What is the evidence for continental drift and plate tectonics? Flashcards
(26 cards)
Basic structure of the Earth
- There is a CONCENTRIC (circular) structure to the Earths interior
Three primary concentric layers:
- Core
- Mantle
- Crust
Crust
Two types:
CONTINENTAL:
Density ~ 2.6-2.7 kg/m^3
Thickness ~ mean 35km
Mineral composition ~ granitic, silicon, aluminium
OCEANIC:
Density ~ 3.0kg/m^3
Thickness ~ 5-10km
Mineral composition ~ basaltic, silicon, magnesium
(Together the lithosphere and crust make up these oceanic and continental plates)
Moho
- The distinct boundary that separates the CORE and MANTLE
- DEPTH ~ 2900 km
Depth below:
Continents ~ 35km
Ocean ~ 5-15km
Mantle
Thickness ~ depth of 2900 km
Density:
At Moho ~ 3.3 kg/m^3
At core ~ 5.6 kg/m^3
Mineral composition ~ Magnesium & iron
Mantle ~ TWO LAYERS
ASTHENOSPHERE:
- Extends from 100km down to 300km
- Solid but flows under pressure
CONVECTION CURRENTS within it:
- caused by heat generated deep in the mantle
- causes the asthenosphere to flow, carrying the solid lithosphere & crust with it
LITHOSPHERE:
- RIGID layer sandwiched between the crust and asthenosphere
- Varies in thickness
BOUNDARY BETWEEN THEM:
- Difficult to define as pressure increases with depth
- The rock becomes less rigid & able to flow
Who was ALFRED WEGENER and what did he propose?
- A meteorologist 1912-1930
- Made a comprehensive case for CONTINENTAL DRIFT
- Proposed that in the CARBONIFEROUS period, 250 million years ago, a large single continent existed ~ PANGAEA
- This slowly broke apart into TWO large land masses
- Movement continued to the present day as the continents separated & spread across the globe
- REJECTED by geologists ~ believed in their own ‘FIXED EARTH’ model
Alfred Wegener Theory ~ GEOLOGICAL & BIOLOGICAL evidence
GEOLOGICAL:
- The fit of continents ~ e.g South America & Africa on either side of the Atlantic
- Mountain chains and rock sequences are either side of oceans are similar
BIOLOGICAL:
- Similar FOSSIL animals found in South America and Australia
- Fossils from BEFORE Carboniferous period show FEWER similarities, suggesting they followed different evolutionary patterns , e.g Australia & India
- Similar fossil MARINE SHELLFISH found in Australian & Indian LIMESTONE.
Palaeomagnetism
MID-ATLANTIC RIDGE ~ discovered accidentally by echo sounders used to locate submarines
Subsequently, a PATTERN OF MAGNETISM was discovered in the basaltic rocks making up the sea floor
Showed mirror images of repeating STRIPES as you move away from mid-Atlantic ridge
- The stripe reflected that the Earth’s polarity is NOT CONSTANT
- Every 400,000-500,000 years , its polarity changes ORIENTATION, which is recorded within the rocks of ocean floor
-
Continental Drift ~ EVIDENCE
- IRON minerals within rock, are positioned to indicate the NORTH pole at the time of their formation
- This is ‘locked in’ as the rock SOLIDIFIES
- Rocks that formed at the same time on either side of the ATLANTIC.
