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
2
Q
What separates the core and the mantle?
A
The Mohorovičić discontinuity (MOHO)
3
Q
What two layers make up the mantle?
A
- Lithosphere
- Asthenosphere
4
Q
How far down is the Moho discontinuity?
A
2900km
5
Q
What are the features of the asthenosphere?
A
- Solid layer
- Flows under pressure
6
Q
What sections of the earth make up oceanic and continental plates?
A
- Lithosphere and crust
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
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.
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)
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.
11
Q
Who proposed the theory of continental drift?
A
Alfred Wegener
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.
12
Q
What were the two evidence categories that Wegener used for his theory of continental drift?
A
- Geological evidence
- Biological evidence
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.
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
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.
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.
16
Q
Define sea-floor spreading
A
- The lateral movement of new Oceanic crust away from a mid-ocean ridge.
17
Q
On what type of plate boundary are mid-ocean ridges on?
A
- Divergent plate boundaries.
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.
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.
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.
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.
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.
23
List evidence for plate tectonics and continental drift.
- Geological and biological evidence (Wegener)
- Paleomagnetism
- Sea-floor spreading
- Age of sea-floor rocks
- Evidence from ancient glaciation
24
Outline the global pattern of plates and plate boundaries.
- 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.
25
What are the three types of plate boundary?
- Divergent
- Convergent
- Conservative
26
What MAIN process is associated with divergent plate boundaries?
- 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.
27
What landforms are associated with divergent plate boundaries and how are they formed?
- **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.
28
Explain the process of forming underwater rift valleys
- 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.
29
What occurs at mid-ocean ridges when seawater seeps into rifts?
- 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**.
30
What are the three different combinations in which plates may converge in convergent (destructive) plates?
- Oceanic-oceanic
- Continental-Oceanic
- Continental-continental
31
List processes involved in divergent plate boundaries
- Overall formation of new crust
- Formation of mid-ocean ridges
- Formation of rift valleys
- Formation of black smokers
32
What processes occur at oceanic-continental **convergent** plate boundaries
- **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?
33
What is an ocean trench?
An ocean trench are asymmetrical long, narrow depressions formed at the plate boundary, usually at depths of 6000-11000 m deep.
34
What are landforms associated with oceanic-continental margins?
- Oceanic trenches
- Mountain chains
35
What can cause earthquakes in convergent plate margins?
- 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.
36
What processes occur at Oceanic-Oceanic **convergent** plate margins?
- **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.
37
At what type of convergent plate boundary do island arcs form?
Oceanic-Oceanic convergent plates boundaries.
38
What are landforms associated with Oceanic-Oceanic **convergent** plate boundaries?
- Oceanic trench
- Island arcs
39
In more depth, how are island arcs formed?
- 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**
40
What is an example of an island arc?
- The Antilles in the Caribbean.
- The North American plate subducted under the smaller Caribbean plate.
41
What is an example of an oceanic trench?
- The Mariana trench.
- The Pacific plate is subducted under the Philippine plate.
42
What processes are associated with Continental-Continental **convergent** plate boundaries?
- Formation of fold mountains: impact and pressure tends to form fold mountains (e.g- The Himalayas)
43
Does any subduction occur at Continental-Continental **convergent** plate boundaries?
- No.
- Little, if any subduction takes place as the two plates have similar densities.
44
What type of convergent plate boundary are fold mountains found in?
- Continental-Continental convergent plate boundaries.
45
List landforms associated with convergent plate boundaries, and the crust combination that forms these.
- Marine trenches ------> Oceanic-Oceanic and Oceanic-Continental
- Island arcs ------> Oceanic-Oceanic
- Mountain chains ------> Oceanic-continental
- Fold mountains ------> Continental-Continental
46
What is another word for Continental-Continental convergent boundaries?
- Collision boundaries
47
What is another word for Oceanic-Oceanic convergent boundaries?
- Destructive boundaries
48
What occurs at conservative plate margins?
- 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.
49
Do conservative plate margins have volcanic activity?
- No, no subduction takes place.
