Topic 1 - EQ1 - Tectonics

Question 1

What are the standard tectonic hazards?

Answer 1

Earthquakes and volcanic eruptions (includes secondary hazards such as tsunamis)

Question 2

What are the 2 ways to classify tectonic hazards

Answer 2

Seismic or volcanic

Question 3

Where do earthquakes generally occur?

Answer 3

Earthquakes generally occur on and around plate boundaries where tectonic plates are moving, as we can see around the pacific region where earthquakes are concentrated and clustered around the border of the pacific place. About 70% are clustered are around the pacific ‘Ring of Fire’.

Question 4

What are the anomalies where earthquakes happen away from plate boundaries?

Answer 4

Intra-plate earthquakes (ancient fault far from plate boundaries responsible for minor earthquakes in the North of the UK)

Non-tectonic earthquakes can happen (Groningen in the Netherlands there are increasingly frequent non-tectonic earthquakes as a result of drilling in the nearby Groningen gas field)

Question 5

What are the two main zones that earthquakes are distributed within?

Answer 5

Oceanic fracture zone (OFZ) - a belt of activity through the oceans along the mid- ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and California.

Continental fracture zone (CFZ) - a belt following the mountain ranges from Spain, via the Alps, to the Middle East, the Himalayas to the East Indies and around the pacific.

Question 6

Where do the most powerful earthquakes occur?

Answer 6

The most powerful are at convergent or conservative boundaries.

Question 7

Where do volcanoes generally occur?

Answer 7

Volcanoes clustered around plate boundaries. E.g. high concentration of volcanoes in Iceland by the boundary between the NA and Eurasian plates.

-Main volcanic zones are found on or near to plate boundaries, depending on the margin type.
-A strong presence on the Pacific ‘Ring of Fire’, like earthquakes.

-Rare intra-plate volcanoes (hotspots) can happen near the middle of plates. These are associated with upwelling or mantle plumes, e.g. Nyos volcano in West Africa or Hawaii.

Question 8

What are the 7 major plates?

Answer 8

Indo-Australian, Eurasian, African, South American, North American, Antarctic, Pacific

Question 9

What are examples of 3 minor plates?

Answer 9

Arabian, Caribbean, Nazca

Question 10

What are examples of 2 micro plates?

Answer 10

Banda Sea plate, Easter plate

Question 11

What is an example of a diverging plate boundary

Answer 11

NA and Eurasian plate boundary

Question 12

What is an example of a converging plate boundary?

Answer 12

Indian and Eurasian plate boundary

Question 13

What is an example of a conservative plate boundary

Answer 13

NA and Pacific plate boundary

Question 14

How do earthquakes happen at intra plate boundaries?

Answer 14

Intra-plate earthquakes are caused by stresses within a plate. Plates moves over spherical surface so zones of weakness are created. Intra-plate earthquakes happen at this zones of weakness. Intra-plate earthquakes may take place along ancient fault lines or rift zones.

Question 15

What are mantle plumes?

Answer 15

Especially hot areas of the mantle that move upwards underneath the crust. They are constant sources of heat and can cause weak points in the crust which become hot spots.

Question 16

What are the world’s two huge mantle plumes?

Answer 16

Pacific and African

Question 17

What are hotspots?

Answer 17

Spots in the world’s crust where magma from the mantle has broken through a weak point in the crust. There appears to be two types - one associated with individual upwelling from near the core-mantle boundary, and the second from the top of large mantle plumes.

Question 18

How are hotspot volcanoes formed

Answer 18

-The heat of a magma plume causes melting of the upper mantle and lithosphere and magma rises through weaknesses in the crust
-Magma erupts onto the seafloor forming a seamount which builds up with continuous eruptions supplying more magma until the seamount breaks the surface of the water
-The lava at oceanic hotspots is basaltic creating shield volcanoes

Question 19

How are hotspot volcanoes formed

Answer 19

-The heat of a magma plume causes melting of the upper mantle and lithosphere and magma rises through weaknesses in the crust
-Magma erupts onto the seafloor forming a seamount which builds up with continuous eruptions supplying more magma until the seamount breaks the surface of the water
-The rising magma continues to erupt out of the top the sea mount/new volcanic island
-At oceanic hotspots the result is a shield volcano with basaltic lava and at continental hotspots the result is a very explosive volcano with viscous lava

Question 20

How are island chains such as Hawaii and the Azores formed?

