Geophysical systems D1.1

Earth Structure

Earth Structure

Inner core

• Solid metal (iron) and nickel

Outer core

• Liquid metal (iron and nickel)

• Spins around inner core to create earth magnetic field

• Heat of core is result of frictional forces + radioactive decay of elements - generates 44 trillion watts of heat to flow away from core into mantle

Mantle

• Solid rock

• Upper mantle = asthenosphere

  • Is ductile/malleable and in places it is semi-molten

Crust

• Made up of oceanic and continental plates

• Varies in depth 3-70km

• Oceanic crust = thin, young, dense, heavy - made from basalt rock

• Continental crust = thicker, older, lighter - made from different rock types (mostly granite)

Plate movement

• Earth crust = made from number of large plates + small plates

• Plates are in constant movement - in a process called continental drift

• Plates move at slow speed - 2-5cm a year

• 3 key factors that drive plate movement

  • convection cells

  • ridge push

  • slab pull

Convection cells

• Mantle convection is slow creeping motion of earth solid rock caused by convection currents carrying heat from the interior to the planets surface

• Earth surface + Lithosphere are dragged by frictional forces as convection moves sideways through the asthenosphere

Lithosphere

• Made up of rigid crust and the more ductile upper part of the asthenosphere

Ridge push

• At rift zones, crust is moving apart under tensional stresses

• The shearing of the crust reduces pressure on asthenosphere that allows mantle rock to form magma

• Which rises upwards through the rift zone to form new ocean crust in the lithosphere

• Lithosphere thickens with distance + time away from the mid-ocean ridge

• This is because it cools as it moves away from the ridge + the boundary between the solid lithosphere and the plastic asthenosphere becomes deeper

• Result of thickening away from ridge is a downward slope away from ridge

• The weight of lithosphere on slope produces a downslope force allowing for it to push the older crust creating tis movement away from the ridge.

Rift zones

• Found at ocean ridges and in continents

• Rift zones are areas of weakness in the volcano which form early in its lifetime

• likely due to spreading of the volcano as it settles

• This linear area that is being rifted, or pulled apart, remains active through most of the volcano's building stages.

Mantle Plumes

• In some locations in the mantle, there are upwellings of hot rock

Hotspots

• As the heads of mantle plumes can partly melt when they reach shallow depths, they are thought to be the cause of volcanic centres known as hotspots and probably also to have caused flood basalts.

Flood Basalts

• A flood basalt is the result of a giant volcanic eruption or series of eruptions that cover large stretches of land or the ocean floor with basalt lava

• Many flood basalts have been attributed to the onset of a hotspot reaching the surface of the earth via a mantle plume

• An example of a hotspot can be seen below in Hawaii.

Slab Pull

• As lithospheric plates move away from mid-ocean ridges they cool and becom denser

• Eventually they become denser than the hot mantle

• During subduction — cool, dense lithosphere sinks into the mantle under its own weight

• Because it is denser than mantle around, process of subduction continues

• So, weight of subducting plate + force of gravity drives continental movement = slab pull

• Slab pull considered most important factor causing plate movement

Subduction zone

• At a subduction zone where two plates meet the denser of the two plates sinks under the other in a process called subduction.

Where do volcanoes occur

• destructive (convergent) plate boundaries

• constructive (divergent) plate boundaries

• hotspots associated with mantle plumes

Destructive Plate Boundaries - What type of volcanoes

• Associated with composite or strato-volcanoes that produce more explosive eruptions

Hotspots - What type of volcanoes

• Associated with shield volcanoes - but can result in all forms of volcanoes

Types of volcanoes

1. Cinder Cone

   1. When runny, low silica magma contains a lot of disolved gas, it could form a cinder cone

   2. Gassy eruptions - sputter like soda spraying from a shaken can

   3. They give loose volcanic fragments called cinders which settle and stack in a conical shape

   4. Eg. Paricutin - Mexico

2. Composite/strato

   1. Gas pressure in viscous, high silica magma can trigger explosive eruptions of loose magma fragments.

   2. Less explosive eruptions create short lava flows that cover the fragments

   3. Layers of lava and fragments build into a quintessential volcanic shape

   4. Most often found on the edges of continents where oceanic crust descends beneath the thicker continental crust

   5. Magma rising from deep boundary = plenty of time to drop out silica poor components and pick up silica rich ones

   6. Eg. Mount fuji - Japan + Mount St Helens - Washington

3. Lava Dome/volcano

   1. large depression formed when a volcano erupts and collapses

   2. During volcanic eruption - magma present underneath the volcano is gotten rid of, often forcefully

   3. when magma chamber empties - support that magma provided disappears

   4. Found everywhere

   5. Eg. Yellowstone supervolcano = Hawaii

4. Shield

   1. Low silica magma - flow readily and puddle on land

   2. Over time - eruptions form layers of lava like paint - creating a broad flat shield volcano

   3. Often found in oceans

   4. Since oceanic crust is thinner than continental, magma does not have to traval as far to reach surface - less time to change its composition and become silica rich

