D.1 - Geophysical Systems Flashcards
What are the 3 main parts of the earth and what sections fall into these
- Mantle
- Outer core
- Inner core
What is the mantle
Semi solid rock sourounding the core. More solid closer to the surface
What is the core
Centre of the earth
Very high temp
Rock material is melted into flowing liquid
Inner core is hottest but is solid due to pressure
What are the 2 main flows of heat from earths interior to the surface
radiogenic (that is, radioactive decay of materials in the mantle and the crust)
primordial heat (that is, the heat lost by the Earth as it continues to cool from its original formation).
What are the 3 ways earths heat transport occurs
Earth heat transport occurs by:
- convection
- conduction
- volcanic advection
How does a large scale convection current occur in earths interior
Hot magma rises through the core to the surface and then spreads out at mid-ocean ridges. The cold solidified crust sinks back into the Earth’s interior because it is heavier and denser than the surrounding material. The cause of the movement is radioactive decay of uranium and potassium in the mantle.
Define subduction
Subduction refers to the plunging of one plate beneath another.
When does a subduction zone form (explain in 4 parts)
- Subduction zones form where an oceanic lithospheric plate collides with another plate - whether continental or oceanic.
- The density of the oceanic plate is similar to that of the aesthenosphere, so it can be easily pushed down into the upper mantle.
- Subducted (lithospheric) oceanic crust remains cooler, and therefore denser than the surrounding mantle, for millions of years; so once initiated, subduction carries on, driven, in part, by the weight of the subducting crust.
- The subducting plate drags or pulls the rest of the plate behind it. Plates are hot at the mid-ocean ridge but cool as they move away.
Define plume
A plume refers to a small area of unusually high heat flow. Plumes or hotspots can cause movement, that is, the outward flow of viscous rock from the centre may create a drag force on the plates and cause them to move.
Where are plumes found
Most plumes are found near plate margins and they may be responsible for the original rifting of the crust. However, the world’s most abundant source of lava, the Hawaiian Hotspot, is not on the plate margin.
Where does rifting occur
Rifting occurs at constructive plate boundaries, for example the East Africa Rift Valley or the rift at Thingvellir, Iceland, where the North American Plate and the Eurasian Plate are moving away from each other.
What is the mian cause of rifting
In each case, hotspot activity is believed to be the main cause of rifting. The rift valleys created consist of rock that is hotter and less dense than the older, colder plate. Hot material wells up beneath the ridges to fill the gaps created by the spreading plates.
What rocks make up the crust
Sial and sima
Describe sial
Lighter rocks in the upper crust . Makes up the continents
It’s composed of silicates and aluminium
Describe sima
Heavier rock in lower crust. Makes up ocean beds
Composed of silicates and magnesium
Describe tectonic plates
- make up the crust
- move, carried by currents moving slowly by liquid mantle
- dragged over the asthenosphere in different directions
- float as they are less dense
What are 3 causes of plate movement
- convection currents
- ridge push
- slab pull
How does a convection current cause plate movement
- Occur in mantle due to radioactive decay of elements in upper mantle, which releases heat
- Allow the earth to dissipate heat from its hot core
- Temperature rises so liquid rocks expand and rise to surface, forming currents
- Upward movement occurs where there is the greatest radioactive decay, which is unevenly distributed according to the composition of the earth
- Heat rises from the core and spreads at the mid-Atlantic ridges
- This causes them to pull apart
- The cold solid crust sinks as it is denser than the surroundings
How does a ridge pull cause plate movement
- occurs at mid ocean ridges at divergent plate boundaries
- due to rigid lithosphere sliding down hot, raised asthenosphere between the ridges
- gravitational forces slide the lithosphere away from the high ridge towards subduction zones
How does slab pull cause plate movement
- tech tonic plate motion caused by its subduction
- cold, dense slabs of plates sink deep into the mantle
- this pulls on the rest of the plates
- occurs at convergent margins at the subduction zones
- reduces pressure on underlying asthenosphere, which leads to partial melting
- the resulting magma wells up under divergant margins, cools, and rises
- it is added to the plate as new lithosphere
What are the 3 types of plate boundaries
- divergant/ constructive
- convergent/ destructive
- transform/conservative
What is the movement of a divergant plate
Spread apart, creating new material
What causes a divergant plate boundaries
They are formed by plumes pushing plates apart
What does divergant plates create
New oceanic crust is added as pillow lava
Also occurs as rift valleys where continents are splitting apart
What is the lava flow of a divergent plate
Magma rises through shallow crust and results in low silica effusive lava
Creates low viscosity, basaltic flow
What volcanos do divergent plate boundaries create
- gentle sloping/shield volcanos
