Tectonic Processes & Hazards - EQ1 - 1.2 Flashcards
How has the Earth’s structure has been established
- By studying the movement of seismic waves through the planet
- Seismic waves travel at different speeds through material of different densities, and some types (for example, S waves) do not travel through liquids
What is the thickness of the Earth’s crust and what does it form
- This is the thinnest layer and forms the outer shell on which life exists
- It varies in thickness from just 1km in some places to more than 80 kilometres in others.
What temperature is the earth’s crust
400°C at base
What is the density of the earth’s crust
less dense
What is the earth’s crust made up of
Granite/basalt
What state of matter is the earth’s crust
solid
Decribe the 2 types of crust’s depth, rock type & density - continental
- a thicker (45-50km), less dense layer (mostly granite)
Decribe the 2 types of crust’s depth, rock type & density - continental
- a thinner (6-10km), denser layer (mostly basalt)
The crust consists of……&…….&….tectonic plates
7 majorandseveral minor and dozens of small microplates
The……………….. …………….- is the boundary between the crust and the mantle is also known asMoho
Mohorovičićdiscontinuity
What is the Mohorovičićdiscontinuity
the boundary between the crust and the mantle is also known asMoho
The mantle is the …………. layer, being approximately ………. km thick.
The mantle is the thickest layer, being approximately 2,800 km thick.
What is the density of the mantle
- Mixed density layers (less to medium);
- Its density is 3.4g/cm3 at the top
- & 5.6g/cm3 at the bottom where it changes into the outer core
what are the states of matter in the mantle
- It is a solid layer but acts like a viscous liquid due to temperatures being close to the melting point of key minerals in this layer
- The lowermantle is ……. and …….. than the upper mantle
- The intense …….., at depth, keeps the lower mantle …..
- Thelowermantle is hotter and denser than the upper mantle
- The intense pressure, at depth, keeps the lower mantle solid
What is the types of rock in the mantle
- All of this mantle rock is comprised of a variety of oxides
- An example is peridotite
- Their atomic elements include oxygen, silicon and magnesium
The upper mantle has … layers
two
What are the 2 layers of the upper mantle
- The rigid layer above the asthenosphere, which together with the crust, makes up thelithosphere
- Theasthenosphereis a semi-molten, plastic type layer, which moves under high pressure
At the centre of the earth lies the …………
This is split into two sections: ……………….
At the centre of the earth lies the core This is split into two sections: theinner and outer core
State of matter & rock type of the inner core
Theinner coreis solid and is composed of iron and nickel
State of matter & rock type of the outer core
Theouter coreis composed of liquid iron (88%) and nickel
How thick is the inner core
1,221 km
How thick is the outercore core
2,259km
What is the density of the inner core
very dense - 12.8g/cm3 at the top of the section and 13.1g/cm3 at its deepest point
What is the density of the outercore
dense - 9.9g/cm3 at the top and 12.2g/cm3 at the bottom where it changes into the inner core
What is the temperature of the inner core
5150-7000°С
What is the temperature of the outercore
4400°C to 6100°C
Earth’s tectonic plates move at a speed of ……. per year
2-5 cm
Plate tectonics all fit together in a…
constantly moving jigsaw of rigid lithosphere
Each plate is about …… km thick. (With thinner ………………./…………. crust on top.)
Each plate is about 100 km thick. (With thinner oceanic/continental crust on top.)
what do plate tectonics consist of of layers of the earths structure
- Its lower part consists of upper mantle material whilst its upper part is either oceanic or continental crust.
Who discovered the Continental Drift Theory
Alfred Wegner
When did Alfred Wegner discover his contienntal drift theory
1912
What was Aflred Wegner’s continental drift theory
- now-separate continents had once been joined
- This was on the basis that fossils had been found oceans apart
- The fit of continents
- Similar rock formations found on different continents
Who came up with the theory of Mantle Convection & when
Arthur Holmes and Inge Lehmann in the 1930s
In the past, the theory of ………….. ……..was used as the primary explanation for tectonic plate movement.
