Hazardous Earth SG6 Flashcards
How long ago were the continents a single land mass called Pangaea?
200 million years ago
How long ago did Pangaea split into two continents - Laurasia and Gondwanaland?
120 million years ago
What is Alfred Wegener’s theory called?
Continental Drift
When did Alfred Wegener publish his theory?
1915
What four pieces of evidence did Alfred Wegener use for his theory?
Jigsaw fit of continents, Geological sequences (matching mountain belts), Glacial deposits, Fossil evidence
What were the two fossils Wegener used as evidence?
Mesosaurus and Glossopteris
Why were Alfred Wegener’s ideas dismissed?
Couldn’t understand how the continents could travel through the solid sea floor
Define Echo Sounding
Send pulses of sound which travel to the sea floor and bounce back up again, The time it takes to come back up is how deep the sea floor is in that location
What did Bruce Heezen and Marie Tharp discover from the echo sounding data?
Vast mountain chain running down the bottom of the ocean
How long was the mountain chain found by Tharp and Heezen?
60,000km
What did Tharp and Heezen notice about the crest of the mountain chain?
There was a cleft - a mid ocean ridge
How did the mid ocean ridge found by Tharp support Wegener’s theory?
The ridge would be formed naturally if the continents were pulled apart
Define Continental Drift
The theory that the continents are mobile and have moved across Earth’s surface through geological time
Define Seismic Profiling
Dropping explosives off a ship and recording how long it takes for sound waves to return
What was Harry Hess’ theory for continental drift?
Seafloor spreading
Define Seafloor Spreading
Lateral movement of new oceanic crust away from a mid ocean ridge
What evidence did Hess use for his idea of sea floor spreading?
The age of ocean floor basalt
What data from the ocean floor did Vine use?
Magnetic reversals of polarity on the sea floor
Define Core
The metallic centre of the earth, composed of mainly iron and nickel
What state is the inner core?
Solid state (despite being more than 3,000 degrees)
What state is the outre core?
Liquid state
How thick is the mantle?
2,900km
What is the mineral composition of the mantle?
Iron and magnesium rich
What is the main rock type of the mantle?
Peridotite
Define Crust
The outer layer of the earth which varies in characteristic depending on whether it is continental or oceanic
What is the thickness of continental crust?
35km
What is the thickness of oceanic crust?
5-70km
What is the density of the mantle?
3.3g/cm3
What is the composition of continental crust?
Granite
What is the composition of the oceanic crust?
Basalt
What is the density of the continental crust?
2.7g/cm3
What is the density of the oceanic crust?
2.9g/cm3
Define Asthenosphere
The semi-molten or viscous layer that extends from 100km to 300km deep
Define Lithosphere
The rigid layer comprising of crust and the upper mantle above the asthenosphere
Define Mohorovicic Discontinuity (Moho)
The boundary between the crust and the mantle
How old is continental crust?
Up to 3.8 billion years old
How old is oceanic crust?
Up to 220 million years old
Define 1300 Degree Isotherm
The boundary between the lithosphere and asthenosphere
Who developed the convection current theory?
Arthur Holmes
What are the two sources of internal heat to generate convection currents in the mantle?
Heat remains from the formation of the Earth, Decay of radioactive elements - U, Th, K
Define Ridge Push
Upwelling of magma at the mid ocean ridges creates new ocean crust causing the plates to push apart
Define Plate Drag
Convection currents in the asthenosphere drag the overlying plate with it
Define Trench Pull
The weight of the dense, cold oceanic crust subducting into the asthenosphere at an oceanic trench pulls down the rest of the plate with it
Define Basaltic Magma
A type of magma which is low in silica and flows easily
Define Decompressional Melting
A reduction in overlying pressure enables the peridotite to melt, leading to magma formation
Define Fault
Cracks in rock due to stresses in the earth’s crust
What four features are associated with divergent plate boundaries?
Mid Ocean Ridges, Rift Valleys, Pillow Lavas, Black Smokers (Hydrothermal Vent)
Define Mid Ocean Ridge
The boundary between two diverging plates consisting of two parallel chains of submarine mountains separated by a rift valley
How are MORs formed?
As magma rises to the surface, the overlying rock thermally expands and is forces upwards into a dome
What is the total length of a MOR?
60,000km
What is the approximate height of a MOR?
300m
What is the approximate width of a MOR?
1,000km
What is the spreading rate of the Mid Atlantic Ridge?
2-3cm/year
What is the spreading rate of the East Pacific Rise?
16.5cm/year
What faults occur at MORs?
Transform faults
Define Transform Fault
Large-scale faults in the crust at right angles to a mid ocean ridge, Earthquakes occur along their lengths as they slip
Define Seamount
Submarine volcano
Define Pillow Lava
Rounded mounds of lava erupted along mid-ocean ridges which cool rapidly on contact with sea water
Define Black Smoker
Hydrothermal vent on the ocean floor, The water carries high amounts of metal sulphides
Define Rift Valley
A valley formed by down faulting between parallel faults on MORs and at a divergent boundary on land
What faulting occurs at rift valleys?
Normal faulting
Define Normal Fault
A fault in which blocks of rocks slip straight down due to tension in the lithosphere
Define Graben
The downfaulting section of a rift valley
Where do grabens occur?
Rift valleys
What % of the world’s active volcanoes occur at convergent plate boundaries?
80%
Define Andesitic and Rhyolitic Magma
Two types of magma which are high in silica and viscous (slow moving)
What faulting occurs at convergent plate boundaries?
Reverse faults
Define Reverse Faults
A fault in which one block of rock slides over another due to compression in the lithosphere
What four features are associated with O/C convergent plate boundaries?
Earthquakes, Ocean Trenches, Fold Mountains, Volcanoes
Define Ocean Trench
A narrow, deep depression on the ocean floor adjacent to a subduction zone
How deep do ocean trenches go?
6,000 - 11,000m
What are the dimensions of the Peru-Chile trench?
5900km long, 8km deep, 64km wide
Define Benioff Zone
Boundary between a subducting ocean plate and the over-riding continental plate at a destructive boundary where earthquakes take place, Has an angle of 45 degrees
Define Fold Mountains
A mountain built up by the crumpling (folding) of crustal rocks, Associated with convergent boundaries
What four features are associated with O/O convergent plate boundaries?
Ocean Trench, Island Arc, Earthquakes, Volcanoes
Define Island Arc
Chain of volcanic islands formed along a subduction zone
What two features are associated with C/C convergent plate boundaries?
Fold Mountains, Earthquakes
Define Tectonic Plate
A large slab of the Earth’s lithosphere and crust
Define Subduction
The tectonic process found at destructive plate margins where an oceanic plate descends into the Earth’s mantle and is destroyed
Define Divergent Boundary
Two tectonic plates move away from each other
Define Conservative Boundary
Two tectonic plates slide past each other
Define Convergent Boundary
Two tectonic plates move towards each other