   5. Lava in shield volcanos most resembles magma in earth

   6. Eg. Muana Kea + Mauna Loa = Hawaii

Types of magma

• 3 types of magma = different characteristics

  • Basaltic forms at constructive boundaries and hotspots over oceans - temperature is hottest + has lowest silica content

  • Rhyolitic is common at destructive plaete boundaries is cooler + greater silica content = makes it thicker and more viscous which is resistant to flow. — This magma traps gas and produces highly pressurised eruptions

Magma

• Magma is produced when rock in the upper mantle melts

• Because it is less dense than the rock around it - it rises upwards

What does low silica content mean

• This means that it is more fluid in form and produces faster-flowing lava

• It also allows for gas to more easily escape and so eruptions tend to be less violent with smaller ash columns.

Ocean-to-oceanic margins + Oceanic-to-Continental margins

Two types of destructive plate boundaries where subduction occurs

1. How island arcs form when the denser oceanic crust subducts under lighter.

2. Subduction of dense oceanic crust under lighter continental crust

In both = subducting crust under sinks into the softer asthenosphere

Carries hydrated sediments (matter that settles to the bottom of a liquid) and rock - so this water lowers melting point of rock found in the mantle wedge

• This process causes melting of the mantle wedge and partial melting of the crust leading to magma

What is silica?

• Most important compound in magma

• Influences the explosiveness of eruptions

• When magma rises up through crust, silica compound increases in concentration

  • this happens for 2 reasons

    • 1. silica-poor compounds crystallize, gain density and sink

      • silica rich compounds crystalize slower

    • 2. when magma rises it assimilates more silica-rich compounds in the surrounding rock + higher up in the crust = more silica rich rock is found

      • magma rising upwards in continental crust - due to greater depth - will get more silica

• Silica rich compounds increase the viscosity of magma + trap more gas

• Thus, eruptions at destructive plate boundaries are more explosive

2 types of hazards from a tectonic event

• Primary Hazards

• Secondary hazards

Associated primary hazards from a volcano

• Lava

  • 2 types

    • Pahoehoe

      • Smooth in its appearance

      • Low in viscosity and travels slowly

    • A’a

      • More rough in appearance

      • Higher viscosity and carries laval blocks called clinkers

  • Lava flow can lead to some secondary hazards like snow and glacier melt that can cause floods or glacial outburst floods called jokulhlaup and forest fires

• Ash and Gas

  • Powerful eruptions create ash columns that can shoot to max 12km

  • Then produce ash plumes that extend away from the volcano and result in ash fall that can affect places many hundreds of kms away

  • Ash can impact global climate if sufficient ash reaches top of stratosphere

  • Ashfall can asphyxiate (kill) people and livestock

  • When ash mixes with rainfall the flanks (side of volcano) of the volcano become unstable - creating rapid moving mudflows called lahars

  • Gases emitted include a lot of co2, sulfur dioxide, hydrogen sulfide, and hydrogen halides

  • depending on concentration, gas = potentially hazardous to people, animals, agriculture and property

  • Eg. 1986 Lake Nyos in cameroon - Large volumes of volcanic gas made out of co2 were suddenly released from depth of lake which killed livestock and people

• Pyroclastic Flows

  • Occurs when superheated gas, pumice and ash fly down flank of volcano

  • These flows can reach speeds of several hundred km/h

  • They vary in speed - 20-70km per hour

  • Temperatures vary from 200-700 degrees celsius

  • They form when the upward force of the blast is weak + part of ash column collapses down the flank of the volcano

  • Composition has a surge (Pyroclastic surges are low-density currents of ash, pumice, crystals, and volcanic gases that are more dilute than pyroclastic flows) at the front, heavier blocks and ash at the base and lighter ash and gas above

  • Pyroclastic flow will knock down nearly anything in its path

Associated secondary hazards from a volcano

• Landslides

  • When pressure within magma chamber is large - leads to lateral blasts

  • Where flank of volcano becomes unstable and collapses

  • leads to block and ash flow called rock avalanches

  • Scale of events can be extreme

  • Eg. 1980 - Mount St Helens eruption = 2.3 billion cubic meters fell from mountain destroying more than 600km2 of territory and millions of trees - Rock avalanches can also cause floods and tsunamis when they hit rivers/water

• Lahars

  • Occurs when large quantities of water mix with fallen ash

  • Water can come from torrential downpours (rain that comes down very fast) caused by ash particles in the atmosphere but also from snow melt or dam failure

  • When water mixes with ash - rivers will turn into mud and stones

  • Lahars can run for a long distance away from volcano - destroying everything in their way

