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Geography > Hazardous earth > Flashcards

Hazardous earth Flashcards

1
Q

What is the basic structure of the earth (list in order)

A
  • Lithosphere
  • Asthenosphere
  • Moho discontinuity
  • Inner core
  • Outer core
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2
Q

What separates the core and the mantle?

A

The Mohorovičić discontinuity (MOHO)

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3
Q

What two layers make up the mantle?

A
  • Lithosphere
  • Asthenosphere
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4
Q

How far down is the Moho discontinuity?

A

2900km

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5
Q

What are the features of the asthenosphere?

A
  • Solid layer
  • Flows under pressure
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6
Q

What sections of the earth make up oceanic and continental plates?

A
  • Lithosphere and crust
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7
Q

How are convection currents generated AND where do they occur?

A
  • Convection currents are generated by heat from the earths core.
  • They occur in the asthenosphere
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8
Q

What is the mechanism of convection currents?

A
  • The core heats the asthenosphere
  • As the asthenosphere heats up, material becomes less dense and rises towards the crust.
  • As it rises, it cools and descends due to becoming denser.
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9
Q

How do convection currents influence tectonic plate movement?

A
  • Rising hot material pushes plates apart (in divergent boundaries)
  • Sinking and cooling material can pull plates together (in convergent)
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10
Q

What is the difference between the asthenosphere and lithosphere?

A
  • The lithosphere is a rigid, solid outer later.
  • The asthenosphere is semi-fluid and able to flow under pressure.
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11
Q

Who proposed the theory of continental drift?

A

Alfred Wegener

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11
Q

What was included in Alfred Wegener’s main ideas?

A
  • In the Carboniferous period, a large single continent existed: Pangea.
  • Pangea slowly broke apart into two land masses, and this movement and breaking of land continued to the present day as continents have separated and spread across the globe.
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12
Q

What were the two evidence categories that Wegener used for his theory of continental drift?

A
  • Geological evidence
  • Biological evidence
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12
Q

What was some of Wegener’s geological evidence for continental drift?

A
  • The complementary shapes of South America and Africa, separated by the Atlantic.
  • Mountain chains / rock sequences on either side of the ocean having similarity, for example the Scottish Highlands and North East Canada.
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13
Q

What were some of Wegener’s biological evidence for continental drift?

A
  • Similar fossil brachiopods (marine shellfish) found in Australian AND Indian limestones.
  • Similar fossil animals found in South America and Australia, especially marsupials
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14
Q

What evidence from ancient glaciations supports the theory of continental drift?

A
  • There is evidence from around 290 million years ago of the effects of contemporaneous glaciation in Southern Africa, South America, India and Antarctica.
  • This suggests that these land masses were joined at this time, located near to the South Pole.
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15
Q

Define paleomagnetism

A
  • Traces of changes in the Earth’s magnetic field in the alignment of magnetic minerals within sedimentary and igneous rocks.
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16
Q

Define sea-floor spreading

A
  • The lateral movement of new Oceanic crust away from a mid-ocean ridge.
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17
Q

On what type of plate boundary are mid-ocean ridges on?

A
  • Divergent plate boundaries.
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18
Q

How does paleomagnetism support the theory of continental drift?

A
  • Paleomagnetism shows that magnetic orientations of rocks on either side of a mid-ocean ridge are symmetrical.
    This symmetry suggests sea-floor spreading.
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19
Q

What is evidence of sea-floor spreading?

A
  • Magnetic field data showing that fresh molten rock from the asthenosphere reached the seabed, pushing older rock away from the ridge.
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20
Q

How is seafloor spreading important to the theory of continental drift and plate tectonics?

A
  • As fresh molten rock pushed away older rock, it became clear that plates were being forced to move by sea-floor spreading.
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21
Q

What is the significance of paleomagnetism?

A
  • The magnetic orientation of iron filaments indicate the direction of Earth’s magnetic field.
  • It became apparent that Earth’s polarity is NOT constant - it changes every 400,000-500,000 years.
  • Paleomagnetism led to the proposal of sea-floor spreading.
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22
Q

How does the age of seafloor rocks support continental drift / plate tectonic theory?

A
  • The age of sea-floor rocks acts in support of sea-floor spreading
  • It was found in the 60s through an ocean drilling program that the thickest and oldest sediments were near the continents whereas younger sediment deposits were further out in the oceans.
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23
Q

List evidence for plate tectonics and continental drift.