- convection currents
What is the theory of convection currents 1. heat…
- Heat from radioactive decay of isotopes such as uranium-238 and thorium-232 in the inner core rises into the mantle
What is the theory of convection currents 2. Heat from radioactive decay of isotopes such as uranium-238 and thorium-232 in the inner core rises into the mantle……
- This heat generates convection currents, as the mantle behaves like a viscous fluid, exhibiting both solid and liquid properties under pressure
What is the theory of convection currents 3. This heat generates convection currents, as the mantle behaves like a viscous fluid, exhibiting both solid and liquid properties under pressure……
- The mantle is complicated, consisting of multiple layers with density and mineral differences, so its been suggested that convection is occuring in multiple layers or a single layer
What is the theory of convection currents 4. The mantle is complicated, consisting of multiple layers with density and mineral differences, so its been suggested that convection is occuring in multiple layers or a single layer…..
- As convection cells reach the base of the crust (the Moho), they create friction, which contributes to ridge push—the force that drives mid-ocean ridges apart
- This movement helps drag the crust along in the direction of the convection currents.
What is the theory of convection currents 4. As convection cells reach the base of the crust (the Moho), they create friction, which contributes to ridge push—the force that drives mid-ocean ridges apart
This movement helps drag the crust along in the direction of the convection currents……
- The interior of the Earth is therefore dynamic rather than static
Without this process of mantale convection….
- the geological evidence for ‘continental drift’ was not enough to support the theory
Most tectonic hazards occur…..
- at or near tectonic plate boundaries
What are convection currents
- convection currents are formed due to the hottest molten rock (magma) rising due to extra energy it received in the form of heat and cooler molten rock falls because of the low energy state
- This creates a current as the magma rises, cools then sinks back towards the core
- This process takes place continuously
What is paleomagnetism the study of
- Palaeomagnetism is the study of Earth’s past magnetic field changes, recorded in the orientation of magnetic minerals within rocks
Paleomagnetism
- The Earth’s…..
…… outer core, composed mostly of liquid iron, generates a magnetic field through convection. This field reverses approximately every 100,000 years.
Paleomagnetism
2) …..outer core, composed mostly of liquid iron, generates a magnetic field through convection. This field reverses approximately every 100,000 years……
- In the 1960s, scientists discovered symmetrical magnetic striping on the ocean floor—evidence of past magnetic reversals
- These palaeomagnetic signals confirm that new oceanic crust forms at mid-ocean ridges through seafloor spreading
Paleomagnetism
3) These palaeomagnetic signals confirm that new oceanic crust forms at mid-ocean ridges through seafloor spreading….
- As lava cools and solidifies, its minerals align with the Earth’s magnetic field at that time.
Paleomagnetism
4) As lava cools and solidifies, its minerals align with the Earth’s magnetic field at that time…
- Since new rock is constantly forming on the ocean floor, it preserves a timeline of past magnetic reversals, providing further evidence of seafloor spreading at constructive plate boundaries
Paleomagnetism
5) Since new rock is constantly forming on the ocean floor, it preserves a timeline of past magnetic reversals, providing further evidence of seafloor spreading at constructive plate boundaries
- the mirrored pattern of magnetic directions on either side of mid-ocean ridges proves that the oceanic crust has been continuously diverging
Paleomagnetism
6) The last recorded magnetic reversal occurred …………. years ago, as indicated by ……………..
- 780,000
- geological records
What is the process of subduction
- Convectioncurrents in the mantledragthe overlying lithosphere towards each other
- Asubduction zoneis formed when two plates meet
- The heavier, denser plate subducts under the lighter, less dense plate
- At destructive plate boundaries, denser oceanic plate is subducted under less dense continental plate,
- e.g. at deep ocean trenches, volcanic activity 100 km from the plate boundary on the continental side, and earthquake focal points as deep as 700km along the Benioff zone.
