Hazards- Plate Tectonics Flashcards
Lithosphere
The solid top layer of crust in which plates are formed
Consists of crust and upper mantle
Asthenosphere
Soft, plastic like rock in the upper mantle just below the lithosphere
Earths heat source
- Earth was hot when it formed
- formed when “planetesimals” condensed and stick together
- earth makes some of its own heat through RADIOACTIVE DECAY
- seismic waves and seismic data show what materials make up the core
Continental crust
30-50km thick 4 billion years old Density of 2.7*/cm3 Silicon, aluminium and oxygen Granite (metamorphic rock)
Oceanic crust
7-10km thick 200 million years old Density of 2.9*/cm3 Silicon, aluminium and oxygen Basalt (igneous rock)
Jigsaw fit
Similarities in outline of the coastlines of Eastern South America and west Africa
Best fit when coastlines match at 100km below sea level
Areas where there are gaps/overlaps may be explained by:
-coastal erosion
-coastal deposition
-changes in sea level
-changes in land level
Geological fit
- geology of eastern South America and west Africa mapped
- ancient rock outcrops over 2000 million years old were continuous from one continent to the other
Tectonic fit
- fragments of old fold mountain belt (400-450 million years ago) found on widely separate continents today
- when fragments are reassembled, the mountain belt forms a continuous linear feature
Fossil evidence
Many examples of fossils found on separate continents and nowhere else, suggesting continents were once joined
What is Paleomagnetism?
When new material is made at mid-ocean ridges (this is a constant process), the new material takes the current magnetism of the earth into it at the time
Paleomagnetism- process
RIDGE PUSH:
-as the sea floor is spreading there is a symmetrical pattern in the rock which is created as the rock gets increasingly far from the ridge
-due to the magnetism of the Earth changing (flipping), the striped patterns which are created in the rock can help tell the age of the rock
SLAB PULL:
-as new material is constantly being created at ridges for the Earth to remain how it is materials are destroyed at subduction zones
So.. SLAB PULL=RIDGE PUSH
Subduction zones
- Where gravity pulls the oceanic plate down into the mantle (at destructive boundaries)
- This destroys crust material (helping maintain the Earths shape)
Constructive boundary
- two plates are moving apart leaving a gap in the middle for magma to rise up through
- volcanoes form but don’t erupt with much force (magma can’t escape easily)
- rift valleys form
Destructive boundary
Oceanic plate descends below the less dense continental plate
- as the plate descends it is melted due to friction (forms magma)
- magma then rises through cracks in the continental plate and forms volcanoes if it reaches the surface
Collision boundary
- two plates of similar density (eg. Continental and continental plates)
- causes material to buckle up forming fold mountains eg. Himalayas
Conservative boundary
- two plates don’t directly collide but slide past each other along a fault
- no volcanoes are found here but earthquakes do occur eg. San Andreas Fault
How do rift valleys form? Eg. East African Rift Valley
- form at divergent plate boundaries
- basaltic lava erupted from hundreds of parallel fissures on the ocean floor
- convection currents cause the plates to move apart
- elevated heat flows from the mantle caused by thermal bulges
- as the bulges form they stretched and fractured the crust into a series of faults
- convection currents in the atmosphere causes the extension of the crust
- as the crust extends, graben (down dropped blocks) and horts (upthrown blocks) are formed
- rift continues to extend, part of the crust becomes thin enough for oceanic type basalts to be erupted, signalling the formation of new oceanic crust
Destructive boundary: oceanic-continental
-denser oceanic plate subducts beneath the less dense continental plate
Fold mountains:
-pressure put on the continental plate causing it to buckle upwards- fold mountains form
-the descending oceanic plate is scraped off and folded to form fold mountains
Benioff zone (oceanic-continental)
- the further the rock descends, the hotter the surroundings become
- this, together with the heat generated from friction, begins to melt the oceanic crust into the magma
- as it is less dense than the surrounding asthenosphere, the molten material begins to rise in great plumes up through fissures/faults in the buckled continental plate by burning their way through overlaying rock
- eventually these reach the surface to form explosive volcanoes (with viscous lava)
Destructive boundary:oceanic-oceanic
- two oceanic plates moving towards each other
- the crust that is subducted may be marginally denser of the two plates, or it is the one moving quicker
- occurs in the same way as oceanic-continental but is less clear which is subducted
- earthquakes are frequent- subduction produces shallow-to-deep focus earthquakes (some very powerful sub sea ones may cause a tsunami)
- ocean trenching and island arcs are the main features
Destructive boundary: continental-continental
- continental plates are of lower density than the asthenosphere beneath them
- so subduction can’t occur
- instead they collide with each other
- these boundaries are prone to very powerful earthquakes
- fold mountains also form