  • Lahars vary in size + speed

  • Small lahars are less than a few meters wide + several centimeters deep - can travel at slow speeds

  • Large lahars = hundreds of meters wide + tens of meters deep - can travel at fast speeds

  • Can go up to 100km per hour in speed

  • Flow distances of more than 300km

  • Eg. 1985 - Nevado del Ruiz eruption - lahar caused by snow melt - in Colombia - Deaths = 23,000

Earthquake characteristics + why/how do they occur - stress

• Take place most often at conservative plate boundaries where two plates slide past each other rather than away or towards each other - but can happen at all types

• Those with the highest magnitudes occur at destructive plate boundaries where subduction takes place - tend to be less frequent

• Occurs when the fault suddenly releases stored stress

• When stress is released from the focus of the fault - seismic waves transfer the energy in all direction

• Focus - area where stress is released

• Epicentre - point directly above the focus and is where the energy on the surface will be greatest

• Seismic waves = move away from the focus

Types of faults

• Normal

• reverse

• strike-slip

Why do earthquakes occur - magma

• Earthquakes are also caused by rising magma in chambers within volcanoes

• Increased frequency of earthquakes in volcanoes = known to be important warnings before eruption

• Earthquake frequency = increase with magma height within the conduit

Why do earthquakes occur - human activity

• Human activity also cause earthquakes

• Mining activities disturb rock structure

• Use of dynamite and heavy machinery = cause earthquakes

• Dam construction for reservoirs adds significant weight and loads to faulys and can also cause earthquakes

• Fracking - shale gas is extracted from rock seams by injecting high pressures = increase frequency in earthquakes

Types of seismic wave

• Can be scene in seismography

• 3 distinct types

  • primary/body

  • secondary/shear

  • surface

    • has two types:

      • love

      • rayleigh

Primary waves

• Alternating compressions (pushes) and dilations (pulls) move in the same direction

• P motion travels fastest in materials

  • 5-7km per s in crust

  • 8km per s in earth’s mantle and core

• P wave is the first arriving energy on seismograph

• P—waves are generally smaller and have a higher frequency than the S and surface waves

Secondary waves

• Particle motion is in both vertical and horizontal planes

• S-waves do not travel through fluids, do not exist in earth’s outer core or in air, water, or magma.

• S-waves travel slower than p-waves in a solid

  • 3-4km per s in earth’s crust

  • 4.5 km per s in mantle

  • 2.5 km per s in inner core

• arrive after p wave

Surface waves = Love waves

• Generally parallel to earth’s surface

• They exist because of earth’s surface

• Largest at the surface and decrease in amplitude and depth

• They are dispersive

• Wave velocity and depth are dependent on frequency

• Generally, low frequencies propagating (growing) at a higher velocity and greater depth

• velocity varies from 2-4.4km per s

Surface waves = Rayleigh waves

• Motion is generally elliptical

• Waves are also dispersive

• amplitudes generally decrease with depth in the earth

• appearance and particle motion are similar to water waves

• Depth of penetration of waves = dependent on frequency

• Lower frequency penetration toa greater depth

• velocity varies from 2-4.2km per s

Primary associated hazards of earthquakes

• Directly linked to seismic waves

• Earthquakes can cause movement of the surface - rolling and shaking

• Brittle surfaces may fracture and infreasture = collapse

• Collapsing buildings + bridges = considered primary hazard

Secondary associated hazards of earthquakes

• Tsunami

  • Classified as a shallow water wave bc the wave is bigger than the depth of the ocean it moves over

  • Earthquakes cause tsunami - as a result of fault movement in a subduction zone

  • As oceanic plate = subducted — typically locks on to part of the continental plate and drags it with it

  • At some point the connection = broken + crust spring back in the direction of the ocean — this is a process called elastic rebound

  • Sudden movement of ocean floor around the fault leads to ocean displacement + series of tsunami waves

  • Eg. 2011 Japan tsunami - fault movement can be as much as 10meters = means that 10 meters of water is pushed upwards

• Fires

• Liquefaction

  • Soil liquefaction also called earthquake liquefaction

  • Similar impact to landslide

  • Ground failure or loss of strength

  • Leads to soil behaving temporarily as a viscous liquid

  • Occurs in water-saturated soils affected by s waves

  • These waves cause ground vibrations during earthquakes

  • areas with poorly drained find-grained soils (sandy, gravelly soils) = most susceptible to liquefaction

  • Eg. 2010 earthquake Christchurch, New Zealand — Large parts of the centre liquefied close to the river banks - ground sank leading roads and buildings to subside

  • Eg. Indonesia, Sulawesi - Palu = liquefaction largely impacted

• Landslides/avalanches

  • The stress exerted on unstable slopes leads to mass movement

  • Eg. Island of Hokkaido, Japan = 6.7 earthquake in 2018

Deep ocean vs Coastal tsunamis

Tsunamis behave differently in open ocean compared to coast (where water is shallow)