What is the process of slap pull
- As oceanic crust cools, it becomes denser and thicker, and gravity forces the lithosphere down into the subduction zone
- As it sinks, it drags or pulls the plate with it
- This is known asslab pull
What are transform zones
- Conservative plate boundaries consist of transform faults
- These faults ‘join up’ sections of constructive plate boundary as they traverse the Earth’s surface in a zig-zag pattern
How do transform zones cause tectonic hazards
- In some locations, long transform faults act like a boundary in their own right,
- most famously in California where the fault zone (including the San Andreas fault) creates an area of frequent earthquake activity
Earthquakes along conservative boundaries often have…………….., meaning……………
Earthquakes along conservative boundaries often have shallow focal depths, meaning high magnitude earthquakes can be very destructive
Volcanic activity at transform zones?
- Volcanic activity is absent
- The friction causes earthquakes but not volcanoes
- Land is neither made or destroyed
- Because no subduction occurs, there is no melting of the crust and so no volcanic activity
Define ridge push
- Ridge push at a constructive plate boundary may help to move the plates away from the point of divergence
Define mantle super plumes
- Mantle super-plumes - may be responsible for pushing up the crust, causing it to move or split (perhaps splitting Africa and Europe from the Americas)
At destructive plate margins, the plates…..
- move towards each other (converge)
At destructive plate margins, the plates move towards each other (converge). There are …. types of destructive plate margin
3
destructive plate boundaries - oceanic meets continental
- Why does the oceanic plate slide beneath the continental
- Oceanic plate is denser than continental plate, so, when the plates collide, the oceanic plate subdcuts under the continental plate into the mantle and melts
destructive plate boundaries - oceanic meets continental
- What marks the place where the oceanic plate starts to sink
- Deep ocean trenches mark the place where the oceanic plate starts to sink beneath the continental plate (subduction)
destructive plate boundaries - oceanic meets continental
- what forms as a result of subduction
- This subduction also leads to the formation of fold mountains (as the two plates collide, the continental plate is folded and slowly pushed up, forming chains of fold mountains)
- Since the plates are constantly moving towards each other, most fold mountains will continue to grow
destructive plate boundaries - oceanic meets continental
Earthquakes?
- The friction created between the colliding plates (and the resultant subduction) causes intermediate and deep earthquakes in an area called the Benioff Zone
- Earthquakes here occur at a range of focal depths from 10 km to 400 km, following the line of the subducting plate. The Benioff Zone can yield very large earthquakes up to magnitude 9.0
destructive plate boundaries - oceanic meets continental
What are the characteristics of fold mountains
- Fold mountains form the highest of the world’s mountain ranges
- They are long, relatively narrow belts of mountains
- The main fold range is made up of a series of smaller ranges
destructive plate boundaries - oceanic meets continental
Volcanoes?
- Volcanic eruptions occur when magma, formed by the melting of the subducting oceanic plate, rises through faults in the continental crust
- This melting, known as wet partial melting, produces magma rich in gas and silica, resulting in explosive eruptions when it reaches the surface
destructive plate boundaries - oceanic meets continental
These two features make destructive plate margins amongst the….
- …..most seismically active areas of hazard
destructive plate boundaries - oceanic meets continental
What are deep sea trenches
- Deep sea trenches are long, narrow depressions in the ocean floor with depths of over 6km and up to 11km
- Trenches are found adjacent to land areas and associated with island arcs
- The boundary between the Nazca plate and the South American plate is an example
descructive plate boundaries - oceanic meets oceanic
When two oceanic plates collide….
- one plate (the denser or faster) is subducted beneath the other
- As explained above, deep ocean trenches form where this occurs, and the subducted plate then melts - creating magma, which rises up from the Benioff Zone to form underwater volcanoes
descructive plate boundaries - oceanic meets oceanic
What are Island arcs?
- Over millions of years, these growing volcanoes rise above sea level to form separate island volcanoes, which are usually found in curved lines called island arcs
descructive plate boundaries - oceanic meets oceanic
Earthquakes?
