tectonics Flashcards
intra plate earthquakes
These occur in the middle or interior of tectonic plates and are much rarer than boundary earthquakes
volcanic hazards
Associated with eruption events
volcano
A landform that develops around a weakness in the Earth’s crust from which molten magma, volcanic rock and gases are ejected and extruded
seismic hazards
Generated when rocks within 700km of the Earth’s surface come under such stress that they break and become displaced
tectonic hazards
These include earthquakes and volcanic eruptions as well as secondary hazards such as tsunami and represent a significant risk in some parts of the world in terms of loss of life, livelihoods and economic impact
what % of earthquakes are found along plate boundaries
95%
what 3 patterns does the distribution of earthquakes show
The oceanic fracture zone (OFZ)
The continental fracture zone (CFZ)
Scattered earthquakes in continental interiors
The oceanic fracture zone (OFZ)
activity found in mid-ocean ridges. E.g. the mid-Atlantic ridge.
The continental fracture zone (CFZ)
activity found in mountain ranges e.g. across the Himalayas.
Scattered earthquakes in continental interiors
along old fault lines.
E.g. the Church Stretton Fault in Shropshire
what does the type of plate boundary determine about a volcano
whether a volcano exists and what type it is
how many volcanoes are there globally
500
50 erupt each year
what is the voilence of a volcanic eruption determined by
by the number of dissolved gases in the magma and how easily they can escape.
what is a volcano called that forms away from a plate boundary
hot spots
where do intra plate earthquakes happen
Intra-plate earthquakes happen in the middle of plates.
why do intra plate earthquakes happen
Scientists think that they occur when stresses build up in ancient faults - causing them to become active again.
why are intra plate earthquakes harder to predict
they don’t occur in well-defined patterns along plate margins
what is a volcanic hotspot
an area in the mantle from which heat rises as a hot thermal plume from deep in the Earth – often called a ‘magma plume’.
High heat and low pressure at the base of the lithosphere enable melting of the rock
what % of earthquakes happen in the Ring of Fire
70%
example of a hotspot/magma plume
Hawaii
what 4 scientists produced the theories of plate tectionics
Harry Hess
Alfred Wegener
John Tuzo Wilson
James Hutton
what theory did Harry Hess propose
proposed that ridges on the ocean floor were the result of molten rock rising from the asthenosphere.
what theory did Alfred Wegener propose
published two articles about a concept called continental drift.
what theory did John Tuzo Wilson propose
proposed that volcanic island chains (e.g. Hawaii) are created by fixed ‘hotspots’.
what theory did James Hutton propose
that the processes of erosion, deposition and uplift were connected and operated continuously – driven by the Earth’s internal heat.
what was James Huttons theory called and what year
Theory of the Earth
1785
what did the Theory of the Erath suggest
the processes of erosion, deposition and uplift were connected and operated continuously, driven by the Earth’s internal heat.
what cycle did James Hutton come up with
the rock cycle
what did the rock cycle suggest
where rock particles were transported to sea, buried, solidified and then later lifted back to the Earth’s surface through tectonic processes.
what was the name of Alfred Wegeners theory and what year
Continental drift
1912
what did continental drift suggest
200 million years ago, a supercontinent called Pangaea began to break into pieces, its parts moving away from one another. The continents we see today are fragments of that supercontinent.
how did Alfred Wegener support his continental drift theory
by pointing to matching rock formations and similar fossils across continents.
what was the name of Harry Hess’ theory
seafloor spreading
what did sea floor spreading suggest
discovered that the oceans were shallower in the middle and identified the presence of mid-ocean ridges which were as high as 1.5km above the flat sea floor.
He envisaged that oceans grew from their centres, with molten material (basalt) oozing up from the Earth’s mantle, creating seafloor spreading either side.
what did john tuzo wilson come up with
hotspot theory
hotspot theory by wilson
John proposed in 1963 that plates might move over fixed ‘hotspots’ in the mantle, forming volcanic island chains.
