Plate tectonics Flashcards
Athenosphere
The part of the mantle which lies directly beneat the lithosphere made of molten and semi molten rocks
Lithosphere
The layer of earth consisting of the rigid crust and upper section of the mantle
Key facts about structure of earth
- Helps to understand why tectonic hazards occur
- Theory of plate tectinics has revolutionised
- Earths outershell is the curst and thickness between 5 and 10km beneat the oceans to nearly 70km under the continents
- Rocks in upper mantle are solid anf sit on top of athenosphere, softer almost plastic like rock which moves very slowly
- As earth rotates liquid outercore spills, earths magenetic field
- Cores internal heat is major cause fo earths tectonic activity
- Some heat may primeval - retained from the ball of dust and gas from which the earth evolved
- Earths greatest source of heat energy is from radioactivedecay (natural radioactive decay from euranium, thorium, potassium and other elements provides a continoous buy slowly diminishing heat
- Heat at core generates convection currents within mantle
oceanic crust
the outermost layer of Earth’s lithosphere that is found under the oceans and formed at spreading centres on oceanic ridges, which occur at divergent plate boundaries. Oceanic crust is about 6 km (4 miles) thick. It is composed of several layers, not including the overlying sediment.
An occasionally broken layer of balsaltic rocks known as sima ( made up of silicia and magenesium)
continental crust
Thicker crust up to 70Km Below continents
Bodies of mainly granictic rocks known as sial (silia and alumnumin)
core
The core is the centre and hottest part of the earth - reaches up to 6000 degrees celcius and is mostly made up of iron and nickel and is 4x more dense than the crust
inner core
inner core = solid and made up of iron nickel alloy
What is sea floor spreading?
Sea-floor spreading = Movement of oceanic crustal plates away from constructive plate margins
Who produced the idea of sea floor spreading and when?
-Harry Hess in 1962 and this helped support wegeners theory of continental drift
-Hess discovered that rocks increased in age with distance from mid ocean ridges and that new oceanic crust were therefore being formed when plates diverged
- Hess theory was backed by the magnetic dating of these rocks (paleomagnetism)
-
Facts about sea floor spreading?
- 12,000ft long mountain range formed as mid alantic ridge a ridge of underwater volcanoes
-Molten rock pushed up from inside earth at the ridge new crust new ocean floor - Rate of spreading is expected to be up to 5cm per year
What is Contitnental drift?
Continental drift = the movement of tectonic plates due to varying weights of crust
Slab pull is the primary driving force of plate movements
What is paleomagnetism?
Paleomagnetism = Measurement of the magnetism preserved in older rocks
Facts about paleomagnetism?
-Approximatley every 400,000 years the earths magnetic field swtitch polarity between north and south
-Studies in 1960s of magnite ( iron oxide) produced from the balsatic lava along mid ocean ridge records the earths magentic orientation at that time
-Both age and magnetic orientation and symetrical suggesting that the oceanic crust is slowly spreading away from this boundary and therefore supporrs theory of contineyal drift
-
What is mantle convention?
Mantle convention = the rising and falling of magma within the asthenosphere
Facts about the asthenosphere
part of the mantle that lies directly beneath the lithosphere
Gravitational sliding
Gravitational sliding = the movement of tectonic plates as a result of gravity at mid-ocean ridges and deep sea trenches (ridge push and slab pull)
Ridge push
Ridge push = the higher elevation at an ocean ridge causes gravity to push down and drag crust away from the ridge
Slab pull
Slab pull = The process whereby , following subduction, the lithosphere sinks into the mantle under its own weight ‘pulling’ the rest of the plate with it
How does ridge push occur?
Ridge push is a result of gravitational forces acting on young oceanic lithosphere, causing it to slide down the asthenosphere and push on the lithospheric material further away from the ridges
Magma rises as the plates move apart. The magma cools to form new plate material. As it cools It becomes denser and slides down away from the ridge. This causes other plates to move away from each other.
How does slab pull occur?
Slab pull is a force that results from denser oceanic plates subducting beneath less dense continental plates. The gravitational force of the sinking oceanic plate drags the rest of the plate along with the portion experiencing slab pull
What are plate margins?
Plate margins = the place where two plates meet and the crust either side of this is the junction
What are constructive (divergent) plate margins?