- The subduction also produces shallow- to deep-focus earthquake: (some of which can be very powerful)
- The 2004 Indian Ocean tsunami was caused by an earthquake that occurred when two oceanic plates collided (with the Indian Plate sinking beneath the Burma Plate, a part of the Eurasian Plate).
Destructive (collision) - continental meets continental
When two continental plates meet, a….
collision margin occurs
Destructive (collision) - continental meets continental
What happens at collision margins
- As both plates have about the same density, and are less dense than the asthenosphere beneath them, neither plate is actually subducted.
- Instead, they collide and sediments between them are crumpled and forced up to form high collision fold mountains, which are still being formed upwards, at a rate of 1cm/annum
Destructive (collision) - continental meets continental
What is an example of a landform at collision margins
- e.g. the Himalayas are formed by the collision of the Eurasian and Indian plates
Destructive (collision) - continental meets continental
subduction?
- However, inevitably there can be some subduction, caused when the compressed (and therefore denser) sediments result in plate subduction beneath them
Destructive (collision) - continental meets continental
tectonic activity?
- There is no volcanic activity as there is no subduction,
- but collision zones are cut by huge thrust faults that generate shallow, high-magnitude earthquakes
- such as in Kashmir in 2005 and Nepal in 2015
Transform (conservative) plate boundary
What is formed at conservative plate boundaries
- Along some boundaries, two plates slide past each other, forming a conservative plate margin
- This results in a major break in the crust between them as they move
- The break itself is called a fault, and where it occurs on a large scale is known as a transform fault, which affects a wider area
Transform (conservative) plate boundary
Tectonically active?
- Although no crust is made or destroyed here (and there is no volcanic activity),
- this type of plate margin is tectonically very active - and can be associated with powerful earthquakes
Transform (conservative) plate boundary
What forces build up at conservative plate boundaries
- Plates move slowly past each other – they do not have to be in different directions but at a different rate of travel in the same direction
- Transform margins are offset at angles, creating zigzag patterns to accommodate movement
- They become stuck and pressure builds, the plates eventually ‘snap’ past each other
- These can be called ‘strike-slip’ faults as they strike/stick and then slip/release past each other
Transform (conservative) plate boundary
What happens when pressure builds up here
- stress and pressure to build up, which is suddenly released as a strong shallow-focus earthquake
Give an example of a conservative plate margin
- San Andreas Fault in California, which has generated significant earthquakes
What are the different landforms which form at constructive plate margins
- In oceans, this divergence forms mid-ocean ridges, and on continents it forms rift valleys.
What happens at constructive (divergent) plate boundaries
- At constructive plate margins, two plates are moving apart (diverging) - which leads to the formation of new crust
constructive (divergent) plate boundaries
Describe the formation of mid-ocean ridges
- Mid-ocean ridges of underwater mountains extend for over 60 000 km across the world’s ocean floors
- Transform faults cut across these ridges as they spread at different rates
constructive (divergent) plate boundaries
Describe the formation of rift valleys
- When plates move apart on continents, the crust stretches and breaks into sets of parallel cracks (faults)
- The land between these faults then collapses, forming steep-sided valleys called rift valleys
constructive (divergent) plate boundaries
What earthquakes occur along mid ocean ridges
- Shallow-focus earthquakes (at a depth of less than 70 km) occur, but they pose little risk to humans because the shocks are minor and occur underwater
constructive (divergent) plate boundaries
What volcanoes occur along mid ocean ridges
- Regular volcanic eruptions also create submarine volcanoes along these mid-ocean ridges, some of which grow above sea level to create new islands such as Iceland on the Mid-Atlantic Ridge
At constructive plate boundaries, do earthquakes occur
- Minor, shallow earthquakes
At constructive plate boundaries do volcanoes occur
- Basaltic volcanoes - Eruptions are small and effusive, as the erupted basalt lava has a low gas content and high viscosity
- Minor earthquakes