- erupt with regular and continuous frequency
- associated with Icelandic and Hawaiian eruptions
What is the movement and result of convergent plates
They collide into each other and cause the destruction of material
What processes do convergent plate boundaries create
- subduction can occur leading to the partial melting of mantle as saturated oceanic crust enters it
- part of crust melts too, causing magma to rise up
What is the lava flow in a convergent plate
- magma rises through more crust and results in high sill a magma
- creates high viscosity andesitic magma
What volcanos do convergent plate boundaries create
- forms composite cone volcanos
- magma produces tephra and pyroclastics
- associated with vulcanícele and plinian eruptions
What is the movement and result of a transform plate boundary
They slide laterally and do not create nor destroy
What does a transform plate boundaries create
Earthquakes as the plates move past each other and generate friction
Summarise the subduction zone
- region of crust where tectonic plates meet
- the oceanic crust usually sinks into the mantel beneath light continental crust
- earthquakes are a common feature
- sima is cooler/denser than asthenosphere
- its downward movement experts drag on the asthenosphere causing fricación and therefore earthquakes
- the plate is forced down and heated up by the surrounding mantle
- eventually malts, giving off gasses with rise tp the surface as liquid rock
- this is released as a volcanic eruption
What are the 3 types of collisions between plates of different material
- sial + sial
- sima + sima
- sial + sima
What is the collision between a sial + sial plate
This is light continental material with similar density. At this collision folding will occur as plates crumple into each other and are forced upwards forming mountain ranges
What is the collision between a sima and sima plate
Here heavy oceanic material with similar density will collide causing both plates to subduction forming deep ocean trenches
What is the collision between a sial and sima plate
Here one sial and one sima plate collide with different density’s causing the denser sima plate to subduct. The lighter sial plate will fold and fault upwards against the other plate forming a volcano.
Define magma
Molten rock found beneath earths surface, stored int he crust
Define lava
magma that reaches the surface of the earth through a volcano vent
Define tephra
solid rock material ejected by volcanic eruptions
What are the 2 types of lava
• Andesite/dacite - silica rich lava which is more viscous, generally erupt more explosively
• Basaltic - runny and relatively fluid lava which contains less silica
What are the 3 types of volcano
- cinder cone
- composite cone
- shield
What plate boundarie does a cinder cone volcano occur at
Any of them - occurs whenever magma gets near surface
What plate boundaries does a composite cone volcano occur at
Convergent - where oceanic crust subducts beneath continental
What plate boundaries does a shield volcano occur at
Divergant or hot spot
What is the shape of a cinder volcano
Steep, straight sides
Cone-shaped hill
Much smaller
Bowl-shaped crater at summit
What is the shape of a composite cone volcano
Steep, upwardly concave sides
Much taller
Have a central vent or cluster of vents at summit
What is the shape of a shield volcano
Gentle slope
Looks like shield resting on ground
Very low
What is a cinder volcano composed of
Layers of ash deposited from successive explosive eruptions
What is a composite cone volcano composed of
Alternating layers of hardened lava and proclastic material
What is the process of formation for a cinder cone volcano
Occur when mix of gases and magma rise to surface and blow violently into air
Lava is blasted into tiny fragments which solidify as ash and cinders
Form symmetrical cone around the crater
Constructed of loose tephra
What is the process of formation for a composite cone volcano
Formed as eruptions spew out and deposit combinations of ash lava, pumice, and tephra at various times, forming a cone
How is lava ejected from a cinder cone volcano
Eject basaltic lava
Relatively thick with trapped gases
What is the process of formation for a composite cone volcano
Formed as eruptions spew out and deposit combinations of ash lava, pumice, and tephra at various times, forming a cone
What is the lava flow from a composite cone volcano
Eject gas-rich andesitic lava
What is the lava flow from a shield volcano
Very fluid basaltic lava
Travels long distances before solidifying
What is the eruption process of a cinder cone volcano
Often produce lava flows from base
What is the eruption process for a composite cone volcano
Produce lava flows and proclastic materials
Produce large eruptive columns which inject gases and particles high into atmosphere
Associated with lahars and lava flows
What is the eruption process for a shield volcano
Produce lava flows and proclastic materials
Produce large eruptive columns which inject gases and particles high into atmosphere
Associated with lahars and lava flows
What is a Strombolian eruption
Strombolian eruptions are explosive eruptions that produce proclastic rock. Eruptions are commonly marked by a white cloud of steam emitted from the crater. Frequent gas explosions blast quantities of runny lava into the air, and when these settle and cool they form the cone.