A
  • Geological and biological evidence (Wegener)
  • Paleomagnetism
  • Sea-floor spreading
  • Age of sea-floor rocks
  • Evidence from ancient glaciation
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24
Q

Outline the global pattern of plates and plate boundaries.

A
  • The lithosphere is divided into 7 large and 3 smaller tectonic plates - so all together there are 10 tectonic plates operating on the lithosphere.
  • Most seismic activity is spatially concentrated in narrow bands, whilst large areas generate few earthquakes in between.
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25
Q

What are the three types of plate boundary?

A
  • Divergent
  • Convergent
  • Conservative
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26
Q

What MAIN process is associated with divergent plate boundaries?

A
  • The formation of new crust.
  • As the crust diverges (moves apart) plumes of magma rise through the asthenosphere and erupt at the surface, creating new crust.
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27
Q

What landforms are associated with divergent plate boundaries and how are they formed?

A
  • Ocean ridges:
  • Formed when plates move apart in oceanic areas. The space between the diverging plates fills with basaltic lava to form a ridge (volcanoes can exist along the ridge)
  • Rift valleys:
  • Formed when plates move apart in continental areas. Occasionally, the brittle crust fractures as sections of it move, so areas of crust drop between parallel faults to form a valley.
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28
Q

Explain the process of forming underwater rift valleys

A
  • Eruption of magma (divergent) onto the seabed means magma is cooled rapidly, forming rounded mounds called pillow lavas.
  • When magma rises towards the surface, the pressure reduces and it liquefies.
  • The overlying rocks are forced upwards as the lithosphere is placed under stress, so it eventually fractures.
  • This produces underwater rift valleys found along mid-ocean ridges.
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29
Q

What occurs at mid-ocean ridges when seawater seeps into rifts?

A
  • When seawater seeps into rifts, it becomes superheated and rises to the surface.
  • This causes a chemical change to the basaltic rocks.
  • Super-heated jets of water sometimes re-emerge at the ocean floor, containing metal sulfides - called black smokers.
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30
Q

What are the three different combinations in which plates may converge in convergent (destructive) plates?

A
  • Oceanic-oceanic
  • Continental-Oceanic
  • Continental-continental
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31
Q

List processes involved in divergent plate boundaries

A
  • Overall formation of new crust
  • Formation of mid-ocean ridges
  • Formation of rift valleys
  • Formation of black smokers
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32
Q

What processes occur at oceanic-continental convergent plate boundaries

A
  • Subduction: the oceanic crust is denser than the continental plate so the oceanic plate subducts under the continental plate to form an ocean trench.
  • Formation of mountain chains: sediments and rocks fold and are uplifted along the leading edge of the continental plate - the continental crust buckles and mountain chains form (such as the Andes).
  • Faulting at the *Benioff zone: occurs during subduction. As the oceanic plate descends, the plate comes under immense pressure and friction. Faulting and fracturing occur in the Benioff zone - releasing immense seismic energy in earthquakes?
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33
Q

What is an ocean trench?

A

An ocean trench are asymmetrical long, narrow depressions formed at the plate boundary, usually at depths of 6000-11000 m deep.

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34
Q

What are landforms associated with oceanic-continental margins?

A
  • Oceanic trenches
  • Mountain chains
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35
Q

What can cause earthquakes in convergent plate margins?

A
  • Faulting and fracturing occurring in the Benioff zone at Oceanic-Continental plate boundaries.
  • This process occurs at descending angles of 45°, and releases considerable energy in the form of earthquakes.
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36
Q

What processes occur at Oceanic-Oceanic convergent plate margins?

A
  • Subduction: The slightly denser oceanic plate will subduct under the less denser oceanic plate - creating a trench.
  • Formation of island arcs: Descending plates melt when subduction occurs, magma rises and chains of volcanoes - island arcs form, such as the Antilles.
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37
Q

At what type of convergent plate boundary do island arcs form?

A

Oceanic-Oceanic convergent plates boundaries.

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38
Q

What are landforms associated with Oceanic-Oceanic convergent plate boundaries?

A
  • Oceanic trench
  • Island arcs
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39
Q

In more depth, how are island arcs formed?

A
  • As a denser oceanic plate subducts below another, dehydration occurs which causes partial melting of the mantle wedge in the plate above.
  • This magma generated rises to the surface and forms chains of volcanic islands called Island arcs
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40
Q

What is an example of an island arc?