Answer 20

-Once a volcano rises above the water at a hotspot a new island has been created
-As the plate moves, the island is carried beyond the hotspot, cutting it off from the supply of upwelling magma and volcanic activity stops
-As the plate moves new islands are created over the hotspot resulting in the creation of a chain of volcanic islands

Question 21

Assess the significance of plate boundaries in relation to the distribution of earthquakes and volcanoes (12) - what are the supporting points?

Answer 21

P1 - Significant for volcanoes as 95% of volcanoes happen at plate boundaries especially within the pacific ring of fire e.g. Mount St Helens in Washington state (80% at subduction zones where sub-ducted plate melts and creates magma to supply the volcano)

P2 - Significant for earthquakes as 95% of all earthquakes occur along plate boundaries - 70% of all earthquakes happen on a pacific plate boundary - and earthquakes can occur at all 3 different main types of plate boundaries but conservative (San Andreas) and convergent (Nepal) plate boundaries produce the strongest earthquakes. Building friction (and its release) and the collision of plates causes tremors and quakes.

Question 22

Assess the significance of plate boundaries in relation to the distribution of earthquakes and volcanoes (12) - what are the challenging points?

Answer 22

P1 - Magma plumes and hotspots do also cause intra plate boundary volcanic activity as mantle plumes cause the heating of the upper mantle and lithosphere causes magma to rise through the crust and erupt onto the sea floor. It then builds up and creates volcanic islands and volcanic island chains. Hawaii and the Azores are evidence of this volcanic activity as they are away from plate boundaries.

P2 - Intra plate earthquakes often have two explanations. Intra-plate earthquakes can be due to ancient fault far from plate boundaries, these are responsible for minor earthquakes in the North of the UK. Or, intra-plate earthquakes can be due to gas drilling or hydraulic fracturing for shale gas, Groningen in the Netherlands has been a province ravaged by numerous earthquakes despite its position firmly within the eurasian plate and away from ancient fault lines because of gas drilling in the province which is home to the EU’s largest continental gas field.

Question 23

What is plate tectonics?

Answer 23

Plate Tectonics refers to a set of concepts and theories that explain the formation and distribution of the Earth’s major structural features and seismic and volcanic events. It involves the structural layers of the Earth, existence of major lithospheric plates that are moving and movement of plates by various mechanisms.

Question 24

What are the 4 main layers of the Earth?

Answer 24

-Crust
-Mantle
-Outer Core
-Inner Core

Question 25

What seismic waves can pass through each of the 4 levels?

Answer 25

Crust - surface and body waves
Mantle - body waves pass through at variable rates due to density changes
Outer Core - Only P waves able to pass through (S wave shadows zone at about 105degrees from the focal point)
Inner Core - Only P waves reach the inner core and pass through

Crust least dense and inner core is the most

Question 26

What is the theory of continental drift?

Answer 26

Wegener’s theory that the continents all used to be one (Pangea) and they have since shifted into their modern day positions and continue to drift constantly in different directions and velocities. The evidence of this is the jigsaw fit of South America and Africa, coal deposits, biological evidence (fossils) and geological evidence (Appalachian and Caledonian mountains have same geological feature).

Question 27

What is sea floor spreading?

Answer 27

Henry Hess developed after discovery of the Mid Atlantic Ridge. Explains that at divergent plate boundaries oceanic crust spreads due to the convection currents in the mantle creating openings in the crust where molten basalt from the Earth’s mantle rises up through the gap to create new sea floor.

Evidence of this is that the youngest oceanic rock is found at the centre of oceans such as around Iceland and the oldest is found towards the USA.

Question 28

What are convection currents?

Answer 28

Currents in the mantle which cause plates to move through frictional drag on the Earth’s lithosphere, the direction of the convection currents causes the types of plate boundaries. Caused because lower mantle experiences higher temperatures than upper mantle. There is rising and spreading of heat inside the mantle at the mid ocean ridges creating currents.