- Pointed to different location separated by thousands of Km
- These locations could be pulled together by closing the Atlantic Ocean
The GLOBAL patterns of plates and plate boundaries ~ 3 Types
- SEISMIC data worldwide showed that most earthquakes, were SPATIALLY concentrated in NARROW BANDS
- Suggested that the rigid LITHOSPHERE and CRUST were broken up into TECTONIC PLATES
THREE TYPES :
- Divergent/constructive
- Convergent/destructive
- Conservative
Distinctive landforms & landscapes develop along these
Divergent/constructive Plate boundaries
- Locations where plates are MOVING APART
- Magma is rising through the asthenosphere and forcing its way to the surface
Landforms:
- Mid-ocean ridges
- Transform faults
- Pillow lavas
- Black smokers
- Underwater rift valleys
- Rift zones
Mid-ocean ridges
- Remain hidden at an average depth of 2.5km below the ocean
- Consist of very long chains of MOUNTAINS ~ rise 300m above sea bed
- These SUBMARINE MOUNTAIN chains have a combined length of 60,000km
Transform Faults
- Break mid-ocean ridges into SEGMENTS at frequent intervals
- Displace the ridge LATERALLY
- Volcanic activity is ABSENT ~ it is on conservative margins
- As they slip, energy is released in the form of EARTHQUAKES
Rate of spreading at Mid-ocean Ridges
- Determines their TOPOGRAPHY
- Depends on the amount & rate of magma brought to the surface by CONVECTION CURRENTS
FAST:
- 16.5 cm per year
- No Rift Valleys
- Smooth mountain chains
- Example ~ East Pacific Rise
MEDIUM:
- 5-10 cm per year
- Poorly defined Rift Valleys
- Relatively smooth mountain chains
- Example ~ Galapagos Ridge
SLOW:
- 2-3 cm per year
- Clearly define central Rift Valleys
- Steep slopes to mountains chains
- Examples ~ Mid Atlantic Ridge
Pillow lavas
- Eruption of magma occurs mostly UNDERWATER
- Maga erupting directly onto the sea bed is COOLED RAPIDLY
- Forms ROUNDED MOUNDS
Underwater Rift Valleys
- As magma rise towards the surface, pressure reduces & it LIQUEFIES
- LITHOSPHERE is placed under STRESS and eventually FRACTURES
- Overlying rock is forced upwards
- Produces underwater Rift Valleys , found at MID-OCEAN RIDGES
Example ~
North Atlantic ~ The worlds largest Volcanic Island ~ ICELAND
Black Smokers
- At mid-ocean ridges, sea water seeps into RIFTS and is SUPERHEATED
- As it rises towards the surface, CHEMICAL CHANGES occur in the basaltic rocks
- Superheated JEST of water sometimes RE-EMERGE on the ocean floor
- Contain METAL SULPHIDES
- These support organisms & ecosystems
Rift Zones ~ Continental Crust
Example
- Rift zones are NOT confined to the ocean floor
- The continental crust must THIN considerably for RIFTING to occur
Example ~ Rift stretching from THE RED SEA northwards to TURKEY
- Crust has been uplifted & stretched
- Caused FAULTING
- Formation of a sunken valley ~ GRABEN
- As the rift WIDENED, magma erupted at the surface
- Eventually , the Rift Valley sank below sea level, forming the present day RED SEA
Convergent/Destructive Boundaries
- When plates MOVE TOWARDS each other
THREE TYPES:
- Oceanic-continental
- Oceanic-oceanic
- Continental-continental
Oceanic-Continental Plate Margins
SUBDUCTION ~ the DENSER oceanic plate is forced under the continental plate
ANGLE ~ 30-70 degrees
- The oceanic plate comes under intense PRESSURE and FRICTION
BENIOFF ZONE ~ faulting & fracturing occurs, releasing energy in the form of EARTHQUAKES
LANDFORMS:
- Ocean Trench
- Fold mountains & Volcanoes
Ocean trenches
- A DEEPENING of the ocean at the plate boundary
- Long, narrow depressions with depths of 6000-11,000 m
- ASSYMETRIC ~ steepest side towards the continent
- SLAB PULL ~ where the oceanic crust descends into the asthenosphere, pulling the rest of the ocean plate with it
- The LEADING EDGE of the overriding plate is buckled to form a TRENCH
Fold Mountains & Volcanoes
- Oceanic plates contain substantial amounts of WATER
- During subduction , DEHYDRATION takes place
- Triggers the MELTING of the overlying plate
- This MOLTEN material rises towards the surface as PLUTONS of magma
- The huge INTRUSIONS of magma create further uplift of FOLD MOUNTAINS & form VOLCANOES
Oceanic-Oceanic Plate Margins
- The OLDER and therefore DENSER plate is SUBDUCTED under the other
- Forms a TRENCH
- The plate descends & DEHYDRATION takes place.
- Partial MELTING of the mantle wedge in the plate above
- MAGMA rises to the surface forming …
- ISLAND ARCS ~ chains of volcanic islands
- EARTHQUAKES are extremely COMMON
Ring Of Fire
- The northwest zone of the pacific
- Contains Island arcs
Example ~ Aleutian Islands ~ extends westwards from Alaska