A
  • The Antilles in the Caribbean.
  • The North American plate subducted under the smaller Caribbean plate.
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41
Q

What is an example of an oceanic trench?

A
  • The Mariana trench.
  • The Pacific plate is subducted under the Philippine plate.
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42
Q

What processes are associated with Continental-Continental convergent plate boundaries?

A
  • Formation of fold mountains: impact and pressure tends to form fold mountains (e.g- The Himalayas)
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43
Q

Does any subduction occur at Continental-Continental convergent plate boundaries?

A
  • No.
  • Little, if any subduction takes place as the two plates have similar densities.
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44
Q

What type of convergent plate boundary are fold mountains found in?

A
  • Continental-Continental convergent plate boundaries.
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45
Q

List landforms associated with convergent plate boundaries, and the crust combination that forms these.

A
  • Marine trenches ——> Oceanic-Oceanic and Oceanic-Continental
  • Island arcs ——> Oceanic-Oceanic
  • Mountain chains ——> Oceanic-continental
  • Fold mountains ——> Continental-Continental
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46
Q

What is another word for Continental-Continental convergent boundaries?

A
  • Collision boundaries
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47
Q

What is another word for Oceanic-Oceanic convergent boundaries?

A
  • Destructive boundaries
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48
Q

What occurs at conservative plate margins?

A
  • Plates slide past each other, either in the same direction or opposite directions.
  • Earthquakes occur here due to frictional resistance as a result of movement - causing the build up and release of pressure —-> This causes rocks to fracture, releasing enormous amounts of energy.
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49
Q

Do conservative plate margins have volcanic activity?

A
  • No, no subduction takes place.
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50
Q

What are features associated with conservative plate margins?

A
  • They aren’t associated with spectacular landforms
  • It is possible to observe active conservative plate boundaries as they can appear as a giant tear extending through the landscape.
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51
Q

What is an example of a conservative plate margin?

A
  • San Andreas fault.
  • This occurs due to the movement of the North American and Pacific plates that are moving
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52
Q

What are examples of the THREE types of convergent plate margin?

A
  • Oceanic-Continental: The Andes mountains (Nazca subducted under South American plate)
  • Oceanic-Oceanic: The Antilles (North America subducted under Caribbean plate)
  • Continental-Continental: The Alps (African and Eurasian plate collision)
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53
Q

What is an example of a divergent plate margin?

A
  • The Mid-Atlantic ridge
  • The North American plate and Eurasian plate are moving apart
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54
Q

What are the two main types of volcanic eruption?

A
  • Explosive
  • Effusive
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55
Q

Briefly describe effusive eruptions

A
  • Effusive: gentle, free-flowing basic eruption of lava (basaltic)
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56
Q

Briefly describe explosive eruptions

A
  • Explosive: violent, caused by a build up of pressure, with viscous magma ( such as andesite) that prevents the escape of gases
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57
Q

What plate boundaries do explosive eruptions occur at?

A
  • Convergent plate boundaries
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58
Q

What plate boundaries do effusive eruptions occur at?

A
  • Divergent plate boundaries
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59
Q

What type of lava is associated with explosive reactions and what are their characteristics?

A
  • Rhyolitic (acidic)
  • Andesitic (less acidic)
  • Characteristics:
  • Acidic (high silica content),
  • high viscosity,
  • lower temperature at eruption
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60
Q

What type of lava is associated with effusive reactions and what are their characteristics?

A
  • Basaltic
  • Characteristics:
  • Basic (low silica content - so lower acidity)
  • Low viscosity
  • Higher temperature at eruption
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61
Q

What materials do explosive eruptions eject?

A
  • Gas
  • Dust
  • Lava bombs
  • Ash
  • Tephra
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62
Q

What materials do effusive eruptions eject?

A
  • Gas
  • Lava flows
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63
Q

What are the frequencies of eruption for effusive AND explosive eruptions?

A
  • Effusive: Tends to be more frequent, and can last for months at a time
  • Explosive: Tends to have long periods of inactivity.
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64
Q

What 2 volcanic features are associated with explosive eruptions?

A
  • Steep-sided strato-volcanoes
  • Calderas
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65
Q

What 2 volcanic features are associated with effusive eruptions?