Question 29

What is paleomagnetism?

Answer 29

It is the study of the history of changes to the Earth’s magnetic filed (polarity).

Question 30

How does paleomagnetism support tectonic theory?

Answer 30

Confirms Hess’ theory of sea floor spreading. Material from mantle rises up through mid ocean ridges and cools which preserves a record of the polarity of the Earth’s magnetic field - as the magnetic iron in the basaltic lava will align with the N Pole when it cools - but when polarity switches so too will the orientation of the lava. Vine and Matthews noticed a symmetrical pattern of magnetic stripes on either side of the mid ocean ridges. In addition to seeing changes in polarity, basalts on the sea floor were dated - it was confirmed that sea floor on opposite sides of mid ocean ridges at the same distance were the same age. Confirms that the Atlantic ocean floor was created at the mid ocean ridges, then moved sideways.

Question 31

What is the direction of plate movement at divergent/constructive plate boundaries? Which hazards exist at this boundary?

Answer 31

The plates move away from one another. E.g. NA and EU plate in Iceland.

Hazards = Earthquakes and volcanoes.

Also form mid ocean ridges when two oceanic plates are diverging and rift valleys when two continental plates are diverging.

Question 32

What is the direction of plate movement at transform/conservative plate boundaries? Which hazards exist at this boundary?

Answer 32

The plates move past each other. E.g. NA and PAC at San Andreas Fault in Cali.

Hazards = Strike slip earthquakes with shallow focuses along faults (major breaks in the lithosphere from the plates moving past each other) powered by friction from plates rubbing past each other.

Question 33

What is the direction of plate movement at convergent plate boundaries? What are the two types of convergent boundaries? Which hazards exist at this boundary?

Answer 33

Two plates move towards one another. At collision boundaries both plates are forced up creating fold mountains (e.g. Himalayas which are still growing over 2cm a year due to plate convergence) but also island arcs often because both plates are continental or oceanic so have the same density. Causes earthquakes at an intermediate depth.

At destructive boundaries one plate (the more dense plate) is forced under another (less dense plate) creating a subduction zone (which results in fold mountain, volcanic activity as the subducted crust melts into magma, and earthquakes as the plates lock and release), usually continental and oceanic plates colliding - but can also be two oceanic plates colliding (this causes ocean trenches).

E.g. India and EU is a convergent collision but Pacific and Phillipine (or India-Australia plate, just general E Asian plate boundaries) is a destructive collision.

Question 34

What actually is a (tectonic) earthquake? How does it happen?

Answer 34

An earthquake is ground shaking caused by shockwaves emanating from a sudden slip on a fault.

Caused when plates move against each other they stick causing pressure to build but when pressure is too much the rock fractures along cracks called faults and energy is released as seismic waves causing the ground to shake.

Question 35

What is the focus in an earthquake?

Answer 35

The point in the crust in which strain is released

Question 36

What is the epicentre in an earthquake?

Answer 36

It is the earth’s surface directly above the focus - it is the point where the earthquake is felt the strongest. Most damage occurs at the epicentre and further from the epicentre the less damage as the seismic waves dissipate.

Question 37

What is a locked fault in an earthquake?

Answer 37

A fault that is stuck.

Frictional resistance on fault is > stress across the fault.

When the frictional resistance is overcome, the strain and pressure is eventually released as a large magnitude earthquake.

It is common in subduction zones for the leading plate to lock under high friction causing powerful ‘mega thrust’ earthquakes.

Question 38

At which plates are you most likely to find shallow focus earthquakes?

Answer 38

Divergent and conservative plate boundaries - neither plate subducts. Also convergent collision boundaries.

Question 39

Are greater magnitude seismic waves at shallower or deeper focuses?

Answer 39

A shallower focus = greater magnitude seismic waves

Question 40

Where are the greatest depth focus earthquakes? At this boundary are earthquakes still of great magnitude?

Answer 40

Destructive convergent boundaries.

They result in deep seismic activity in a place known as the benioff zone as the plate is thrust deep down. Oceanic-oceanic convergent boundaries produce particularly deep focused earthquakes.