What is a Vulcanian eruption
Vulcanian eruptions are violent and occur when the pressure of trapped gases in viscous magma becomes sufficient to blow off the overlying crust of solidified lava. Often the eruption clears a blocked vent and spews large quantities of volcanic ash into the atmosphere. Violent gas explosions blast out plugs of sticky or cooled lava. Fragments build up the cone of ash and pumice.
What is a Vesuvius eruption
Vesuvian eruptions are characterized by very powerful blasts of gas that push ash clouds high into the sky. Lava flows also occur. Ash falls to cover the surrounding area.
What is a plinian eruption
Plinian eruptions are extremely violent eruptions characterized by huge clouds of pulverized rock and ash that are kilometres thick. Gas rushes up through the sticky lava and blasts ash and fragments into the sky in huge explosions. Gas clouds and lava can also rush down the slopes. Part of the volcano may be blasted away during the eruption.
What are the 3 primary hazards of a volcano
- Pyroclastic flows
- Pyroclastic surges
- Lava flows
What is a Pyroclastic flow
Fast mixtures of lava, pumice, ash, and gases extruded from volcanoes during eruptions
Flow downhill, blown by high pressure gases from the eruption
Destroys everything in their path, either by burning, knocking over, shattering, or burying
What is a Pyroclastic surge
Fast clouds of hot ash that travel with proclastic flows
Fall back to earth as they cool, covering vast areas with a layer of ash
What is a lava flow
Occur when molten rock pours and oozes from erupting volcanoes
Cools and solidifies as it flows across ground
Typically not too dangerous, as molten lava is so viscous it flows slowly
However, can be hard to stop or divert
Significant threat to property
What are 5 secondary hazards of a volcano
- Lahars
- landslides
- floods
- fires
- tsunamis
What are Lahars
Liquid mudflow made of a slurry of pyroclastic materials, rocks, and water
Typically flow down sides of volcanoes
Move quickly, destroying everything in its path Especially prominent in cold areas where eruptions melt snow/ice which gather extra material as they flow downhill
What are landslides
Large rock/soil masses separate from sides of mountains and slip downhill due to gravity Can be initiated by earth tremors/quakes, leakage of gases beneath the ground, or explosive equations
Occur very quickly
What is a Strombolian eruption
Strombolian eruptions are explosive eruptions that produce proclastic rock. Eruptions are commonly marked by a white cloud of steam emitted from the crater. Frequent gas explosions blast quantities of runny lava into the air, and when these settle and cool they form the cone.
What is a Vulcanian eruption
Vulcanian eruptions are violent and occur when the pressure of trapped gases in viscous magma becomes sufficient to blow off the overlying crust of solidified lava. Often the eruption clears a blocked vent and spews large quantities of volcanic ash into the atmosphere. Violent gas explosions blast out plugs of sticky or cooled lava. Fragments build up the cone of ash and pumice.
What is a Vesuvius eruption
Vesuvian eruptions are characterized by very powerful blasts of gas that push ash clouds high into the sky. Lava flows also occur. Ash falls to cover the surrounding area.