A
  • Gently sloping volcanoes
  • Shield volcanoes
  • Lava plateaux (when eruption occurs from multiple fissures)
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66
Q

List the products of explosive eruptions

A
  • Composite cone volcanoes (strato-volcanoes)
  • Sills and dykes (from internal lava flow networks)
  • Calderas (deep craters formed when eruption destroys the cone)
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67
Q

List the products of effusive eruptions

A
  • Lava plateaux (vast area covered by free-flowing lava) –> event called flood basalts.
  • Shield volcanoes (gently sloping sides, usually formed in ocean)
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68
Q

What is meant by viscosity?

A
  • How well a substance flows
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69
Q

How do calderas form?

A
  • An explosive eruption destroys much of the cone
  • The underlying magma chamber is largely emptied.
  • Without support of underground magma, the sides collapse to form a caldera.
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70
Q

Why are explosive eruptions violent?

A
  • The magma is acidic, and therefore high in viscosity as it does not flow easily.
  • The vents often fill with masses of solidified magma, preventing magma from rising freely from the depth.
  • As a result, enormous pressures can build up inside a volcano until it explosively erupts.
71
Q

What are calderas?

A
  • Volcanic craters, usually more than 2km in diameter.
72
Q

Give an example of a caldera

A
  • The 1883 eruption of Krakatoa left a 7km wide caldera.
73
Q

What are strato-volcanoes (composite cone volcanoes)

A
  • A type of volcano, associated with explosive eruptions, made up of layers of ash and acid lava - having concave symmetrical profiles.
74
Q

What is a sill and what is a dyke?

A
  • Sill: A minor, approximately horizontal, intrusion of magma into surrounding older rocks
  • Dyke: A minor, approximatelyvertical intrusion of magma through surrounding older rocks.
75
Q

Where are sills and dykes found?

A
  • In complex internal networks of lava flows, found within strato (composite cone) volcanoes.
76
Q

What is an example of an archipelago of strato-volcanoes?

A
  • Indonesia, with 130 active stratovolcanoes along its archipelago.
77
Q

How are lava plateaux formed?

A
  • Formed when basic magma erupts from multiple fissures, covering vast areas with free-flowing and low viscosity lava.
78
Q

What is an example of a lava plateaux?

A
  • The Deccan Plateau in central India, covering over 500,000km² - known as a LIP: Large igneous province)
79
Q

How do flood basalts (lava plateaux) affect ELSS - SYNOPTIC LINK

A
  • When the flood basalt event occurs, they have a devastating impact on the atmosphere and hydrosphere, affecting the biosphere consequentially.
80
Q

How are shield volcanoes formed?

A
  • Eruptions of basic lava result in volcanoes with gently sloping sides
  • Due to low viscosity, successive flows accumulate and form huge volcanoes, extending horizontally for tens of kilometers.
81
Q

What is an example of a shield volcano?

A
  • Mauna Loa (Hawaii)
82
Q

Where do shield volcanoes usually form and what is an exception?

A
  • Shield volcanoes usually form on the ocean floor, due to the boundaries coinciding with mid-ocean ridges on divergent plate boundaries.
  • An exception is Iceland - with many shield volcanoes such as Skjaldbreidur
83
Q

What are eruptions NOT at plate boundaries associated with?

A

Hot-spots.

84
Q

What are hotspots?

A
  • A fixed area of intense volcanic activity where a plume of magma rises from the mantle and erupts at the surface.
85
Q

Where are hotspots located?

A
  • In intra-plate areas, away from boundaries such as Hawaii.
86
Q

How have some Hawaiian volcanoes become extinct as a result of the hotspot?

A
  • Over millions of years, the Pacific plate moves North-West away from the hotspot.
  • The volcanoes lose their source of magma and become extinct.
87
Q

What happens to volcanoes after they become extinct on a hotspot?

A
  • Weathering and erosion breaks down volcanic rock into deep and fertile soils.
  • Further along the chain to the NW, volcanic islands have sunk below the surface of the Pacific to form underwater mountains or sea mounts.
88
Q

How do hotspots form volcanoes, using Hawaii as an example?

A
  • In Hawaii, to the South East, positioned directly above the hotspot - the next volcano in the chain, Loihi is rising from the ocean floor. It’s position above the hotspot means it’ll continue to grow
89
Q

What two locations witness volcanoes from hotspots?

A
  • Hawaiian chain
  • East African Rift Valley
90
Q

How did the East African Rift Valley form as a result of a hotspot?