Constructive plate margins = when two plates separate (diverge) and form a constructive margin
What are the two types of divergence?
mid - ocean ridges , rift valleys
How do mid ocean ridges form?
- Forms thousands of kilometres across the floor of submarine mountains
- Transform faults cut across the ridges, occurring at a right angle to the plate boundary, separating sections of the ridge
- Can rise up to 4000m above ocean floor
-Volcanic eruptions along ridges build
How are rift valleys formed?
- Formed when the lithosphere stretches causing it to fracture into sets of parallel faults
- The land between these faults then collapse into deep wide valleys that are separated by upright blocks of land called horsts
What are destructive (convergent) plate margins?
Destructive plate margins = when two plates collide they form a destructive plate margin
What are the three types of destructive plate margins?
- Oceanic meets continental
-Oceanic meets oceanic
-Continental meets continental
Oceanic meets contitnental?
- The meeting of two plates is associated with subduction
- Oceanic plate is denser than lighter contitnental plate so it subducts beneath it
- Exact point of collision is marked by bending of the oceanic plate to form a deep ocean trench (e.g. peru -chile trench)
- As two plates converge the contitnental land mass was uplifted, compressed, bucked and folded into **chains of fold mountains **
- Continuous compressing, simple folding can become assymetrical then overfolded making a **recumbent fold **
- Nappe can occur if increased compressing makes middle section thin
- Descending oceanic plate melts at depth beyond 100km and completley destroyed by 700km
- Friction may lead to stresses building up which may be released as **intermediate or deep focus earthquakes **
- Melted oceanic creates magma which rises in great plumes passing through cracks and faults
What is the zone of melting?
Benioff zone - caused by both increasing depth and friction
Oceanic plate meeting oceanic plate
- One plate (faster or denser) subducts beneath the other
- Leads to formation of a deep ocean trench and melting
- Rising magma from benioff zone forms crescents of submarine volcanoes along plate margins which may grow to form **island arcs **
Mantle convention
The rising and falling of magma within the athenosphere
Two plates sliding **past **eachother
Conservative plate margin
- Crust is not destroyed or subducted
- No melting rock which results in no volcanic activity or formation of new crust
- Very active margins and associated with powerful earthquakes
- Friction between two moving plates leads to stresses building up wherever sticking occurs
- Stresses may be released as shallow focus earthquakes such as LA (1994), SF (1906, 1989)
What is Magma
Magma= extremley hot liquid and semi-liquid rock located under earths surface
Magma plumes
- Hotspots form around core if decay is concentrated
- Heat in lower mantle creates localised thermal currents where magma plumes rise vertically
Usually located at plate margins but can be risen within centre of plates and burn through lithosphere to create volcanic activity on the surface - Hotspots remain stationary, movement of overlaying plate results in formation of chain of active and extinct volcanoes as plate moves away
What are conservative plate margins?
Conservative plate margins = where two plates slide past eachother causing earthquakes to occur
Continental meets continetal
- Continental are of lower density than the athenosphere beneath them which results in no subduction occuring
- Colliding plates become uplifted and buckle to form** high fold mountains **
- No volcanic activity as there is no subduction but **shallow focus earthquakes **can be triggered
- Young fold mountains continually growing and compressing
What is a hotspot?
An intraplate volcano caused by a magma plume
What are transform faults?
The great african rift valley
An example of divergent plate boundary that occur in the middle of continents
May eventually form a new ocean as eastern africa splits away from the rest of the continent
outer core
outercore = semiliquid and is mainly iron
2400-kilometre-thick band around the internal center. Rock matter at the highest point of the external center has a thickness of around 10 grams for every cubic centimetre
What is the collison zone?
Collison zone = A type of convergent plate margin where two continental plates collide to form mountains
Extrusive rocks
A type of igneous rock formed on the surface of the Earth from lava, which is magma that has emerged from underground
Intrusive rocks
Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet.
Igneous rocks
Form from the crystalisation of magma
Plate margin
The place where two plates meet and the crust either side is the junction
Plate tectonics
Plate tectonics = A theory developed more than 60 years ago to explain the large scale movements of the lithosphere
supported by evidence of sfs
Transform faults
A type of strike slip fault and the boundary between two plates of the fault
Example for hotspots
Hawaii around pacific ring of fire
Example of fold mountains
The himalayas
Move 5cm every year