What is a plinian eruption
Plinian eruptions are extremely violent eruptions characterized by huge clouds of pulverized rock and ash that are kilometres thick. Gas rushes up through the sticky lava and blasts ash and fragments into the sky in huge explosions. Gas clouds and lava can also rush down the slopes. Part of the volcano may be blasted away during the eruption.
What is a mantle plumes
- Mushroom-shaped zones within the mantle consisting of abnormally hot material
- Form where radioactive decay is particularly strong
- Not associated with plate boundaries
- So hot that they tend to rise and spread out in asthenosphere beneath the crust, as a geological intrusion forcing overlying crust upwards
What are some hotspots
• Areas above plumes, fed by underlying mantle
• Anomalously hot compared with surrounding mantle
• E.g. Hawaii is far from plate boundaries but is situated on a hot spot o Still has many active volcanoes
• As the Pacific Plate passes over the hot spot, new volcanoes are created
Define earthquake
An earthquake is a series of seismic vibrations or shock waves which originate from the focus - the point at which the plates release their tension or compression suddenly (Figure D.5). The epicentre marks the point on the surface of the Earth immediately above the focus of the earthquake.
What is a shallow focused earthquake
- where are they found
- what is there affect
Shallow-focus earthquakes occur relatively close to the ground surface, whereas deep-focus earthquakes occur at considerable depth under the ground. Shallow-focus earthquakes have greater potential to do damage as less of the energy released by the earthquake is absorbed by overlying material.
How can a large earthquake be predicted
A large earthquake can be preceded by smaller tremors known as foreshocks and followed by numerous aftershocks.
What are the 2 shock wave types
- body wave
- surface wave
What are body waves §
Body waves are transmitted upwards towards the surface of the earth from the focus of the earthquake
What are the 2 types of shockwave
- primary and secondary
What is a primary shock wave
Primary (P) waves or pressure waves are the fastest and can move through solids and liquids - they shake the earth backwards and forwards.
What is a secondary shock wave
Secondary (S) waves or shear waves move with a sideways motion and are unable to move through liquids - they make the ground move horizontally, causing much damage.
What are the 2 surface waves
- love wave
- Rayleigh wave
What’s does a love wave do
Cause the ground to move sideways
What does a Rayleigh wave do
Cause the ground to move up and down
How do love and Rayleigh waves move
Love waves and Rayleigh waves travel slowly through the crust, but they cause the most damage.
What is the contrast between body and surface waves
P-waves and S-waves are transmitted from the earthquake focus to the surface. By contrast, surface waves are produced in the ground by the transformation of some body waves once they reach the surface.
What is the focus/hypocentre
Point of fracture where stored seismic energy is released from when a break occurs
what is the epicentre
Point on earth’s surface directly above focus
What is the fault line
Break in the ground occurring when tectonic plates move
Are areas where EQs are likely
What is wave motion
When crust ruptures, seismic waves move outwards in all directions
Shocks waves usually quickly dissipate, as the crust is so dense
Generates three types of waves during an EQ
What is the speed ranking on waves
Primary - fastest
Secondary - second fastest
Surface - slowest
What is the frequency ranking on waves
Primary and secondary - high - cause low buildings to vibrate
Surface - low - cause high buildings to vibrate
What are the 2 types of boundaries
- shallow focus
- deep focus
Where are shallow boundaries found and what do they do
- transform/divergant
- cause extensive damage due to little depth of rock to absorb seismic energy
Where are deep focus found and what do they do
- Convergent
- Occur as mountains are built up through folding and faulting, or subduction as oceanic plate is forced under continental
What stress is found at convergent plate boundaries
- compression
- meaning they push together
- found at a normal fault
What stress is found at a divergant plate boundary
- tension
- meaning they pull away from each other
- this is found at a fault which is reversed or thrust
What kind of stress is found at a transform plate boundary
- shear
- push in opposite directions
- found at a strike - slip fault
What are seismic belts and what are the 2 types
• Narrow geographic zones on surface where most EQ activity occurs
• Broad belts - associated with convergent boundaries and subduction zones
• Narrow belts - associated with divergent boundaries
What are 3 human causes of earthquakes
- large dams in earthquake prone areas
- mining
- fracking
How does dams cause earthquakes
Weight of water places downward pressure on fault lines beneath
Water can seep down and lubricate rock movement
Over time, stresses in fault line build up
Can destabilise them
How does mining cause earthquakes
Removal of material from ground can cause instability
Leads to sudden collapses that trigger EQs
How does fracking cause landslides
Directly from drilling/blasting/excavating
Disposal of high-pressure wastewater can crack rocks and lubricate faults
Fracking (also known as hydraulic fracturing) is a process in which water, containing certain chemicals, is injected at very high pressure into rocks in order to open up their pore spaces and release natural gas contained within the rocks. Fracking has been associated with the triggering of earthquakes in the UK.