In 1965, he followed this discovery with the idea of a third type of plate boundary – transform faults (conservative plate boundaries) – regarded as the missing piece in the puzzle for plate tectonic theory.
what is the lithosphere broken up into
into seven major and several minor parts – tectonic plates.
These plates move relative to each other over the asthenosphere.
There are a number of processes which drive their movement
asthenosphere definition
the part of the mantle below the lithosphere, where the rock is semi-molten.
what processes drive the movement of tectonic plates
mantle convection
slab pull
subduction
seafloor spreading
paleomagnetism
mantle convection
Heat produced by the decay of radioactive elements in the Earth’s core heats the lower mantle – creating convection currents.
These hot, liquid magma currents are thought to move in circles in the asthenosphere – thus causing the plates to move.
slab pull
Newly formed oceanic crust at mid-ocean ridges becomes denser and thicker as it cools.
This causes it to sink into the mantle under its own weight –pulling the rest of the plate down with it.
subduction
Subduction is the process of a plate being destroyed.
As two oceanic plates OR an oceanic and continental plate move towards each other, one slides under the other into the mantle – where it melts into an area called the subduction zone (the circle on the diagram).
seafloor spreading
This is the process of new crust pushing tectonic plates apart.
In the middle of many oceans there are mid-ocean ridges, or underwater mountain ranges.
These are formed when hot magma (molten rock) is forced up from the asthenosphere and hardens – forming new oceanic crust.
paleomagnetism
1950’s, studies of palaeomagnetism confirmed that the sea floor was spreading.
Every 400,000 years or so, the Earth’s magnetic fields change direction i.e. the magnetic north and south swaps.
When lava cools and becomes rock, minerals inside the rock line up with the Earth’s magnetic direction (polarity) at the time.
what theory is now less accepted
mantle convection
when two tectonic plates meet what do they form
a plate boundary
what are the areas next to plate boundaries called
plate margins
what are the 3 plate boundaries
convergent
divergent
conservative
what are convergent also known as
destructive margins
what are divergent also known as
constructive
what are conservative also known as
transform
what happens at a divergent plate boundary
two plates are moving apart (diverging) – leading to the formation of new crust.
In oceans, this divergence forms mid-ocean ridges and on continents it forms rift valleys.
how far can Mid-ocean ridges of underwater mountains extend for
60,000 km
eg of a divergent plate boundary with oceanic crusts
mid-atlantic ridge
earthquake hazards at mid ocean ridges
Shallow-focus earthquakes (less than 70km into the crust) occur frequently, but they pose little threat to humans as they are small and underwater.
volcanic hazards at mid ocean ridges
Submarine volcanoes can occur, some which grow above sea level to create new islands. E.g. Iceland on the mid-Atlantic ridge. These are generally less explosive and more effusive, especially when they occur underwater.
effusive meaning
a type of eruption in which lava steadily flows out of a volcano.
what are rift valleys (divergent)
When continental plates move apart, 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.
example of a divergent plate boundary with continental crusts
east african rift valley
earthquake hazards at rift valleys
Similar to mid-ocean ridges – shallow and low magnitude.
volcano hazards at rift valleys
Yes! The thinning crust allows magma to rise
the subduction zone
Broad areas where two plates are moving together. Often the thinner, more dense oceanic plate descends beneath the continental plate.
locked fault
In a subduction zone, as plates move together they can get stuck due to frictional resistance. Such faults may store strain for extended periods, that is eventually released in a large magnitude earthquake.
the benioff zone
An area of seismicity corresponding with the slab being thrust downwards in a subduction zone.
what happens at a convergent plate boundary
plates move towards each other
what are the three types of convergent plate boundary dependent on
the type of plate found there
what are the 3 types of continental plate boundaries
Oceanic meets continental
Oceanic meets oceanic
Continental meets continental
destructive convergent boundaries
oceanic meets continental
oceanic meets continental convergent boundaries
Oceanic crust is more dense than continental, so when they collide, the oceanic crust subducts underneath into the mantle. This is marked by deep ocean trenches
As the oceanic plate subducts, the continental plate is folded and slowly pushed up, forming a chain of fold mountains
earthquake hazards at oceanic meets continental convergent boundaries
Friction between colliding plates causes intermediate and deep earthquakes in the Benioff Zone.