A
  • In the past 30 million years, the crust has been uplifted and stretched –> causing tension in local rocks. (AS A RESULT OF HOTSPOT)
  • Rifting occurs as a result, and magma forces it’s way up to the surface and creates a line of several active volcanoes.
  • Mt Kilimanjaro was formed in this way.
91
Q

Do hotspots exclusively lead to shield volcanoes?

A
  • No.
  • El Teide, Tenerife in the Canary Islands is a strato-volcano formed by hotspot.
92
Q

Define ‘super-volcanoes’

A
  • Super-volcanoes are volcanoes that erupts more than 1000km³ of material in a single eruption.
93
Q

What is evidence of past super-volcano activity?

A
  • Giant calderas.
94
Q

What is an example of a super volcano?

A
  • Yellowstone super-volcano in Wyoming
  • It has a caldera of 75km in diameter.
95
Q

What are the 6 basic classifications of volcano?

A
  • Icelandic lava eruptions
  • Hawaiian eruptions
  • Strombolian eruptions
  • Vulcanian eruptions
  • Vesuvian eruptions
  • Plinian eruptions
96
Q

What are Icelandic lava eruptions characterized by?

A
  • Characterized by persistent fissure eruption, in this, large quantities of basaltic lava build up on vast horizontal plains (FLOOD BASALT EVENTS)
97
Q

What are Hawaiian eruptions characterized by?

A
  • Characterized by:
  • More noticeable central activity.
  • Runny, basaltic lava travels down the sides in lava flows
  • Gas escapes more easily.
  • Occasional pyroclastic flows occur
98
Q

What are Strombolian eruptions characterized by?

A
  • Characterized by:
  • Frequent gas explosions - blasting fragments of runny lava to form cones.
  • Very explosive, with large quantities of pyroclastic rock thrown out.
  • Marked by a white cloud of steam emitted from the crater.
99
Q

What are Vulcanian eruptions characterized by?

A
  • Characterized by:
  • Violent gas explosions, blasting out of plugs of sticky or cooled lava
  • Fragments build up into cones of ash and pumice
  • Occur when there is viscous lava that solidifies rapidly after explosion
  • Eruption clears blocked vents and spews large volumes of ash into the atmosphere.
100
Q

What are Vesuvian eruptions characterized by?

A
101
Q

What are Plinian eruptions characterized by?

A
102
Q

What is used to measure volcanic eruptions?

A
  • The Volcanic Explosivity Index —-> VEI
103
Q

How does the Volcanic Explosivity Index work?

A
  • The VEI combines magnitude and intensity into a single number from 1-8.
  • 0= least explosive, 8= the most explosive.
  • It is logarithmic, each number increase represents a ten-fold increase in explosivity.
104
Q

List factors taken into account under the Volcanic Explosivity Index

A
  • Volume of erupted material
  • The height the ejected material reaches
  • The duration in hours
  • Qualitative descriptions
105
Q

What is a limitation to using the Volcanic Explosivity Index?

A
  • The VEI is not that useful for effusive eruptions, such as in Hawaii as it’s factors accounted for are more related to explosive eruptions
106
Q

What is a strength of using the Volcanic Explosivity Index?

A
  • It is valuable in suggesting the relative impacts volcanoes might have at different geological sites, having practical application.
107
Q

Define what is meant by ‘magnitude’ and ‘intensity’ in relation to the VEI

A
  • Magnitude: The amount of material erupted.
  • Intensity: The speed at which the material is erupted.
108
Q

List the names of hazards created by volcanic activity

A
  • Lava flows
  • Pyroclastic flows
  • Tephra and ash
  • Toxic gas emission
  • Lahars and Jökulhlaups (associated with melting of ice)
  • Tsunami
109
Q

Define lava flows (volcanic hazard)

A

Flows or streams of molten rock that pour from an erupting vent.

110
Q

Why is lava flow a hazard?

A

Lava is destructive so will burn, bury or bulldoze infrastructure, property, natural vegetation and agricultural land.
Despite this, lava flows rarely cause injuries or fatalities

111
Q

What makes an event a hazard?

A

When they interact with human communities and activities.

112
Q

Define pyroclastic flow (volcanic hazard)

A

A combination of very hot gases (500 degrees+), ash and rock fragments travelling at high speeds of over 100km/h that follow the contours of the ground, destroying everything in its path.

113
Q

Why are pyroclastic flows a hazard?