What are 3 secondary hazards of earthquakes
- landslides
- liquefaction
- tsunamis
Why are landslides a secondary hazard of earthquakes
EQs create stresses which can make weak slopes fail and dislodge rocks
Can even cause a new fault to fracture the hillside
Huge rock masses may slide downhill and destroy everything in its path
Accompanied by a destructive rush of air
Why is liquefaction a secondary hazard of earthquakes
EQs shake an area of soil and separate its solid and liquid components
Causes it to lose structural integrity and behave like a liquid, flowing downhill
Causes collapse of areas with a soil foundation
Why are tsunamis a secondary hazard of earthquakes
Very large sea surface waves caused by underwater EQs/landslides and volcanic activity
After the shock, waves travel outwards in all directions
In the open ocean they travel extremely fast but are not felt
Approaching coasts, they slow down and increase in amplitude, affected by submarine topography
Lead to severe damage and death
Most death is from drowning
Includes secondary risks like flooding, contamination of drinking water, fires from ruptured gas lines, and loss of infrastructure
Can be monitored by networks of tidal gauges which provide countries with warning messages
define mass movement
Downslope movement of weathered rock materials due to gravity
Occurs to establish an equilibrium between weathering and erosion on sloping ground
Occurs when equilibrium is disturbed as:
- Weathering material accumulates on upper slope faster than it can be transported away by agents of erosion
- Agents of erosion undercut weathered materials
• Causes rocks to break free and tumble downwards due to pull of gravity
• Soils/weathered rocks are more vulnerable than bedrock as they are held together more loosely
• Contains many types which are not always separate
• Can easily transition into different forms as environment changes
What are 4 factors which promote mass movement
- water
- initial impetus
- texture
- slope
How does water promote mass movement
Rainfall adds weight to weathered material and disrupts equilibrium
Water lubricates rocks and soil particles, destroying cohesion
Water pushes particles apart by exerting pressures on spaces between them
How does initial impetus promote mass movement
Triggering events upset equilibrium
May be physical (e.g. EQs, rainfall) or human (e.g. blasting for roadworks/mining)
Can even be caused by lightning, trains, gun shots, burrowing animals, removing vegetation etc.
How does texture promote mass movement
Coarser, rougher particles resist mass movement more effectively than smooth particles, as they have more opposing frictional forces
How does slope promote mass movement
Gravity works more effectively on steeper slopes, so mass movements is more rapid
Erosion of slope bases steepen them, increasing the gravitational forces, disrupting equilibrium, and thus initiating mass movement
What are the four processes of each a deform of mass movement
- creep
- flow
- slides and falls
- subsidence
What are the three process of creep
- creep
- terrace totes
- rock/talus creep
What is creep
Slow downhill movement of surface material due to gravity
Continuous movement
Too slow to see with the eye
Posts/poles/walls develop a marked lean
Tress develop bent trucks as their roots shift downhill
What is terracettes
Form when saturated soil on steep slopes expand during rain, then contract as it dries Causes small steps to form which dislodge from underlying soil and move downhill from gravity
What is rock/talus creep
Under gravity, talus slopes are formed at the foot of steep slopes that have shed talus If talus slopes become steeper than their critical angle, equilibrium is lost and mass movement will occur to restore it
What are 3 types of rapid flow
- earthflows
- mudflows
- debris avalanches
What are earthflows
Like slow moving landslides
Occur on slopes when earth is saturated with water and becomes capable of slow flowage
Usually too slow to see
Leave scars on hillside, usually marking a point where it began
Broaden out to a wider slump of earth at the foot of the slope
What are mud flows
Occur when heavy rains destabilise a slope and cause soil/rocks/boulders to slide downhill
Deserts - develop alluvial fans at foot of slope
Mountains - occur on steep slopes when sudden thaw melts snow
Volcanic areas - known as lahars, occurring commonly on loose cinder cone slopes