Earthquakes in subduction zones often still have some of the greatest magnitudes because of the sheer frictional stress of locked plates in subduction zones.

Locked plates can last for hundreds of years and build up enormous stress as frictional resistance in the benioff zone keeps the plates locked. When the stress exceeds a threshold, a sudden failure occurs releasing centuries of stress in a mega thrust earthquake of huge magnitude no matter the depth of focus. E.g. earthquake that triggered 2004 tsunami.

Question 41

What is magnitude?

Answer 41

The amount of energy released by an earthquake.

Question 42

What are the three types of seismic waves?

Answer 42

Primary (P) Waves

Secondary (S) Waves

Surface waves (Rayleigh and Love Waves)

Question 43

What are P Waves?

Answer 43

Primary Waves - Fast waves (less damage, travel further and can travel through solid rock without fluid and even the core) - they travel from the focus and are caused by compression. They push in a backwards and forwards motion.

Question 44

What are S Waves?

Answer 44

Secondary Waves - Slower waves (medium damage, cause crustal fracturing, ground shaking and liquefaction, landslides and tsunamis, can move through mantle) - they travel second from the focus at a slower rate due to longer wavelength and move in a more up/down shaking motion.

Question 45

What are surface waves?

Answer 45

Surface waves - Surface waves (high damage, cause all the same as S waves but to a higher degree, cannot move beyond crust) - surface waves divided into rayleigh waves (rolling motion moving ground up and down and side to side so responsible for shaking felt) but also love waves (moves side to side as it moves forward though at a faster rate than rayleigh waves)

Question 46

How is variation in magnitude linked to seismic waves?

Answer 46

It is linked to variations in the amplitude and frequency of waves - ground surface may be displaced horizontally, vertically or obliquely depending on the strength of the waves.

Question 47

What are the primary hazards of earthquakes?

Answer 47

Ground shaking - causes buildings, bridges, roads… to collapse killing and injuring people

Crustal fracturing - energy released during earthquake causes earth’s crust to crack leaving gaps

Question 48

What are the secondary hazards of earthquakes?

Answer 48

Liquefaction - loosely packed, water logged sediments at/near ground surface lose strength in response to ground shaking. Shaking brings water to the surface making this soft sediment act like quicksand. Liquefaction occurring beneath buildings and other structures can cause sinking toppling and general major damage.

Landslides - shaking triggers collapse of loose ground (can occur when ground is too saturates w rain to maintain stability. Same applies to avalanches triggered by earthquakes.

Question 49

What are the three measures of magnitude of earthquakes?

Answer 49

Moment Magnitude Scale (MMS) measures total amount of energy released by all shockwaves at moment it occurs, size of seismic waves and rock movement. It is infinite (but generally goes on a scale of 1-10) and scale is logarithmic so each number is 10x the magnitude of the number before.

Richter scale - Largely fallen out of use as only uses P and S waves to calculate

Modified Mercalli Intensity Scale - does not measure energy released by the earthquake but rather the effects on people as it takes observations from those who have experienced the earthquake ranking them I (hardly noticed) to XII (catastrophic). Usually magnitude = higher score BUT it is heavily reliant on local conditions (depth of focus, development level, location of epicentre…)

Question 50

What is a tsunami?

Answer 50

A series of waves with deep troughs in between. Created by water column displacement, mostly by undersea plate movements where part of seabed is thrust upwards (or down) very quickly OR by a volcanic eruption on a volcanic island OR by underwater landslides (e.g. at continental shelf edges)

Question 51

How would a tsunami happen due to water displacement at a subduction zone?

Answer 51

-Tectonic plate (pacific) descends/subducts beneath an adjoining plate (NA) in a strike slip fashion with friction building
-At shallow depth the plates are cool and brittle and stick together and they squeeze with overriding plate thickening
-During earthquake the leading edge of overriding plate breaks free springing upwards and plate stretches - its surface falls and the vertical displacement sets off a tsunami
-After earthquake occurs and the water is initially vertically displaced, water is displaced in all directions from the site of the earthquake

Question 52

What happens to tsunami waves as they approach the coastline?

