Plate Tectonics, Volcanoes & Earthquakes

Question 1

what is the structure of the Earth? (in order)

Answer 1

• solid iron inner core
• liquid iron outer core
• semi-solid mantle (where convection currents take place)
• solid crust

Question 2

how can we measure the rate of seafloor spreading?

Answer 2

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

Question 3

what is a constructive boundary?

Answer 3

where crust is being made

Question 4

what is a destructive boundary?

Answer 4

where crust is being destroyed

Question 5

what is causing tectonic plate movement (movement of the tectonic plates by a few cm each year)?

Answer 5

convection currents in the mantle

Question 6

what is a subduction zone?

Answer 6

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

Question 7

at a destructive boundary, what forms?

Answer 7

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

Question 8

what are the two types of crust?

Answer 8

continental and oceanic crust

Question 9

what is an earthquake?

Answer 9

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

Question 10

why is the movement of plates at faults not smooth?

Answer 10

movement is resisted by friction

Question 11

what is the centre of the slipped zone called (earthquakes)?

Answer 11

the focus

Question 12

what is the point directly above the focus on the surface called?

Answer 12

the epicentre

Question 13

what is a divergent boundary?

Answer 13

where the plates are moving away

Question 14

what is a convergent boundary?

Answer 14

where the plates are moving towards each other

Question 15

what scale is used to measure the magnitude of earthquakes?

Answer 15

the Richter scale

Question 16

what are the 3 main types of seismic waves?

Answer 16

P-waves, S-waves and surface waves

Question 17

what are P-waves?

Answer 17

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

Question 18

what are S-waves?

Answer 18

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

Question 19

what are surface waves?

Answer 19

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

Question 20

why are earthquakes being produced in subduction zones?

Answer 20

because there are multiple areas where the rock is being deformed, the subducted plate is sinking forming high pressures

Question 21

what does the refraction of P-waves and S-waves tell us`?

Answer 21

the structure of the Earth

Question 22

what is the Mercalli Intensity scale?

Answer 22

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

Question 23

what is a transform boundary?

Answer 23

transform boundaries are places where plates slide sideways past each other

Question 24

what is liquefaction?

Answer 24

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

Question 25

how are tsunamis generated?

Answer 25

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)

Question 26

some facts on Basaltic lava?

Answer 26

• 1000-1200 degrees celsius
• mafic
• high temp
• low viscosity

Question 27

some facts on Rhyolitic lava?

Answer 27

• 800-1000 degrees celsius
• highly felsic
• low temp
• high viscosity

Question 28

what does volcanic ejecta include?

Answer 28

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]

Question 29

what is a central eruption?

Answer 29

lava from a central vent

Question 30

what is a fissure eruption?

Answer 30

lava from cracks e.g Iceland 1783, 32 km length, 12km^3 of basalt

Question 31

info on shield volcanoes?

Answer 31

made up of layers of rapidly flowing basalt lava forming gentle slopes e.g Mauna Loa in Hawaii

Question 32

how are cinder cone volcanoes formed?

Answer 32

built from pyroclasts, so they have concave slopes

Question 33

what is a Phreatic explosion? example?

Answer 33

when sea water mixes with magma, violent eruption, for example Krakatoa heard 2000km away

Question 34

what is a lahar?

Answer 34

any mudflow incorporating volcanic debris, with water from e.g ice, crater lake, river, snow

Question 35

why is volcanic ash so dangerous?

Answer 35

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

Question 36

what is pyroclastic flow? why is it so dangerous and give some examples of where it has occurred previously?

Answer 36

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

Question 37

what is a volcanic dome?

Answer 37

a volcano dominated by ejecta and viscous lava and so has steep sides

Question 38

what is a cinder-cone volcano?

Answer 38

a volcano formed entirely by ejecta, made up of layers of ejected material

Question 39

what is a composite volcano?

Answer 39

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

Question 40

how do hot spot volcanoes form?

Answer 40

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

Question 41

can hot spots also lie under continental plate as well as oceanic plates?

Answer 41

yes

Question 42

brief explanation of how hot spot volcanoes are formed

Answer 42

• 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

Question 43

what is the crater of a volcano?

Answer 43

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

Question 44

what is a caldera? how is it formed?

Answer 44

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)

Question 45

why is the centre of an extinct volcano often much stronger than its side slopes?

Answer 45

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

Question 46

what type of volcano often forms at divergent boundaries?

Answer 46

fissure volcanoes - these do not have the typical cone shape you would expect

Question 47

what are some of the main causes of fatalities associated with volcanoes?

Answer 47

• 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

Question 48

lava flows aren’t the main danger of volcanoes, give an example of something that is?

Answer 48

landslides and flows are more dangerous as they can kill people and destroy property

Question 49

why is volcanic ash in some ways advantageous?

Answer 49

ash deposits will produce fertile soils which is often why people want to live and farm in these hazardous locations