50
What are features associated with conservative plate margins?
- 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.
51
What is an example of a conservative plate margin?
- San Andreas fault.
- This occurs due to the movement of the North American and Pacific plates that are moving
52
What are examples of the THREE types of convergent plate margin?
- 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)
53
What is an example of a divergent plate margin?
- The Mid-Atlantic ridge
- The North American plate and Eurasian plate are moving apart
54
What are the two main types of volcanic eruption?
- Explosive
- Effusive
55
Briefly describe effusive eruptions
- Effusive: gentle, free-flowing basic eruption of lava (basaltic)
56
Briefly describe explosive eruptions
- Explosive: violent, caused by a build up of pressure, with viscous magma ( such as andesite) that prevents the escape of gases
57
What plate boundaries do explosive eruptions occur at?
- Convergent plate boundaries
58
What plate boundaries do effusive eruptions occur at?
- Divergent plate boundaries
59
What type of lava is associated with **explosive reactions** and what are their characteristics?
- Rhyolitic (acidic)
- Andesitic (less acidic)
- **Characteristics:**
- Acidic (high silica content),
- high viscosity,
- lower temperature at eruption
60
What type of lava is associated with **effusive reactions** and what are their characteristics?
- Basaltic
- **Characteristics:**
- Basic (low silica content - so lower acidity)
- Low viscosity
- Higher temperature at eruption
61
What materials do **explosive eruptions** eject?
- Gas
- Dust
- Lava bombs
- Ash
- Tephra
62
What materials do **effusive eruptions** eject?
- Gas
- Lava flows
63
What are the frequencies of eruption for effusive AND explosive eruptions?
- **Effusive:** Tends to be more frequent, and can last for months at a time
- **Explosive:** Tends to have long periods of inactivity.
64
What 2 volcanic features are associated with **explosive** eruptions?
- Steep-sided strato-volcanoes
- Calderas
65
What 2 volcanic features are associated with **effusive** eruptions?
- Gently sloping volcanoes
- Shield volcanoes
- Lava plateaux (when eruption occurs from multiple fissures)
66
List the *products* of **explosive** eruptions
- Composite cone volcanoes (strato-volcanoes)
- Sills and dykes (from internal lava flow networks)
- Calderas (deep craters formed when eruption destroys the cone)
67
List the *products* of **effusive** eruptions
- Lava plateaux (vast area covered by free-flowing lava) --> event called flood basalts.
- Shield volcanoes (gently sloping sides, usually formed in ocean)
68
What is meant by viscosity?
- How well a substance flows
69
How do calderas form?
- 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.
70
Why are explosive eruptions violent?
- 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
What are calderas?
- Volcanic craters, usually more than 2km in diameter.
72
Give an example of a caldera
- The 1883 eruption of Krakatoa left a 7km wide caldera.
73
What are strato-volcanoes (composite cone volcanoes)
- A type of volcano, associated with explosive eruptions, made up of layers of ash and acid lava - having concave symmetrical profiles.
74
What is a sill and what is a dyke?
- **Sill:** A minor, approximately *horizontal*, intrusion of magma into surrounding older rocks
- **Dyke**: A minor, approximately*vertical* intrusion of magma through surrounding older rocks.
75
Where are sills and dykes found?
- In complex internal networks of lava flows, found within strato (composite cone) volcanoes.
76
What is an example of an archipelago of strato-volcanoes?
- Indonesia, with 130 active stratovolcanoes along its archipelago.
77
How are lava plateaux formed?
- Formed when *basic* magma erupts from multiple fissures, covering vast areas with free-flowing and low viscosity lava.
78
What is an example of a lava plateaux?
- The Deccan Plateau in central India, covering over 500,000km² - known as a **LIP: Large igneous province**)
79
How do flood basalts (lava plateaux) affect *ELSS* - **SYNOPTIC LINK**
- When the flood basalt event occurs, they have a devastating impact on the atmosphere and hydrosphere, affecting the biosphere consequentially.
80
How are shield volcanoes formed?
- 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
What is an example of a shield volcano?