A

As they have heats of above 500 degrees, they destroy everything in their path.
They pose a threat to human life as inhalation of this hot and poisonous gas causes instant death - such as in Pompeii.

114
Q

Define tephra (volcanic hazard)

A

Any material ejected from a volcano into the atmosphere - ranging in sizes from very fine ash to large volcanic bombs.

115
Q

Why is tephra a hazard?

A

-Tephra has potential to be very hazardous, it buries farmland in layers of ash which destroys crops.

  • Eruption columns of tephra carry material into the stratosphere due to the material temperature being hotter than the surrounding air so it rises, helped by the explosion itself. As a result, human activity such as travelling can be disrupted like the 2010 Iceland eruption that caused the cancellation of 100,000 flights.
  • Buildings can collapse due to the weight of accumulated ash - and it creates issues for those with respiratory diseases.
116
Q

What are lava flows hazards dependent on?

A
  • The type of lava (basaltic or acidic)
  • Acidic lavas such as rhyolite are viscous so do not flow easily.
  • Basic lavas is free-flowing and can run for considerable distances.
117
Q

What gases can be emitted from volcanic eruptions?

A

Gases like carbon monoxide, carbon dioxide and sulfur dioxide.

118
Q

Why are toxic gas emissions a hazard?

A
  • Gases can pose a deadly threat to humans, such as the unexpected death of the population around Lake Nyos in Cameroon due to asphyxiation.
  • When sulfur dioxide combines with water from the atmosphere, ocean or free standing bodies - acid rain is produced. Acid rain further causes weathering, destruction of crops and the pollution of surface waters and soils.
119
Q

Define lahars (volcanic hazard)

A

A type of mudflow generated by volcanic activity with the consistency of wet concrete that travels at speeds of up to 50km/h

120
Q

Define Jökulhlaups (volcanic hazard)

A

A torrent of water that floods as a result of volcanic activity.

121
Q

How does a lahar occur?

A
  • Snow and ice on a volcanoes summit melt during an eruption and flow rapidly down the cone.
  • As the melted snow and ice flows, rock fragments, ash and soil are mixed together - creating a mudflow.
122
Q

Why are lahars a hazard?

A

Lahars can destroy everything in their path or bury them in thick layers of debris.
This is the case in South East Asia, where ash-covered volcano slopes continue to generate lahar hazards after periods of heavy rain.

123
Q

How do Jökulhlaups occur?

A
  • Volcanic eruptions beneath an icefield or glacier causes rapid melting.
  • During this eruption, vast quantities of water accumulate until exiting - creating a resulting torrent of water.
  • They can also be caused by snow melt on summits.
124
Q

Define tsunami

A

A large scale displacement of ocean water that travels in waves of speeds up to 600km/h

125
Q

How are tsunamis generated as a volcanic hazard

A

Violent eruptions, seen in explosive eruptions, can cause land movement which causes displacement of water.
This is believed to be a potential hazard for Las Palmas.

126
Q

What factors can be considered when assessing volcanic hazards

A
  • Whether they are short term or long term (temporal scale)
  • Proximity to humans
  • Whether the eruption was explosive or effusive.
127
Q

How can volcanic hazards interrupt earths life support systems (ELSS SYNOPTIC LINK)

A
  • The ejection of tephra and ash into the atmosphere changes the carbon composition within the atmospheric store.
  • The burial of agricultural land in tephra disrupts the biosphere store of carbon and water in crops.
  • Acid rain from toxic gases emitted pollutes surface water and soils, perpetuating the destruction of crops.
  • Jökulhlaups cause a transfer in water from stores in icefields to free standing water, which is more easily transferred between atmosphere and biosphere as a result.
  • In the event of a super volcano eruption, global temperatures will fall as ash blocks insolation —> as a result all flows are disturbed, as temperature plays a large role in transfers.
128
Q

Define an earthquake

A

The release of stress that has built up within the Earth’s crust caused by tension, compression or the shearing of rocks.

129
Q

What is meant by the ‘focus’ and ‘epicenter’ of an earthquake?

A
  • The ‘focus’ of an earthquake is the location where the stress is released.
  • The ‘epicenter’ of an earthquake is immediately above the focus, on the earth’s surface.
130
Q

Where are the 4 main locations for earthquakes?

A
  • Mid-ocean ridges
  • Ocean trenches and island arcs
  • Collision zones
  • Conservative plate margins.
131
Q

How do foreshocks indicate an earthquake will occur?