Answer 52

Water shallows = waves slow down but increase in height to produce on shore waves of up to 30m high

Question 53

What two features of the coastline will make a tsunami more destructive?

Answer 53

Bay = wave power concentrated all in one area.

Relief of coastal areas = low land at coastline means a lack of topographical protection from high waves.

Land use or manmade alterations to the coastline may also make a tsunami more destructive.

Question 54

What does the destruction of a tsunami rely on besides physical factors?

Answer 54

Human factors. Land use (e.g. nuclear power plant in Japan in 2011 went into nuclear meltdown with radioactive waste leaking), population density, warning given…

Question 55

What is a volcano?

Answer 55

An extrusive feature found on the earth’s surface which allows hot lava, ash and volcanic gasses to escape from a magma chamber below the earth’s surface.

Question 56

How many recorded volcanoes have erupted in history and how many have they killed in the past 100 years?

Answer 56

500 different volcanoes (not eruptions) in recorded history and 93k people have died in past 100 years

Question 57

At which two types of boundaries do volcanoes occur?

Answer 57

Divergent (constructive) and convergent (destructive)

Question 58

What is the measure used to measure explosivity of volcanic eruptions?

Answer 58

The Volcanic Explosivity Index (VEI) measures the size of eruptions on a theoretically infinite scale (no volcano ever recorded at an 8 or higher) based upon the volume of ejecta (gas, ash, lava and tephra) erupted and how high this is thrown into the atmosphere (plume height).

It is a logarithmic scale so eacg step on the scale is 10x the material ejected

Question 59

What are the characteristics of volcanic eruptions at constructive plate boundaries (lava type, explosivity, size, ejecta volume)?

Answer 59

Lava type - basaltic lava which is the hottest lava type, low gas and silica levels, and often flows extensively

Explosivity - very low on VEI and usually non-explosive so less dangerous, they mainly flow instead

Size - Very small

Ejecta volume - More likely to flow instead of being ejected in a blast. Have short plumes, less ejecta volume, minor ash, gas and tephra

Question 60

What are the characteristics of volcanic eruptions at destructive plate boundaries (lava type, explosivity, size, ejecta volume)?

Answer 60

Lava type - Rhyolitic and Andesitic lava which is more viscous with high silica levels and lots of lava fragments due to solidification in the magma chamber

Explosivity - Very high VEIs of roughly 4-8. Blasts from vent instead of flowing. E.g. Mount Tambora eruption 1815 had a VEI of 7

Size = Often large and extrude out beyond surrounding topography

Ejecta volume - Lots of ejecta volume and high plume heights. Lots of ash, gas and tephra often bringing devastating pyroclastic flow

Question 61

What are the primary hazards of volcanoes? Examples of when hazards have been disastrous?

Answer 61

Lava flow (fast moving lava, often basaltic, which can reach 15m/sec) - Disaster at Lakagigar fissure in Iceland killed 22% of Iceland and in 50% of its livestock in the 18thC due to a famine caused by destroyed crops

-Dome collapse and pyroclastic flow (fast moving dense hot ‘cloud’ of ash with volcanic gasses, volcanic glass/pumice and tephra) - Pyroclastic flow smothered city of Pompeii

-Tephra (ejected rock fragments which vary from large bomb like fragments or fine dust) - Mt. Asama in Japan

-Gas release (water vapour, hydrogen, sulphur dioxide and most dangerous is undetectable carbon monoxide which can accumulate in valleys) - Lake Nyos disaster in Cameroon in 1986 killed 1700 people as they were gassed by volcanic gasses in the night

-Flank collapse (lateral blasting of volcano if the main vent is plugged by very viscous lava causing pressure build up) - Mt St Helens 1980 in USA

Question 62

What are the secondary hazards of volcanoes? Examples of when hazards have been disastrous?

Answer 62

Lahars (mudflows/sluury of fine sand and silt material as a result of re-mobilised old tephra deposits by heavy rainfall) - Mt Pinatubo 1991

Jokulhlaups/glacial outburst floods (floods initiated by failure of an ice or moraine dam or volcanoes melting bottom layer of ice which causes an outburst of water with ash, mud and ice) - 2010 Eyjafjallajokull came with a jokulhlaup