- Mauna Loa (Hawaii)
82
Where do shield volcanoes usually form and what is an exception?
- 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
What are eruptions **NOT** at plate boundaries associated with?
Hot-spots.
84
What are hotspots?
- A *fixed* area of intense volcanic activity where a plume of magma rises from the mantle and erupts at the surface.
85
Where are hotspots located?
- In intra-plate areas, away from boundaries such as Hawaii.
86
How have some Hawaiian volcanoes become extinct as a result of the hotspot?
- 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
What happens to volcanoes after they become extinct on a hotspot?
- 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
How do hotspots form volcanoes, using Hawaii as an example?
- 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
What two locations witness volcanoes from hotspots?
- Hawaiian chain
- East African Rift Valley
90
How did the East African Rift Valley form as a result of a hotspot?
- 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
Do hotspots exclusively lead to shield volcanoes?
- No.
- El Teide, Tenerife in the Canary Islands is a strato-volcano formed by hotspot.
92
Define 'super-volcanoes'
- Super-volcanoes are volcanoes that erupts more than 1000km³ of material in a single eruption.
93
What is evidence of past super-volcano activity?
- Giant calderas.
94
What is an example of a super volcano?
- Yellowstone super-volcano in Wyoming
- It has a caldera of 75km in diameter.
95
What are the 6 basic classifications of volcano?
- Icelandic lava eruptions
- Hawaiian eruptions
- Strombolian eruptions
- Vulcanian eruptions
- Vesuvian eruptions
- Plinian eruptions
96
What are **Icelandic lava eruptions** characterized by?
- Characterized by persistent fissure eruption, in this, large quantities of basaltic lava build up on vast horizontal plains (FLOOD BASALT EVENTS)
97
What are **Hawaiian eruptions** characterized by?
- 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
What are **Strombolian eruptions** characterized by?
- 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
What are **Vulcanian eruptions** characterized by?
- 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
What are **Vesuvian eruptions** characterized by?
101
What are **Plinian eruptions** characterized by?
102
What is used to measure volcanic eruptions?
- The **Volcanic Explosivity Index** ----> **VEI**
103
How does the **Volcanic Explosivity Index** work?
- 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
List factors taken into account under the Volcanic Explosivity Index
- Volume of erupted material
- The height the ejected material reaches
- The duration in hours
- Qualitative descriptions
105
What is a limitation to using the Volcanic Explosivity Index?
- 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
What is a strength of using the Volcanic Explosivity Index?
- It is valuable in suggesting the relative impacts volcanoes might have at different geological sites, having practical application.
107
Define what is meant by 'magnitude' and 'intensity' in relation to the VEI
- *Magnitude*: The amount of material erupted.
- *Intensity*: The speed at which the material is erupted.
108
List the names of hazards created by volcanic activity
- Lava flows
- Pyroclastic flows
- Tephra and ash
- Toxic gas emission
- Lahars and Jökulhlaups (associated with melting of ice)
- Tsunami
109
Define lava flows (volcanic hazard)
Flows or streams of molten rock that pour from an erupting vent.
110
Why is lava flow a hazard?
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
What makes an event a hazard?
When they interact with human communities and activities.
112
Define pyroclastic flow (volcanic hazard)
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
Why are pyroclastic flows a hazard?
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
Define tephra (volcanic hazard)
Any material ejected from a volcano into the atmosphere - ranging in sizes from very fine ash to large volcanic bombs.
115
Why is tephra a hazard?
-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
What are lava flows hazards dependent on?
- 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
What gases can be emitted from volcanic eruptions?
Gases like carbon monoxide, carbon dioxide and sulfur dioxide.
118
Why are toxic gas emissions a hazard?
- 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
Define lahars (volcanic hazard)
A type of mudflow generated by volcanic activity with the consistency of wet concrete that travels at speeds of up to 50km/h
120
Define Jökulhlaups (volcanic hazard)
A torrent of water that floods as a result of volcanic activity.
121
How does a lahar occur?
- 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
Why are lahars a hazard?
