, neither plate is subducted.
- instead they collide and sediments between them are crumpled and forced up to form high mountains - e.g. the Himalayas.
- however there can be some subduction such as Nepal earthquake, caused when compressed (therefore denser) sediments result in plate subduction beneath them.
- there is no volcanic activity, but any earthquakes are likely to have a shallow-focus increasing their severity.
Divergent plate margin - diagram?
- see flashcard
Divergent plate margin - description?
- 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?
- mid-ocean dishes of underwater mountains extend for over 60,000km across the world’s ocean floors.
- regular breaks (transform faults) cut across the ridges as they spread at different rates.
- regular volcanic eruptions - create submarine volcanoes, along the ridges, some of which grow above sea level to create new islands such as Iceland on the Mid-Atlantic Ridge.
Rift valleys?
- when plates move apart on continents, the crust stretches and breaks into sets of parallel cracks (faults)
- the land between these faults then collapses, forming steep-sided valleys (rift valleys). E.g. Great Rift Valley - Zambia.
Conservative plate margin - diagram?
- see flashcard.
Conservative plate margin - description:
- two plates slide past each other, forming a conservative plate margin. This results in a major break in the crust between them as they move.
- the break is called a fault, and where it occurs on a large scale is known as a transform fault, which affected a wider area. Although no crust is made or destroyed here (and no volcanic activity - no magma), this type of plate margin is tectonically very active - associated with powerful earthquakes.
- the two plates sometimes stuck as they move past each other, causing stress and pressure to build up, which is suddenly released as a strong shallow-focus earthquake.
Conservative plate margin - example:
- San Andreas Fault in California, which has generated significant earthquakes.