Lithosphere 1- plate tectonics Flashcards
What is plate tectonics?
The concept that the Earth’s lithosphere is divided into separate, rigid plates, that move independently of one another
How many major tectonic plates are there?
12
What are the 3 types of plate boundary?
Transform
Convergent
Divergent
What is the movement at a transform plate boundary like?
plates slip by one another along deep crustal fractures
What happens at divergent plate boundaries?
Creation of oceanic lithosphere by sea floor spreading (mid ocean ridge)
What happens at a convergent plate boundary?
destruction of oceanic lithosphere by subduction along a deep ocean trench
What is the lithosphere?
rocky outer part of earth
What are conditions in the lithosphere like?
coolest
most rigid
What does the lithosphere overly?
weak asthenosphere
that behaves in a ductile way
What areas are encompassed by the lithosphere?
mountains
rocks
soil
minerals
seafloor
Where is the crust?
uppermost part of the lithosphere
What is the difference between the crust and the mantle?
Mantle is Ultramafic
Crust contains higher silica
What is the silica content of the mantle?
<45%
What does ultramafic mean?
igneous rock with very low silica content
What 2 types of crust are there?
Oceanic and continental
What is the thickness of continental crust?
25-60km (thickest in mountain belts)
What can make the complex structure of continental crust vary?
setting/ location
What is the average density of continental crust?
2.7g/cm*3
What is the rock make up of the crust especially at the surface?
Granitic (Si, K, Na)
What does the density of continental crust suggest?
overall intermediate composition
What is the average thickness of oceanic crust?
5km
What is usually covering oceanic crust?
4km of water
What is the average density of oceanic crust?
2.9g/cm*3
What is the general make up of oceanic crust?
Basalt
Gabbro - (mafic/ low silica)
what is isostacy?
state of gravitational equilibrium between Earth’s crust and mantle such that the crust “floats” at elevation that depends on thickness and density
What can the lithosphere be called within the earth system?
central sphere
What is an example for evidence for plate tectonics?
continental drift
What is the old term for continental drift?
the movement of Earth’s continents relative to each other
What is continental drift a result of?
plate tectonics
Who recognised that Africa and south America fit together?
Ortelius (1595)
Who was mainly responsible for the development of the continental drift theory?
Alfred Wegner
What fossil evidence did Wegner have for continental drift?
Mesosaurus
Cynognanthus
Lystrosaurus
Glossopteris (plant)
What 2 types of evidence did Wegner have for continental drift?
Rock
Fossil
How is the mesosaurus fossils evidence for continental drift?
Meso is freshwater reptile which is fossils can be found in Africa and South America which are sperated by saline ocean and have vastly different present habitats
What was the widespread karoo glaciation?
Evidence of ice sheet retreat in Karoo basin central Africa
What rock and fossil evidence is there for continental drift?
Evidence widespread Karoo glaciation
Matching Permian fossils
Matching mountains and faults
Matching climatic evidence
What problem did Wegner have proving his continental drift theory?
could not think of a mechanism to account for his observations (led to geological community generally rejecting his theory)
What did scientists eventually discover about the age of oceanic crust?
ages of ocean crust varied globally but everywhere was typically <200 Million years old
Subduction zone
What did the age distribution of Earths ocean floor reveal?
Divergence at mid-ocean ridges
How will age affect the thickness of seafloor sediment?
Thinnest where youngest
Thickest where oldest
What do magnetic stripes/ minerals provide a record of?
Earth’s reversing magnetic poles
What will new oceanic crust polarity be?
magnetic signature at the time of formation
What is another name for earthquake epicentres?
foci
What can earthquake foci reveal?
plate boundaries
How to divergent boundaries form?
through continental rifting and the opening of a new ocean basin
What is an example of a divergent boundary?
Silfra fissure in Iceland forms part of the boundary between the North American and the Eurasian tectonic plates.
How much do North American and Eurasian plates drift apart per year at the Silfra boundary?
2cm
What is an “on land” example of a divergent plate boundary?
East African rift valley
What is formed when oceanic subducts under oceanic lithosphere?
Island arcs
What forms oceanic- continental subduction?
Continental arc
What occurs at subduction zones?
sinking oceanic oceanic lithosphere
What associated processes occur at subduction zones?
Earthquakes
Metamorphism
Fluid release
Induced convection
Melt generation
What do subduction zones create?
Arc-trench systems
What is the distribution of volcanoes closely associated with?
Trench positions
What happens to oceanic lithosphere during continental collision?
consumed
What is thrust tectonics?
shortening and thickening of the crust or lithosphere common to form mountain belts
What are transform plate boundaries delineated by? (located)
transform faults
What can the effect of transform boundaries do t mid-ocean ridges?
Can offset mid ocean ridges and form major crustal features on land e.g. San Andreas Fault, California
What occurs at transform and sedimentary basins?
localised zones of compression and extension
What are hotspots?
Isolated area of volcanism away from plate boundary associated with mantle plumes
What do hotpot volcanoes form over time?
Ocean island chains like Hawaii
Where can oceanic crust be sampled?
cyprus
How thick can the lithosphere be?
100-200km
What is buoyancy?
natural tendency of thigs to float in fluid
What is isostatic rebound?
When a large ice mass has retreated from an area and the ground “bounces back”
What are diffuse plate boundaries?
Ones which have contact for hundreds if km
Why will the height of oceanic crust reduce as you move further from a mid ocean ridge?
as the rock here has had more time to reach buoyancy
What does it mean to say a subduction zone is dynamic?
there is a flow of energy and matter
How can the groupings of continents affect sea level
if there are grouped closely sea level is low if they are apart sea level is higher
