Tectonics Flashcards
(44 cards)
where are most tectonic hazards found
Generally found on plate margins- a large proportion are found on the Pacific ring of fire
Evidence in support of plate tectonic theory
- Continental Drift and the jigsaw fit of continents
- Geological and Fossil Records
- Palaeomagnetism
How are geological and fossil records evidence for the plate tectonic theory
- Rocks of the same age are found where countries would have fitted together
- Fossil remains in India match that of Australia
- The geological sequence of sedimentary and igneous rocks found in Scotland match those in Newfoundland
- Fossil remains of a small aquatic reptile, Mesosaurus are only found in restricted parts of Brazil and SW Africa and is believed to be a poor swimmer
How is Palaeomagnetism evidence for plate tectonic theory
The discovery of a ridge of mountains running along the middle of the Atlantic Ocean known as the Mid-Atlantic Ridge where magnetic strips running parallel to the ridge were found. These stripes correspond to times when the Earths magnetic field reversed from North to South. magma emerging from spreading centres
was magnetised according to the polarity of the earth and that as this polarity
changed it led to stripes of palaeomagnetism which were replicated at each
side of the spreading centre. This led to the realisation that the plates were
drifting apart.
Characteristics of Continental Plates
continental plates are older, thicker, with a lower
density and composed of silica and aluminium
Describe a constructive plate margin
2 plates are moving away from each other creating a gap between them. Molten rock rises from the mantle to fill the gap forming a mid-ocean ridge
Features at constructive plate margins
Mid-ocean Ridges, Rift Valleys, Volcanoes
What is the difference between a mid ocean ridge and a rift valley
Mid ocean ridges are formed when 2 oceanic plates move apart, rift valleys occur when 2 continental plates move apart- the gap is filled with basaltic lava upwelling from below. Areas of crust drop down between parallel faults to form rift valleys
Describe a destructive subduction plate margin
When a continental and oceanic plate move towards each other, the denser oceanic plate gets subducted under the less dense continental plate
Features at destructive subduction margins
Ocean trenches, island arcs and volcanic activity
What are island arcs and how do they form
Island arcs are long chains of active volcanoes with intense seismic activity found along destructive plate margins. Most originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. It is the main way continental growth is achieved. Can form on oceanic-continental margin or oceanic-oceanic margin
Describe a destructive collision zone
When 2 continental plates collides they crumple, because neither can sink. Instead of being subducted they push into each other forcing material to be folded up into mountain ranges
Features at destructive collision margins
Fold mountains and volcanoes
Formation of a fold mountain
- Predominately occurs when plates are inactive
- The depression between the rocks is a geosyncline, which is filled with sediment and compressed into rocks
- When the 2 plates move towards each other the rock become crumpled
- Sediment is folded into fold mountains
- continued compression from plate movement means eventually rock appears above sea level as a mountain range
Describe a conservative plate boundary
2 plates slide past each other either in opposite directions or in the same direction at different speeds
What is a hotspot and how did it lead to the formation of the Hawaiian islands
- It is an area in the mantle where heat rises as a mantle plume from the core of the earth, causing rock to melt and magma to rise. It is a fixed area with a greater heat flow
- Can form both at plate boundaries and intra-plate, however the majority are intra-plate such as Hawaii
- AS the Pacific plate moves, the volcanic activity on the earths surface changes location. As the ocean floor moves over the hotspot, the upwelling lava plume creates a succession of new volcanoes, that become extinct as they move away from the hotspot
What defines a Super volcano
They must emit at least 1,000km^3 of ash and lava
They occur on hot spots
Formation of a super volcano
- The magma rising from the mantle can’t escape and causes a large bulge which starts to appear on the surface of the crust
- Cracks of weakness begin to show, which are fissures
- Once enough pressure has built open through the gases, ash and lava start to erupt from the magma chamber. This leaves an empty magma chamber
- After the eruption a large depression called a caldera can be seen due to the collapsed magma chamber
Warning signs that the Yellowstone Super volcano may erupt
- The ground temperatures increase and the geysers erupt more frequently
- Increase in the amount of hydrogen sulphide gas and carbon dioxide
- Earthquakes-
- Trees and animals dying from gas poisoning
Impacts of the Yellowstone super volcano eruption
-Pyroclastic flows, 50-100 miles from volcano reaching 100mph
-Ash Clouds, which reach UK 5 days after initial eruption
10,000km of land destroyed
87,000 people killed instantly
Global temperature would fall by 12-15c
What are pyroclastic flows
Hot, highly gaseous and ash-rich, fast-moving cloud. Consist of 2 parts, a lower basal flow of coarse fragments that move along the ground and a cloud of ash that rises above the basal flow. The rapidly expanding mixture of gas creates clouds of volcanic ash
What are Nuees Ardentes
These occur after an eruption, when a volcano’s lava dome collapses and the dense rock avalanches down the volcano
Tephra
When a volcano ejects material such as rock fragments
Lahars
Mudflow- A mixture of meltwater from snow and rock fragments that flow down the slope of a volcano