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
How do Jökulhlaups occur?
- 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
Define tsunami
A large scale displacement of ocean water that travels in waves of speeds up to 600km/h
125
How are tsunamis generated as a volcanic hazard
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
What factors can be considered when assessing volcanic hazards
- Whether they are short term or long term (temporal scale)
- Proximity to humans
- Whether the eruption was explosive or effusive.
127
How can volcanic hazards interrupt earths life support systems (ELSS SYNOPTIC LINK)
- 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
Define an earthquake
The release of stress that has built up within the Earth's crust caused by tension, compression or the shearing of rocks.
129
What is meant by the 'focus' and 'epicenter' of an earthquake?
- 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
Where are the 4 main locations for earthquakes?
- Mid-ocean ridges
- Ocean trenches and island arcs
- Collision zones
- Conservative plate margins.
131
How do foreshocks indicate an earthquake will occur?
- 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
Why do earthquakes occur at mid-ocean ridges?
There is a creation of tensional forces associated with spreading, faulting and rifting.
133
Why do earthquakes occur at ocean trenches and island arcs?
There is a creation of compressive forces associated with the subduction of one plate below another
134
Why do earthquakes occur at collision zones?
There are compressive forces associated with the grinding together of plates carrying continental crust.
135
Why do earthquakes occur at conservative plate margins
There are shearing forces associated with the intermittent movement of one plate past another.
136
Which is more widely distributed and less predictable, earthquakes or volcanoes?
Earthquakes.
137
List the 3 types of seismic waves
- P waves (primary waves)
- S waves (secondary waves)
- L waves (surface waves)
138
What are the characteristics of P waves and how do they move?
- 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
What are the characteristics of S waves and how do they move?
- 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
What are the characteristics of L waves and how do they travel?
- 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
How are earthquakes often categorized?
According to their depth of focus.
142
What are the two types of earthquakes, according to depth of focus?
- Shallow focus
- Deep focus
143
Describe shallow focus earthquakes
- Operates on surface down to 70km
- Occur in cold, brittle rocks
- Very common
- Release only low levels of energy
144
Describe deep focus earthquakes
- 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
Name the 3 ways of measuring earthquakes
- Richter scale
- Moment magnitude scale
- Modified Mercalli scale
146
How does the Richter scale work?
- 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
What is damage from an earthquake dependent on?
- Magnitude
- Population density
- Levels of preparedness.
148
How does the Moment Magnitude scale work?
- 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
How does the Modified Mercalli scale work?
- 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
What is a rift valley?
- A valley formed by downfaulting between parallel faults (landform associated with earthquakes)
151
What are 3 distinct landforms that can be created by earthquakes?
- Mountain ranges (e.g: the Himalayas)
- Rift valleys
- Escarpments
152
What is an escarpment?
- 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
What is an example of a mountain range formed by earthquakes?
The Himalayan-Karakoram range in Asia.
154
How did the Himalayan-Karakoram mountain chain form (a landform associated with earthquakes)
- 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
How are escarpments evidence of distinct landforms caused by earthquakes?
- Inward facing escarpments of rift valleys mark the location of faults caused by *tension* and *compression* in the crust
156
How many were killed in earthquakes between 2000 and 2015 and what is a possible explanation?
- Between 800,000-900,000.
- Many occurred in areas that were not thought to be vulnerable such as Kobe and Sichuan.
157
List hazards produced by earthquakes
- Ground shaking and ground displacement
- Liquefaction
- Landslides and avalanches
- Tsunamis
- Flooding
158
Define ground shaking and ground displacement
The vertical and horizontal movement of the ground.
159
What 3 things does the severity of ground shaking depend on (seismic hazard)
- Earthquake magnitude
- Distance from epicenter
- Local geology.
160
How does ground shaking and displacement act as a hazard?
- 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
How can the seismic hazard of ground shaking and displacement affect the natural environment (ELSS SYNOPTIC LINK)
- 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
Define liquefaction
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
When does liquefaction occur?
When soils and sediments are loosely consolidated, having surface materials of fine-grained sands, alluvium and landfill with a high water content.
