1a+b- Evidence For Continental Drift + Tectonics Flashcards
What are the 3 main components of earth
Crust
Mantle
Core
Describe properties of lithosphere + asthenosphere
Lithosphere = crust + upper solid mantle ( crust - 100km)
- rigid layer
Asthenosphere = below lithosphere (100-300km)
- semi molten
Name of super continent and geographer who came up with idea of Continental Drift
- Pangea
- Alfred Wegner
- idea of continents moving apart from super continent 250 million years ago
What Previous Geological Evidence was there for Plate tectonics
- Shape pf continents together - evidence for Pangea eg: S America and Africa
- Striations of glaciers evident in Africa and S America ect = shows that continents must’ve been at polar regions once
How does palaeomagnetism prove plate tectonics - Recent Evidence
Oceanographers towed magnetometers behind boats
- As plates move apart New land forms
- earths polarity changes every 300,000 years = recorded in new rock formed
- Lava erupting from Constructive margins recorded earths polarity at that time
- strips of rock with same magnetic orientation occur symmetrically either side of ridge = evidence of sea floor spreading
How does Sea Floor spreading give evidence for plate tectonics - Recent Evidence
Older Rock found further inland while newer rock found out to sea = Discovered that Rock in Ocean was no more than 200 million years old which is young compared to the continents
Additionally Discovery of large scale Ocean trenches suggesting that Crust was being subducted and destroyed = provided evidence for why earth was not growing despite new land being formed
What was Hugo benioffs theory
- the reason the world was not growing despite new land being formed (constructive margins)
- oceanic crust was being destroyed/ melted
-“Benioff zone”
Differences between oceanic + continental crust including km of thickness
Oceanic= denser- due to ocean pushing down + compressing
- 6km thick
Continental= less dense= due to high silica content (60%)
-70km thick
3 ways plates move
- convection current
- Slab pull
- ridge push
Why is the core hot
- heat from collisions with rock when formed
- radioactive decay
Mechanism of convection currents
- magma heated by core = particles have more energy + expand= less dense
- less dense mantle rises
- cool down at lithosphere + spread out = move plates
- sinks as cools due to more dense
What is the split mantle theory
- 2x sets of convection currents ( 1 in upper 1 in lower mantle)
- reflects the characteristics
Ridge push theory
- high land at ridge created as magma cools
- heavy land “pushes” land apart
Slab pull theory
- plate subducts
- as plate is “pulled under” next bit is pulled down
- plates are very heavy = drags rest of plate down
What is the biggest component of structure of earth
- mantle
-68%
What happens at destructive boundary
(Convergent)
- subduction occurs due to difference in densities
- oceanic plate subducted= deep ocean trench
- earthquakes on Benioff zone= release of seismic energy from friction
- melting of plate = volcanos
Why are mantle + inner core solid despite high temp
High pressure stops melting
What is required for a volcano
- molten rock
Constructive = magma from mantle
Destructive = magma from melted plate
Why does Rock melt at constructive + destructive boundary’s
Constructive= low pressure in gap between plates = rock can melt
Destructive= plate subduction drags down volatiles with it (CO2 + H2O) to asthenosphere
- volatiles lower melting point of rock
Properties of basic lava
- divergent/ constructive p boundary
- decompression melting
- asthenosphere melts
- low silica content = low viscosity
- gasses vented easily/ no pressure
- balsatic rock
- very high temp = from mantle
- gentle/ effusive + frequent eruptions
- gently sloping shield volcano
Properties of acidic lava
- destructive/ convergent P boundary
- volatiles for melting
- U mantle + crust (asthenosphere, lithosphere, oceanic crust) melt
- high silica content= high viscosity
- gasses trapped until eruption
- Andesitic + Rhyolitic rock
- lower temp for melting
- violent + explosive eruptions
- steep sided + composite volcano
What factors effect explosivity of eruptions
- high viscosity of lava = sticky
- cooler temp of magma
- lots of gas bubbles = high pressure
What is VEI
Volcanic explovity index
- measures magnitude + speed
Magnitude= how much material
Speed of eruption= intensity
Why is there a build up of pressure at destructive boundary’s
- magma must force its way though Lithosphere + crust above = high pressure
Why does acidic lava produce composite volcanoes
- sticky lava moves slowly + does not Travel far from volcano
- build up of material = composite volcano
What happens at a collision boundary
- little difference in density results in no subduction
- plates compressed and folded = fold mountains
- plates can “grind” together causing earthquakes
Why do convection currents move plates
- magma rising up pushes plates apart as convection currents circulate
What happens at conservative plate boundary
- plates slide past each other
- no subduction = no volcanic activity
-friction between rocks = build up of pressure - can leave “tear” in landscape where plates have moved
- can affect river course or drainage patterns
Why does Thickness of Ocean Layer increase as you move further from constructive plate margin
and
Why does Oil/Coal in Antarctica show that plates have moved
Cooling of the magma that comes out of the mid-ocean ridge = As it cools, it becomes denser and thicker
also accumulation of more sediment on plate showing evidence its been there longer
Oil/Coal requires tropical climates and dense vegetation to form = shows its moved
Asthenosphere vs Lithosphere
Both part of Upper Mantle
Asthenosphere is below lithosphere and is more fluid whereas Lithosphere is above and is made of Crust and Upper most mantle - Rigid
Describe a Divergent/Constructive boundary
Magma rising from Lithosphere Forces Plates to move apart as Convection Currents
Gap created has lower pressure due to No rock above = Mantle can melt and act as Liquid rising through Gap
Mantle rises and is instantly Cooled by Ocean = New land
Why can you get earthquakes at Divergent/Constructive margins
Rising magma puts pressure on Dome created by rising magma at C-C , Rocks fracture producing rift valleys = frequent and low energy Es
Transform faults formed from moving plates = conservative margins +Es
Activity at Hotspots
Hot spot is a particularly Hot section of Mantle = Can Melt through crust causing Volcanic activity
Movement of plates transport Volcano and Land form away from Hotspot eg: Hawaii
Previous Biological evidence for Plate Tectonics
Similar Fossils of Marine Shellfish/Brachiopods in Australia and Indian Rock = Brachiopods are small shellfish , Unlikely that they would cross large stretch of ocean = Suggests continents were once much closer together
-
Similar Reptiles/Mesosaurus fossils in S America and S Africa = reptile are freshwater so could not have crossed ocean = Continents must’ve once been together
How Long and tall is the Mid-Atlantic Ridge
1000 miles long and 2500km high
How deep is the Mariana Trench
12km
Describe how Oceanographers discovered Sea Floor spreading and Palaeomagnetism
Paleomagnetism = Towed Magnometers behind boats
Sea Floor spreading = Probed the crust in oceans to discover that it was thicker and Older away from Ridge
