Plate Tectonics, Volcanoes & Earthquakes Flashcards
what is the structure of the Earth? (in order)
- solid iron inner core
- liquid iron outer core
- semi-solid mantle (where convection currents take place)
- solid crust
how can we measure the rate of seafloor spreading?
with magnetic mapping as the minerals in the rock align with Earth’s changing magnetic field - telling us new crust is being formed at mid ocean ridges at different points in time
what is a constructive boundary?
where crust is being made
what is a destructive boundary?
where crust is being destroyed
what is causing tectonic plate movement (movement of the tectonic plates by a few cm each year)?
convection currents in the mantle
what is a subduction zone?
an area where the ocean crust is being pulled under the continent at a destructive boundary, because it is cold and dense and so sinks into the mantle
at a destructive boundary, what forms?
large mountain ranges (fold mountains) are produced in these regions called ‘collision zones’ where there are high horizontal pressures as the continental crust gets increasingly compressed
what are the two types of crust?
continental and oceanic crust
what is an earthquake?
the transmission of shock waves (or seismic waves) through the Earth’s crust caused by the slipping of rocks past each other on a fault - usually in the region of plate boundaries, the most common movements have both horizontal and vertical components in the region of subduction zones
why is the movement of plates at faults not smooth?
movement is resisted by friction
what is the centre of the slipped zone called (earthquakes)?
the focus
what is the point directly above the focus on the surface called?
the epicentre
what is a divergent boundary?
where the plates are moving away
what is a convergent boundary?
where the plates are moving towards each other
what scale is used to measure the magnitude of earthquakes?
the Richter scale
what are the 3 main types of seismic waves?
P-waves, S-waves and surface waves
what are P-waves?
P-waves are compressive waves (longitudinal waves) that travel faster than S-waves, about 5km/s, they can travel through liquids and solids, they are transmitted in the direction of movement of the fault
what are S-waves?
S-waves are shear waves (transverse waves), and their movement can be horizontal or vertical, they are much slower than P-waves (travel around 2.5km/s) and can only travel through solids
what are surface waves?
surface waves involve rotational movement at and near to a free surface like waves in water, they have a slow speed, they can only occur at a free surface
why are earthquakes being produced in subduction zones?
because there are multiple areas where the rock is being deformed, the subducted plate is sinking forming high pressures
what does the refraction of P-waves and S-waves tell us`?
the structure of the Earth
what is the Mercalli Intensity scale?
it is a scale that measures earthquakes based on their effects as opposed to just magnitude - it takes into account magnitude, ground conditions and topographical settings - a more useful scale for engineers/gelogists
what is a transform boundary?
transform boundaries are places where plates slide sideways past each other
what is liquefaction?
often in coastal areas where buildings have been built upon saturated or partially saturated soils/sands, strength and stiffness is lost when there is an earthquake which applies compressive loads and it cannot withstand the shear forces and so behaves like a liquid until the shaking stops
how are tsunamis generated?
they are often generated by earthquakes that occur near subduction zones can often cause tsunamis, out in the ocean a shockwave is sent that travels very quickly radially out through the oceans pushing water with it causing a buildup of large masses of water hitting coastal areas which can affect areas of low-lying ground very drastically - they can also be generated by coastal landslides or landslides under the sea floor (if a major landslide occurs, shock waves can be sent out into neighbouring oceans and causes tsunamis in neighbouring countries)
some facts on Basaltic lava?
- 1000-1200 degrees celsius
- mafic
- high temp
- low viscosity
some facts on Rhyolitic lava?
- 800-1000 degrees celsius
- highly felsic
- low temp
- high viscosity
what does volcanic ejecta include?
water vapour and other gases produce explosive eruptions which can produce pyroclasts (clastic rock from fire), rhyolitic lava, ash, pyroclastic flows (can reach 200km/h), there are high internal temp. as high as 800 degrees [think Vesuvius in Pompeii]
what is a central eruption?
lava from a central vent
what is a fissure eruption?
lava from cracks e.g Iceland 1783, 32 km length, 12km^3 of basalt
info on shield volcanoes?
made up of layers of rapidly flowing basalt lava forming gentle slopes e.g Mauna Loa in Hawaii
how are cinder cone volcanoes formed?
built from pyroclasts, so they have concave slopes
what is a Phreatic explosion? example?
when sea water mixes with magma, violent eruption, for example Krakatoa heard 2000km away
what is a lahar?
any mudflow incorporating volcanic debris, with water from e.g ice, crater lake, river, snow
why is volcanic ash so dangerous?
volcanic ash is one of the most immediate hazards of a volcanic eruption – it travels further and faster than lava, and the ash clouds can be so large that they do not flow along valleys, but with the wind
what is pyroclastic flow? why is it so dangerous and give some examples of where it has occurred previously?
dense cloud of hot gas and ash with temperatures in the flow reaching over hundreds of degrees can cause instant death - Vesuvius destroyed Pompeii and Herculaneum in 79AD and Mount Ontake in Japan in Sep 2014
what is a volcanic dome?
a volcano dominated by ejecta and viscous lava and so has steep sides
what is a cinder-cone volcano?
a volcano formed entirely by ejecta, made up of layers of ejected material
what is a composite volcano?
this is the most widely recognised volcano (e.g Mount Fuji in Japan) and it is made up of alternating layers of lava and ejecta forming a brittle volcano
how do hot spot volcanoes form?
as the plate moves across the hot-spot, active volcanoes form above it, and the old volcanoes that have been moved off the hot-spot become extinct
can hot spots also lie under continental plate as well as oceanic plates?
yes
brief explanation of how hot spot volcanoes are formed
- plumes of hot material rises in the mantle from the outer core, the material has a lower density and rises up through the mantle
- when this reaches the crust and breaks through, it produces flood basalts
- then when the flow subsides, the ‘plume tail’ continues for millions of years, producing hot-spot volcanoes
- as the plates move, the displaced volcanoes become extinct and new ones form above the hot spot
what is the crater of a volcano?
the top of the magma pipe - a crater is formed when lava sinks back into the vent, the steep inner sides are often unstable and collapse so the crater may become wider than the vent e.g Mt Etna (Sicily) 300m diameter, 850mm deep
what is a caldera? how is it formed?
a caldera is the result of when the mountain summit collapses into the empty chamber, large pyroclastic flows accompany the collapse, blanketing the caldera and the surrounding area, a lake then forms in the caldera forming a crater lake (a small volcanic cone forms in the caldera)
why is the centre of an extinct volcano often much stronger than its side slopes?
because the slopes are often made of lava and pyroclasts, so after a long period of erosion all that may be left is the central plug and some radiating dikes
what type of volcano often forms at divergent boundaries?
fissure volcanoes - these do not have the typical cone shape you would expect
what are some of the main causes of fatalities associated with volcanoes?
- tsunamis
- pyroclastic flow/surge
- mudflow
- indirect causes (famine etc.) - a large eruption can send lots of ash to high altitudes which can affect the weather globally
lava flows aren’t the main danger of volcanoes, give an example of something that is?
landslides and flows are more dangerous as they can kill people and destroy property
why is volcanic ash in some ways advantageous?
ash deposits will produce fertile soils which is often why people want to live and farm in these hazardous locations