164
Give an example of an earthquake where liquefaction occurred
- The Kobe Earthquake
- This was because the port had been built on reclaimed land in Osaka bay
165
What two seismic hazards lead to slope failure, and subsequent avalanches and landslides
- Ground shaking
- Liquefaction
166
What increases vulnerability to landslides and avalanches (ELSS synoptic link)
- Deforestation
- Heavy monsoon rains
167
Define what avalanches and landslides are
Slope failure as a result of ground shaking and (potentially) liquefaction.
168
Why are avalanches and landslides hazards?
- 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
Define tsunamis in terms of earthquakes
A giant sea wave generated by shallow focus underwater earthquakes.
170
Describe the characteristics of a tsunami
171
How does seabed uplift form tsunamis?
- 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
In what 3 ways can tsunamis form?
- Displacement of land from explosive volcanic eruptions
- Underwater landslides as a result of an earthquake
- Vertical rising of the seabed which displaces water.
173
What, arguably, is the most extreme seismic hazard and why?
- Tsunamis
- Effective protection is extremely difficult and impact often occurs a long way away from its origin, making warning ineffective.
174
In what ways can earthquakes cause flooding?
- Triggering tsunamis
- Destabilizing dams
- Destroying and/or lowering protective levees.
175
How do underwater landslides cause tsunamis?
- 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
What happens when a tsunami approaches shore?
- 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
What does a volcano need to do to be classified as active
- An active volcano is one that has erupted since the last glacial period or within the past 10,000 years.
178
Differentiate between active, dormant and extinct volcanoes
- Active: has erupted since glacial period or in the last 10000 years.
- Dormant: has not erupted in 10000 years, but is expected to erupt in the future.
- Extinct: is not expected to erupt again
179
List reasons to why people live in tectonically active locations
- High soil fertility
- Economic benefits from tourist industry
- Geothermal power production (Iceland)
- Natural resources for construction (gold and copper extraction for technology)
- Cultural significance
180
What is also a big contributing factor into why people live near active volcanoes?
- Risk perception into whether a volcano is classified as active, dormant or extinct.
181
What are the 3 overarching categories for strategies managing tectonic hazards?
- Modifying event
- Modifying vulnerability
- Modifying loss
182
What is meant by “modifying event” for tectonic hazards?
- Any actions taken during or immediately after a hazard occurs to reduce its impact.
183
What is meant by “modifying vulnerability” for tectonic hazards?
Increasing resilience and a communities capacity to cope with a tectonic hazard before the event occurs.
184
What is meant by “modifying loss” in tectonic hazards?
Reducing the impact of losses experienced from a hazard following the event
185
What is meant by *resilience* in tectonic hazards?
- Resilience is an indication of the rate of recovery from a hazardous event.
186
Give examples of strategies that “modify event” in tectonic hazards
- Lava diversion channels
- Spraying lava to cool and solidify it
- Slowing lava flows through placing concrete blocks
187
Give examples of strategies used to “modify vulnerability” to tectonic hazards
- Education
- Community preparedness (building shelters)
- Prediction and warning
- Aseismic and hazard-resistant building design
- Hazard mapping
- Land use zoning
188
What is meant by “modifying loss” in tectonic hazards?
- Strategies implemented *after* the event has occurred to reduce its adverse effects on people and property.
- These strategies are designed to alleviate suffering and facilitate recovery.
189
Give examples of strategies that “modify loss” in tectonic hazards
- Emergency aid (provision of resources)
- Disaster response teams and equipment deployed
- Search and rescue strategies
- Insurance for buildings and business
- Resources for rebuilding
190
Give examples of how buildings can be aseismically designed
- Adding steel frames and braces capable of moving without collapse
- Foundations containing rubber shock absorbers
- Deep foundations
- Having a 'soft storey' base
191
What is meant by 'hazard mapping' in modifying vulnerability?
Predicting hazard routes, such as lahar routes and ground likely to liquefy.
! NOTE: Land-use zoning is implemented in response to this, avoiding building in locations identified by this.