A
  • Some earthquakes are preceded by foreshocks
  • If foreshocks have the same P and S wave profiles, they may signal that a large event is likely to occur which allows warning.
132
Q

Why do earthquakes occur at mid-ocean ridges?

A

There is a creation of tensional forces associated with spreading, faulting and rifting.

133
Q

Why do earthquakes occur at ocean trenches and island arcs?

A

There is a creation of compressive forces associated with the subduction of one plate below another

134
Q

Why do earthquakes occur at collision zones?

A

There are compressive forces associated with the grinding together of plates carrying continental crust.

135
Q

Why do earthquakes occur at conservative plate margins

A

There are shearing forces associated with the intermittent movement of one plate past another.

136
Q

Which is more widely distributed and less predictable, earthquakes or volcanoes?

A

Earthquakes.

137
Q

List the 3 types of seismic waves

A
  • P waves (primary waves)
  • S waves (secondary waves)
  • L waves (surface waves)
138
Q

What are the characteristics of P waves and how do they move?

A
  • Fast-travelling
  • Low frequency
  • Compressional waves
  • Can travel through solids and liquids, through the earths interior.
  • P waves vibrate in the direction in which they travel.
139
Q

What are the characteristics of S waves and how do they move?

A
  • Half the speed of P waves
  • High frequency
  • Cannot pass through liquids, so cannot travel through outer core.
  • They vibrate at right angles to the direction in which they travel.
140
Q

What are the characteristics of L waves and how do they travel?

A
  • Slowest of the 3 wave types
  • Low frequency
  • Travel through the outer crust only
  • They can move the surface vertically, whilst others can move the ground at right angles to the direction of movement.
141
Q

How are earthquakes often categorized?

A

According to their depth of focus.

142
Q

What are the two types of earthquakes, according to depth of focus?

A
  • Shallow focus
  • Deep focus
143
Q

Describe shallow focus earthquakes

A
  • Operates on surface down to 70km
  • Occur in cold, brittle rocks
  • Very common
  • Release only low levels of energy
144
Q

Describe deep focus earthquakes

A
  • Operates at 70-700km below.
  • With increasing depth, there are high pressures and temperatures.
  • Minerals change type and volume
  • Understanding is still evolving, dehydration in subducting plates.
  • Less frequent but more powerful
145
Q

Name the 3 ways of measuring earthquakes

A
  • Richter scale
  • Moment magnitude scale
  • Modified Mercalli scale
146
Q

How does the Richter scale work?

A
  • It uses the amplitude of seismic waves to determine magnitude.
  • The scale is logarithmic, so each whole-number increase in magnitude represents a ten-fold increase in seismic wave amplitude.
  • It has no upper limit, though the largest earthquakes record a magnitude of around 9 (Tohoku earthquake 2011).
  • It CANNOT be used to express damage.
147
Q

What is damage from an earthquake dependent on?

A
  • Magnitude
  • Population density
  • Levels of preparedness.
148
Q

How does the Moment Magnitude scale work?

A
  • Energy release is measured related to geology, area of fault surface and the amount of movement on the fault.
  • It is also logarithmic, expressed from 1.0-9.0.
  • It provides the most accurate measurement of large earthquakes due to using amount of physical movement, a direct function of energy.
  • It is NOT used for small earthquakes.
149
Q

How does the Modified Mercalli scale work?

A
  • Measures earthquake intensity AND its impact.
  • It relates ground movement to impacts that can be felt and seen by anyone in the affected location
  • It is qualitative, based on observation and description.
150
Q

What is a rift valley?

A
  • A valley formed by downfaulting between parallel faults (landform associated with earthquakes)
151
Q

What are 3 distinct landforms that can be created by earthquakes?

A
  • Mountain ranges (e.g: the Himalayas)
  • Rift valleys
  • Escarpments
152
Q

What is an escarpment?

A
  • A tilt block forming an extensive upland area, with a short and steep scarp slope and a long, gentle dip slope on the other side.
153
Q

What is an example of a mountain range formed by earthquakes?

A

The Himalayan-Karakoram range in Asia.

154
Q

How did the Himalayan-Karakoram mountain chain form (a landform associated with earthquakes)

A
  • This mountain chain was formed by the Northward drift of India into Eurasia, causing the continental collision that led to a complex pattern of folding and faulting of rocks.
155
Q

How are escarpments evidence of distinct landforms caused by earthquakes?

