Tectonics EQ1 Flashcards
What is the asthenosphere?
Close to the surface, the low pressure create the ductile upper zone of the mantle
What is the lithosphere?
The crust and upper mantle together, rock becomes cool and more brittle close to the surface of the mantle.
What are the different types of crust?
Oceanic –> thin
Continental –> thick and lower in density than oceanic crust so it is able to float
What % of earthquakes are found in the “Ring of Fire” in the Pacific Ocean?
75%
Describe the two parts of the core?
Inner Core: Hottest - 6000C, solid, mostly Iron
Outer Core: semi-molten, mostly liquid iron and nickel, temps 4,500-6000C
Describe the distribution of earthquakes?
The main earthquake zones are found, often in clusters, along plate boundaries; typically powerful earthquakes are associated with convergent or conservative boundaries. Apart from at plate boundaries, intra-plate earthquakes do occur along old fault lines, such as the Church Stretton Fault in Shropshire.
The oceanic fracture zone (OFZ) is a belt of activity through the oceans along the mid-ocean ridges, coming onshore in Africa, the Red Sea, the Dead Sea rift and California. Mid-ocean ridges such as the Mid-Atlantic Ridge are places in the Earth’s tectonic plate system where new crust forms and the plates move away from one another The CFZ is a belt of activity following the mountain ranges from Spain to the Pacific via the Alps, the Himalayas etc.
Describe the distribution of volcanoes
- Near subduction zones such as the Pacific ring of fire, this often has the most violent of all activity
- Along divergent ridges e.g Mid-Atlantic ridge
- Along rift valleys (same as above just on land) e.g Mt Kilimanjaro
- A volcanic hotspot = area in mantle where heat rises as hot thermal plume from deep in earth, the magma rises through cracks and erupts to form active volcanoes on the Earth’s surface –> as tectonic plates move over the stationary hotspot, volcanoes also move and are replaced by new ones in their place. As oceanic volcanoes move away from the hotspot, they cool and subside, producing older islands –> can create chains of islands over time e.g Hawaiian Islands
What is a convergent/ destructive plate boundary in simplest terms?
Where two plates colide
What are divergent/constructive plate boundaries in simple terms?
Where two plates move apart
What are conservative/transform plate boundaries in simplest terms?
Where two plates slide past each other
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How many active volcanoes are there in the world and how often do they erupt each year?
There are about 500 volcanoes globally, around 50 erupt each year.
Describe the distribution of Tsunamis
90% of all events occur within the Pacific Basin, in association with the activity at plate margins. Most are generated at subduction zones (convergent boundaries), particularly off the Japan-Taiwan island arc, South America and the Aleutian Islands
What are the causes of tsunamis?
When a sub-marine earthquake displaces the sea bed vertically as a result of movement along a fault line at a subduction zone. This violent movement displaces a large volume of water in the ocean water column, which then moves outwards in all directions from the point of displacement.
Sub-marine earthquakes that occur close to the shoreline can generate intense ground shaking damage, followed by subsequent tsunami
Who coined the term continental drift and what does it mean?
Alfred Wegener in 1912. He believed that there was a single supercontinent called Pangea that existed about 300 million years ago. This was only evidenced in the early 1960s when seafloor spreading was validated by the scientific community
Name the 5 pieces of evidence that support Wegeners’ theory of continental drift (i.e that there was once one supercontinent)
REMEMBER THESE ARE NOT THEORIES, BUT EVIDENCE THAT CONTINENTAL DRIFT HAPPENED
- Jigsaw Fit
- Geological Fit
- Tectonic Fit
- Glacial deposits
- Fossil Evidence
Describe the Jigsaw Fit evidence for plate tectonics
The similarity in outline of the coastlines of eastern South America and West Africa had been noted for some time. The best fit is obtained if the coastlines are matched at a depth of 1,000 metres below current sea level
Why there are gaps or overlaps and not a constant jigsaw fit:
- Coastal erosion since separation of continents
- Coastal deposition since
-Rises in sea level (eustatic change) since seperation
-changes in land level (isostatic change) since separation
Explain the geological fit as evidence for plate tectonics
Ancient rock outcrops over 2,000 million years old were the same in both eastern South America and West Africa
Also, rock sequences in northern Scotland are similar with those of
Explain the geological fit as evidence for plate tectonics movement
Ancient rock outcrops over 2,000 million years old were the same in both eastern South America and West Africa
Also, rock sequences in northern Scotland are similar with those of Eastern Canada, indicating they were laid down under the same conditions in one location
Explain tectonic fit as evidence for continental drift
Fragments of old fold mountain belt between 450 and 400 million years ago are found on widely separated continents today. Pieces of the Caledonian fold mountain belt are found in Greenland, Canada, Ireland, England, Scotland and Scandinavia. When these land masses are re-assembled the mountain belt forms a continuous linear feature
Explain glacial deposits as evidence for continental drift
Today, glacial deposits formed during the Permo-Carboniferous glaciation (about 300 million years ago) are found in Antarctica, Africa, South America, India and Australia.
If the continents havent mood then this would suggest an ice sheet extended form the south pole to the equator at this time - very unlikely.
But if the continents of the southern hemisphere re-assembled near the south pole then the Permo-Carboniferous ice sheet assumes a much more reasonable size. More evidence comes from glacial striations - scratches on the bedrock made my blocks of rock embedded in the ice as the glacier moves. These show the direction of the glacier, and suggest the ice flowed from a single central point
Explain fossil evidence for continental drift
There are many examples of fossils found on seperate continents and nowhere else, suggesting the continents were once joined.
i.e same fossil Mesosaurus can be found on South American East coast and African West Coast
How does the theory of Mantle Convection explain how the plates move
Heat produced by the decay of radioactive elements in the earths core heats the lower mantle, creating convection currents. Heat radiates outwards from the inner core. These hot, liquid magma currents are thought to move in circles in the asthenosphere, causing the plates to move.
Less accepted now as the reason for plate movement as rate of convection is less than observed rates of plate movement.
How does the theory of slab pull and ridge push explain tectonic plate movement?
Convection currents in the mantle move tectonic plates as the asthenosphere circulates due to heat present in the core. Where subduction is occuring, this results in a force known as slab pull. This is created as the plates begin to sink into the earths mantle and serves to pull the lower plate further away from the volcanic ridge where it is formed. Where new oceanic crust is formed at mid-ocean ridges, it pushes the tectonic plates apart.
The newly formed oceanic crust is denser than the asthenosphere that it sits upon so it begins to sink (at subduction zones), pulling apart the mid-ocean ridge allowing for more magma to rise.
How do Subduction Processes and the Benioff Zone theory help to explain tectonic plate movement?
The subduction zone is the broad area where two plates are ,oving together, often with the more dense oceanic plate sinking beneath the continental plate. As a new plate is being created in one place, it is being destroyed in another. Benioff zones are dipping zones of increased earthquake activity produced by the interaction of the sinking oceanic crustal plate with an overriding continental or oceanic plate. They occur at subduction zones
The area of seismicity corresponding with the slab being thrust downwards is the Benioff zone. The different speeds and movements of the rocks at this point produce numerous earthquakes. This theory is important in determining the earthquake magnitude as it determines the position and depth of the hypocentre (the focus point where the strain energy of the earthquake stored in the rock is released)