What are debris avalanches
Occurs mainly in alpine areas
Leaves significant scars on slopes of mountains
What is solifluction
In permafrost areas, upper ground layer is permanently saturated as water cannot soak away
The resulting slushy material moves down, even on very gentle slopes
What are 2 types of slides and falls
- landslides
- rockfalls
What are landslides
Rapid sliding and slipping movement of large bedrock masses
Large rocks tumble downhill and break into smaller talus
Usually down steep slopes or along a fracture plane
Occur when a slip surface develops and there is a lack of support for a large section of land
Slide surface may be a curved shear surface, producing slumps, or a planar shear surface, causing rockslides
What are rockfalls
Even quicker than landslides
One or a few large rocks separate from the slope and drop down
Roll rapidly down after their vertical fall
Accumulate at foot of slope as talus
What are three types of subsidence
- subsidence
- cover subsidence sinkholes
- cover collapse sinkholes
What is subsidence
Downward settling of material with little horizontal movement
Occurs when material is removed fron beneath the surface
E.g. due to erosion by underground streams, rocks like limestone dissolving, or mining
Can also occur when groundwater is pumped out for human use, reducing hydraulic pressure of the aquifer
Localised subsidence forms sinkholes
What is cover subsidence sink holes
Form slowly over decades to centuries
Occurs as soil is transported by drainage channels into an underground reserve like a cave
Cause gradual downward sinking of land
Can damage building foundations, but there is normally enough warning of movement to repair/renovate the buildings
What are cover collapse sinkholes
Appear seemingly without warning - very dangerous
Occur during heavy rain when clay or sandstone particles beneath surface are saturated, causing their glue-like cohesion to disintegrate suddenly and cause a sinkhole
Also develop when roof of underlying caves collapses
E.g. if seepage dissolves underground limestone and collapses suddenly
Can also result from human actions that cause ground collapse, like mine collapses, pipeline breaks, or groundwater over-extraction
What 2 factors cause slope failure
• a reduction in the internal resistance, or shear strength, of the slope
• an increase in shear stress - that is, the forces attempting to pull a mass downslope.
What are 5 factors leading to increased shear stress
- removal of lateral support through undercutting to slope steepening
Erosion by rivers and glaciers, wave action, faulting, previous rockfalls or slides - removal of underlying support
Undercutting by rivers and waver, subsurface solution, loss of strength by exposer of sediment - loading of slope
Weight of water, vegetation, accumulation of debris - lateral pressure
Water in cracks, freezing in cracks, swelling, pressure release - transient stresses
Earthquakes, movement of trees in wind
what are 4 factors leading to reduction in shear stress
- weathering effects
Disintegration of granular rocks; hydration of clay minerals; chemical solution of minerals (that is, the weathering of certain chemicals, notably calcium carbonate), in rock or soil - changes in prone water
Saturation, softening of material pressure - changes of structure
Creation of fissures in clays, remoulding of sands and clays - organic effects
Burrowing of animals, decay of roots
When was the landslide in Nepal
2015
Why is Nepal prone to landslides
Landslides are common in Nepal. The region, with its young geology, seismic activity, steep slopes and intense cloudbursts, has a history of earth-moving events. Factors that trigger landslides include seasonal rainfall, road construction and slopes weakened by previous slides.
What earthquake caused these earthquakes
The Gurkha earthquake of April of 2015
What was the largest landslide in Nepal
One of the largest was an ice and rock avalanche at Langtang. This caused substantial loss of life as entire settlements were buried. The landslide consisted of a large mass of freefall rock and ice, shearing off more rock on its descent.
The vertical distance was 700-800 m over a horizontal distance of 2-3 km. The valley below was severely obstructed and covered by debris. It looks like the threat of landslides will continue to exist in this impoverished and vulnerable country.
