Plate Tectonics Flashcards
What is a Geoid and how is it formed?
Bulging at equator due to centrifugal forces (kids on a roundabout)
What is the oceanic crust?
5-10km, occasionally broken layer of basalt known as ‘Sima’ – Silica and Magnesium
What is the continental crust?
up to 70km, mostly granite known as ‘Sial’ – Silica and Aluminium
Difference between ‘Sial’ and ‘Sima’
‘Sial’ (Conti) is less dense and thicker, found above ‘Sima’ (Ocean)
What is the Lithosphere?
Tectonic plates are formed here
upper-mantle + crust (100km)
What is the Mantle?
(2900km) – great heat and pressure results in liquid dense the deeper you go
Core Characteristics
Core (5000’C) is 4 times denser than crust
Outer Core characteristics
Outer Core – semi-liquid=Iron
Inner Core Characteristics
Inner Core=Iron + Nickel
Convection currents are causes by..?
The heat generated by the core creates convection current
Who and when - Theory of Continental Drift
Alfred Wegner – 1912
6 Things the theory of continental dift is based on:
1) Jigsaw fit of SA and A
2) Matching rock in East Cnd to NW Sco
3) Coal beneath Antarctica
4) Fossils in India matched in Aus
5) Unique Mesosaurus in A + Bra
6) Glacial deposits in Bra match west A
Scientific Community reaction to Theory of Continental Drift
Evidence but no explanation dismissed his theories
Theory - 1950’s?
50s – nuclear submarines – Oceanic Floor Mapping
1962?
Sea floor spreading – youngest rock in the middle of Atlantic, newest in Iceland
Palaomagnitism?
During Polarity every 400,000 years, there were eruptions of magnetite, Symmetrical patterns either side of the Atlantic ridge – confirming sea floor spreading
Explain Convection Currents
Plates are driven by convection currents which rise and spread in the asthenosphere before cooling and sinking back down
Oceanic Crust Contructive Formations
Mid-Ocean Ridge
Faults cut across separating sections of the ridge widening at different rates, creating shallow focus earthquakes
Rift valleys in the middle of ridges
Submarine volcanoes can eventually create volcanic islands
Continental Crust Contructive Formations
Rift Valleys
Lithosphere splits – fractures in parallel faults
Oceanic > Continental Destructive Formations
Denser Oceanic Plate subducts beneath Continental = deep ocean trench
Continental land is uplifted, compressed and folded > chains of fold mountains
Oceanic land is destroyed/melted between 100-700 km (Benioff Zone) due to heat and friction which may also cause deep-focus earthquakes
Turned into magma, oceanic rock is less dense than asthenosphere =rising in faults where the continental plate has buckled, before reaching the surface to form explosive volcanic eruptions
Oceanic > Oceanic Destructive Formations
The faster/ denser plate subducts > deep ocean trench
Rising magma melted in the Benioff zone > volcanic island arcs
Continental > Continental Destructive Formations
Subduction does not occur because continental is not dense enough > High Fold Mountains
Shallow focus earth-quakes can be triggered
Conservative Formations
Crust is not destroyed and no subduction > no volcanic activity + no formation of new crust
Extremely active
Shallow Focus Earthquakes when sticking occurs and stress builds
Contructive Crust and Volcanoes
Oceanic crust is formed by basaltic magma rising from the asthenosphere
Basic submarine volcanoes
Volcanic islands
Destructive Volcanoes
Explosive acid volcanoes where there is subduction
Conservative Volcanoes
No subduction > no Volcanicity
Hot Spots
When radioactive decay at the core is concentrated, it can create localise thermal currents where plumes of magma rise vertically
The hotspot remains stationary, whilst the plate moves resulting in chain islands
Dykes
When magma solidifies in a vertical fissure
Once solidified into igneous rock, it is usually more resistant to erosion, leaving a prominent wall-like feature
Sills
Forms when magma solidifies into horizontal/inclined layers of existing rock
Laccolith
Form where viscous magma forcing into overlying rock strata to arch into a dome
Batholith
Larger scale where often, the other features will feed off of them
Extreme heat/ pressure (metamorphism)= metamorphic aureole
Extrusive Features
Gaseous emissions – steam, carbon monoxide, hydrogen sulphide, sulphur dioxide and chlorine
Solids – ash, dust, glassy cinders
Liquids – lava bombs (tephra/pyroclasts), solidify mid-air
Basic Lava
(basaltic) is low in silica, allowing gas bubbles to move freely – fluid and free-flowing
Acid Lavas
rich in silica – viscous > gas bubbles cannot expand - build up of pressure = violent eruptions
Basic Shield Volcanoes
Mid-ocean ridge, rift valleys and hotspots – direct access to surface
Repeated eruptions
Runny lava forms gentle sloping volcanic cones and shields
Acid Dome Volcanoes
Destructive margins
Viscous lava cannot flow far before cooling = steep convex cones
Rhyolitic lava is thick and unable to flow
Explosive pyroclastic flows of superheated gases, ash and pumice
Dormant Volcano
Hasn’t erupted within historic times
Extinct Volcano
10,000 years dormant
Composite cones
Alternating eruptions of ash, tephra (incl. lapilli), lava building volcano in layers
Often covered by secondary (parasitic) cones and fissures
Ash and Cinder Cones
Ash, cinder and tephra erupted in the central crater
Steep-sided concave in shape
Calderas
Emptying of the magma chamber allows flanks to cave in creating a vast pit which can be flooded as a sea or lake