These earthquakes are some of the largest and most damaging
volcanic hazards at continental meets oceanic convergent boundaries
Explosive volcanic eruptions are generated as magma created from the melting oceanic plate pushes up through the faults in the continental crust to reach the surface. These are generally less frequent than other volcanoes, but are more destructive.
why is there so much tectonic activity around the Pacific Ring of Fire
due to the convergent plate boundary
destructive convergent boundaries
oceanic meets oceanic
what happens when an oceanic plate and an oceanic plate meet at a convergent plate boundary
When two oceanic plates collide, one plate (the denser or faster) is subducted beneath the other – deep ocean trenches occur at this boundary.
earthquake hazards at oceanic meets oceanic convergent boundaries
The subduction produces shallow-deep earthquakes, some of which can be very powerful.
volcanic hazards at oceanic meets oceanic convergent boundaries
As the subducted plate melts, magma rises to form underwater volcanoes. Over millions of years, these grow to above sea level to form separate island volcanoes, called island arcs.
convergent boundaries- collision margin
continental meets continental
what happens during a collision margin
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 fold mountains. E.g. The Himalayas
Inevitably, there may be some subduction caused when the compressed (and therefore denser) sediments result in plate subduction beneath them.
eg of a continental meets continental plate boundary
Nepal sits on the boundary between the Eurasian and Indian tectonic plates. It is a convergent boundary where two continental plates collide
In 2015, Nepal experienced a 7.8 magnitude earthquake.
Name the landforms that are formed when an oceanic crust collides with a continental crust.
deep ocean trenches
fold mountains
what happens at a conservative plate margin
two plates are sliding past one another – resulting 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 it’s known as a transform fault.
earthquake hazards at conservative plate margins
Powerful, but shallow earthquakes can occur due to frictional resistance – the plates stick as they move past one another, causing stress and pressure to build.
volcanic hazards at conservative plate margins
none
eg of a conservative plate margin
San Andreas fault
when was the last major earthquake on the San Andreas Fault
1906 at 8.3 on the Richter scale.
how do earthquakes occur
When the plates move against each other they sometimes stick – causing huge amounts of pressure to build up
When the pressure becomes too much, the rock fractures along cracks called faults
Energy is released as seismic waves, which cause the ground to shake.
where are seismic waves released from
inside of the Earth’s crust, along the fault.
where do seismic waves radiate out from
the focus
what are the three main types of seismic waves
Primary ‘P’ waves
Secondary ‘S’ waves
Love ‘L’ waves
how do primary ‘P’ waves travel
P waves are body waves – they travel through the Earth’s body.
They travel through both solids and liquids.
They move in a backwards and forwards motion.
how fast are P waves
They are the fastest and the first to reach the surface (8km/sec).
when are P waves damaging
only damaging in the most powerful earthquakes
how do secondary ‘S’ waves travel
S waves are also body waves – they travel through the Earth’s body.
They travel through solids only.
They move in an up and down motion, perpendicular to the direction of travel.
how fast are S waves
They are slower than P waves (4km/sec).
how damaging are S waves
They do more damage than P waves.
how do love ‘L’ waves travel
L waves are surface waves – they travel only on the Earth’s surface.
They move in a side to side motion, perpendicular to the direction of travel.
speed of L waves
They are the slowest of the three waves (last to arrive).
damage caused by L waves
They are larger and cause the most damage.
what is the overall severity of an earthquake linked with
linked to the amplitude and frequency of these wave types.
The ground surface may be displaced horizontally, vertically or obliquely (slanted) during an earthquake depending on the strength of individual waves.
what does todays research into predicting earthquakes focus on
focuses on precursors, which may suggest a major earthquake is likely to happen
E.g. Foreshocks (small earthquake that happen before a larger one)
how are earthquakes predicted
Currently there is no method at accurately predicting when or where an earthquake will strike
we can use our knowledge of plate boundaries to forecast where an earthquake is likely to happen. E.g. areas that have had one big earthquake are likely to have another
