Distribution Of Tectonic Hazards Flashcards
(23 cards)
What happens at a destructive plate boundary?
An oceanic and continental plate converge.
What happens at a constructive plate boundary?
2 plates move away from each other.
What happens at a conservative plate boundary?
2 plates move alongside each other.
What happens at a collision plate boundary?
2 continental plates converge.
What happens at a complex plate boundary?
Several types are found along a short distance.
What are intra-plate earthquakes?
EQs that can happen anywhere the crust is cracked, like ancient faults created by previous plate movements. Small movements along these faults, build up friction and strain over time, then it’s suddenly released causing low magnitude EQs. Isostatic readjustment can cause EQs as the crust moves up and down. Human activities can cause EQs.
UK experiences around 25 EQs a year.
What are intra-plate volcanoes?
Volcanic activity found in the middle of oceanic/continental plates.
Oceanic plates are 7km thick and magma rising through the mantle from the edge of the outer core is able to melt through, forcing molten ,material to the surface, creating volcanoes which grow from the seabed.
Continental plates are thicker but get stretched enough for magma from a hot spot, linked to a mantle plume, to break through.
Hawaii islands have 15 volcanoes.
What are mantle plumes and hot spots?
There are 2 mantle plumes in the Earth’s mantle layer, one under the Pacific plate and the other under the African plate. Molten viscous silicate material rises from the outer core/mantle boundary to about 700km beneath the lithosphere.
From these mantle plumes, small hot spots are created where magma rises through the asthenosphere and sometimes breaks through the crust. Isolated hot spots occur where there’s an upwelling of molten material directly from the outer core/mantle boundary to the surface.
Describe the crust.
400 degrees celsius at base.
Less dense.
Granite/basalt.
Solid.
7-70km thick.
Describe the mantle.
870 degrees.
Mixed density layers.
Peridotite.
Solid and liquid layers.
2890km thick.
Describe the outer core.
4400-6100 degrees.
Dense.
88% iron.
Liquid.
2200km thick.
Describe the inner core.
7000 degrees.
Very dense.
80% iron.
Solid.
1300km thick.
How have the characteristics of the earth’s structure been established?
By studying the movement of seismic waves through the planet. Seismic waves travel at different speeds through material of different densities and some don’t travel through liquids.
What is the lithosphere?
The crust and upper mantle. Made of tectonic plates.
What is the asthenosphere?
The lower mantle.
What are the differences between the oceanic and continental plates?
Oceanic:
-Thin.
-Dense.
-6-10km thick.
-Ocean floor.
Continental:
-Thick.
-Less dense.
-45-50km thick.
-Land.
Describe the process of mantle convection.
Heat radiating from the inner core through the mantle causes it to convect, as it behaves like a viscous liquid. Can happen in several layers or 1.
Convection cells make contact with the base of the crust, causing friction and so drag the crust along in the general direction of the convection.
Describe the process of palaeomagnetism and sea floor spreading.
The outer core is mostly liquid iron and it convects. This movement creates a magnetic field for the earth; the direction changes every 100000 years. When new igneous rocks cool to form solid rock, they trap in the magnetic field direction of the time. Geological records of magnetic directions are symmetrical either side of constructive plate boundaries, proving that the oceanic crust has been diverging for a long time.
Describe the process of subduction and slab pull.
At destructive plate boundaries, the denser oceanic plate is subducted under less dense continental. As a slab of oceanic plate descends, it pulls the rest of the plate with it.
What is formed at divergent boundaries (constructive).
Mid-ocean ridges, they extend underwater as mountain chains with transform faults cutting across them. Mild shallow-focus EQs occur. Volcanic eruptions create submarine volcanoes, sometimes above sea level.
Rift valleys, they form on continents where the crust forms parallel cracks and the land between them collapses, creating steep valleys.
Processes here: decompression with magma rising and ridge push.
What is formed at convergent boundaries?
Deep ocean trenches, when oceanic crust subducts into the upper mantle and melts.
Fold mountains, where plate collision causes folding from subduction
At 2 oceanic plates: subduction of the lighter plate = deep ocean trenches and volcanoes. Submarine volcanoes grow to make island volcanoes (arcs). Shallow, deep-focused EQs. Eg. 2004 Asian tsunami.
At 2 continental plates: collision margin occurs where rock is forced up to form high fold mountains. No volcanoes, EQs are shallow-focused. Eg. Himalayas.
What happens at conservative boundaries?
No volcanic activity as no plate is melted.
Powerful EQs as plates have jagged edges, so stick and catch on each other, leading to a build up in stress. Released as a shallow-focused EQ. For example LA, 1994.
What is the link between tectonic process and magnitude of events?
Strongest EQ waves are created where there is most friction and build up of strain between 2 plates. Greatest at destructive plate margins.
Most explosive volcanoes are found at destructive margins as the magma is viscous with high silica and gas content due to subduction.