A
  • Inward facing escarpments of rift valleys mark the location of faults caused by tension and compression in the crust
156
Q

How many were killed in earthquakes between 2000 and 2015 and what is a possible explanation?

A
  • Between 800,000-900,000.
  • Many occurred in areas that were not thought to be vulnerable such as Kobe and Sichuan.
157
Q

List hazards produced by earthquakes

A
  • Ground shaking and ground displacement
  • Liquefaction
  • Landslides and avalanches
  • Tsunamis
  • Flooding
158
Q

Define ground shaking and ground displacement

A

The vertical and horizontal movement of the ground.

159
Q

What 3 things does the severity of ground shaking depend on (seismic hazard)

A
  • Earthquake magnitude
  • Distance from epicenter
  • Local geology.
160
Q

How does ground shaking and displacement act as a hazard?

A
  • Vertical movement can destabilize structures, causing collapse
  • Displacement of rocks along fault lines can rip apart pipelines and sewers, severing rigid structures such as railway tracks and roads.
  • Natural drainage can be effected
161
Q

How can the seismic hazard of ground shaking and displacement affect the natural environment (ELSS SYNOPTIC LINK)

A
  • Surface displacement can disrupt natural drainage
  • Rivers and streams will be diverted and groundwater movement will be effected in aquifers
  • This has serious implications for public water and irrigation.
162
Q

Define liquefaction

A

A process by which sediment and soils lose their mechanical strength from a sudden loss of cohesion, causing these materials to behave like liquids.

163
Q

When does liquefaction occur?

A

When soils and sediments are loosely consolidated, having surface materials of fine-grained sands, alluvium and landfill with a high water content.

164
Q

Give an example of an earthquake where liquefaction occurred

A
  • The Kobe Earthquake
  • This was because the port had been built on reclaimed land in Osaka bay
165
Q

What two seismic hazards lead to slope failure, and subsequent avalanches and landslides

A
  • Ground shaking
  • Liquefaction
166
Q

What increases vulnerability to landslides and avalanches (ELSS synoptic link)

A
  • Deforestation
  • Heavy monsoon rains
167
Q

Define what avalanches and landslides are

A

Slope failure as a result of ground shaking and (potentially) liquefaction.

168
Q

Why are avalanches and landslides hazards?

A
  • Landslides block transport routes in mountainous regions where accessibility is already difficult.
  • Movement of soil and rock on slopes can block rivers, creating natural dams with temporary lakes - threatening downstream areas with floods.
  • In upland areas, dams are also affected as landslides could displace water and generate waves to areas below.
169
Q

Define tsunamis in terms of earthquakes

A

A giant sea wave generated by shallow focus underwater earthquakes.

170
Q

Describe the characteristics of a tsunami

A
171
Q

How does seabed uplift form tsunamis?

A
  • Underwater earthquakes cause the seabed to rise vertically
  • Water above is displaced, producing powerful waves at the surface that spread out at high velocity from the epicenter.
  • As tsunamis approach shore, wave height increases when entering shallow water.
  • Before the wave breaks, water in front of the wave is pulled back out to sea in a process called drawdown
  • Finally, the tsunami hits land as a wall of water that can exceed 25m in height.
172
Q

In what 3 ways can tsunamis form?

A
  • Displacement of land from explosive volcanic eruptions
  • Underwater landslides as a result of an earthquake
  • Vertical rising of the seabed which displaces water.
173
Q

What, arguably, is the most extreme seismic hazard and why?

A
  • Tsunamis
  • Effective protection is extremely difficult and impact often occurs a long way away from its origin, making warning ineffective.
174
Q

In what ways can earthquakes cause flooding?

A
  • Triggering tsunamis
  • Destabilizing dams
  • Destroying and/or lowering protective levees.
175
Q

How do underwater landslides cause tsunamis?

A
  • A large volume of rock is shaken and slides downslope.
  • Water is dragged behind it from all sides and collides in the center, generating a tsunami wave that radiates outwards.
176
Q

What happens when a tsunami approaches shore?

A
  • As tsunamis approach shore, wave height increases when entering shallow water.
  • Before the wave breaks, water in front of the wave is pulled back out to sea in a process called drawdown
  • Finally, the tsunami hits land as a wall of water that can exceed 25m in height.
177
Q
A

Geography (36 decks)

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