192
What is a recurrence interval?
The average time between two events of equal magnitude.
193
How have risks from tectonic hazards increased over time?
- The number of natural disasters have increased overtime, with an increase in frequency and magnitude of some hazards produced such as flooding.
- Despite this, the increase in earthquakes and volcanic eruptions is less pronounced.
194
Give explanations as to why it appears tectonic hazards have increased overtime
- Human factors can have influence on the subsequent effects of hazards, for example with deforestation, this creates more ground instability which leads to lahars.
- A counter argument is that measuring and reporting of these events have become much more reliable over the last century, so these are documented more.
195
Why have tectonic events had a less pronounced change overtime?
Human activities play no part in causing earthquakes and volcanic eruptions
(despite influence on *impacts* of these hazard events*).
196
What is the relationship between recurrence interval and earthquake magnitude?
- The greater the magnitude of an eruption or earthquake, the longer the recurrence interval (it occurs less frequently).
- Vice versa.
197
When do geophysical events become *hazards*?
When they pose a risk to human populations
198
What does the disaster risk equation indicate?
The hazard vulnerability for a location
199
State the formula for the disaster risk equation
- Risk (R) = Frequency or magnitude of hazard (H) x Level of vulnerability (V) / Capacity to cope and adapt (C)
- R = (H x V) / C.
200
What is the difference between a hazard/risk and a disaster?
A disaster is an actual event, usually involving a loss of life and damage to the human and natural environment.
201
Which is more hazardous (leads to more loss of life) - volcanoes or earthquakes?
Earthquakes.
202
What are the possible future strategies to cope w/ risks from tectonic hazards?
- Improving the forecasting of earthquakes.
(This is because, currently, it is not possible to make predictions of where and when an earthquake will happen).
- Alleviation of poverty
(The UN believe that addressing poverty will allow for advanced strategies in LIDCs that reduces effects).
- Assessing and evaluating current strategies
(Such as building design, making more scientific advancements and upgrades in infrastructure to help).
203
What is the disaster response curve also known as?
The Park model (of human responses to hazards).
204
What does the Park model illustrate?
The varying impacts of hazards over time, phased as: before the disaster, the event as it occurs and post event relief, rehabilitation and reconstruction.
205
What graph shape does the Park model take?
A curve.
206
List the stages of the Park model.
- Stage 1: *modifying event and cause*
- Stage 2: *The event*
- Stage 3: *Search, rescue and care*
- Stage 4: *Relief and rehabilitation period*
- Stage 5: *Nature of recovery*
207
What is time (*pre-disaster, relief, rehabilitation and reconstruction*) plotted against in the Park model?
- Quality of life
- Lower down on the axis is deterioration, above that is normality and above that is improvement.
208
What time categories are on the X axis in the Park model (disaster response curve)
- Pre-disaster
- Relief (hours to days)
- Rehabilitation (days to weeks)
- Reconstruction (weeks to years)
209
Describe the normal course of the Park model (disaster response curve)
- Stage 1 is plateaued at 'normality', where modifying the event is occurring.
- Stage 2 is the hazardous event, where there is a rapid decrease in the line where vulnerability strategies are being used.
- Stage 3 is at the lowest point, in the deterioration level where relief is occurring.
- Stage 4 sees a gradual rise in quality of life - however it is slow and extends into the reconstruction period from the rehabilitation period.
(LOOK AT BOOK THING FOR THIS)
210
Suggest a difference between Park models for a LIDC and an AC
- An AC may see a less significant decrease in quality of life following the event occurring due to more advanced 'modifying vulnerability' strategies in place.
- The rate of recovery may be faster due to potential for the 3 modification strategies being successful.
(However, an LIDC may receive aid).
211
What model sets out a theory of the relationship between disaster and response?
- The park model (disaster-response curve).
212
In what overall 3 ways has risks from tectonic hazards changed over time?
- Change in frequency and impacts of hazards temporally
- Degree of risk and probability of hazard occurring (d-r equation).
- Possible future strategies to help cope.