Deep ocean:

• long wavelengths

• low amplitude - meaning wave crests are far apart

• Look at diagram = wavelength is so great that in the deep ocean a ship wont notice the wave has passed

• Also travels very fast

Approaching shore/coastal

• tsunami slows down in speed

• amplitude increases dramatically

• base of the wave = slowed down by friction

• Surface = less affected so amplitude of the wave increases

• speed slows down as it travels into shallower water

Facts:

• Waves that are unnoticeable in the deep ocean can reach 10-50 meters at the shore

• Although they slow down they can still reach 200km per hour

• Tsunamis are very powerful + have the ability to destroy everything in their path

• Arrive in series of wave trains + with alternating deep waves and shallower

Examples

• Indonesian earthquake and tsunami in 2004 = 9.0 on richter scale, major global impact (impact 18 countries) and 250,000 deaths

• Japanese earthquake + tsunami in 2011 = 9.1 on richter scale caused a nuclear power station in Fukushima to leak radioactive waste and 20,000 deaths

• Sulawesi in Indonesia = earthquake and tsunami in 2019 = 7.5 on richter scale, 4,500 deaths + produced dramatic liquefaction

What is a mass movement

• classified in different ways

• downward movement of material on a slope

• consist of unconsolidated (loose) material - made of many individual pieces that act on each other to create a dynamic mass movement

• Rock falls + avalanches = examples of unconsolidated mass movement

• Can also one as one consolidated mass - ie. landslides

• Slope stability depends on the balance between a number of forces

  • gravity - relates to steepness of slope

  • shear strength - relates to structure of slope + forces that maintain its stability

  • shear stress - refers to forces acting to reduce slope stability and lead to movement

  • Simply put - if shear stress is greater than shear strength and gravity and the angle of the slope is sufficient there will most likely be a mass movement - movement might be one bouler falling or a whole section of earth

3 most important factors influencing slope stability

1. gravity

2. slope angle

3. pore water pressure

   1. refers to the weight of water within the soil or rock and its attempt to move downslope

   2. water also acts as a lubricant causing more instability

   3. in some cases, water enters pores between particles forcing them apart and reducing ability to bind

What human causes increase shear stress/reduce shear strength of a slope

• deforestation

  • trees serve two functions

    • bind the soil increasing shear strength through their roots

    • reduce pore water pressure in slope by reducing infiltration by interception

• house construction

  • undermine slope stability

  • be building houses on the slope = adds weight to the slope which increases shear stress

  • water infiltration may also be disturbed - may increase instability

• road construction

  • building at base = undermine stability

  • consider this as removing the foundation of a building

Causes of mass movements

Factors increasing shear stress:

• Removal of lateral support - by undercutting or slope steepening

  • Eg. by rivers, wave action. previous rock falls

• removal of underlying support - by undercutting rivers and waves, road construction

• Loading of slope - through increased weight of water, debris, construction, water injection through mining

• Lateral pressure - water and ice in cracks - swelling and pressure release

• transient stresses - earthquakes, wind moving trees, mining

Factors reducing shear strength

• weathering effects, disintegration of granular rock, chemical weathering processes

• changes in pore water, saturation, softening of material pressure

• changes in structure, creation of fissures in clays, remoulding of sands and clays

• organic effects - biological weathering, decay of roots and burrowing animals

Classification of mass movements

• based on speed of flow and degree of fluidity

• Dry mass movements

  • eg. creep and rock slides which are based on unconsolidated materials are the slowest

• Generally, the wetter the mass movement the faster its speed of flow - although there is a significant range in speed of wet mass movemetns

Slow mass movements

• occur as a gradual process over time

• eg. soil creep, scree creep, solifluction - occurs in regions of permafrost as the active later on the surface becomes more fluid and slowly moves downslope

  • Soil creep and scree creep = unconsolidated

  • solifluction is consolidated

Fast mass movements

• can take different forms - classified as falls, topples, slides and flows

• vary in speed and scale

• rock fall begins as a rock - becomes separated from bedrock - as it falls it can set in motion an unconsolidated movement as one rock knocks into another

• topples occur when the strata of the rock are unstable in their alignment to the angle of the slope

• slides can be divided into

  • rotational

    • occur when a slump block, composed of loosely consolidated sediments slides along a slip plane

    • several characteristic features - slip plane is concave in its shape

    • causes original surface of the block to become less steep and the top of the slump is rotated backwards

    • main slump block often breaks into a series of secondary slumps and an associated scarp marks its original position

  • translational

    • follow a more linear axis of slip

• debris flows = water-laden masses of soil and rock rush down mountainsides, funnel into stream channels and form hick muddy deposits on valley floors

• generally have bulk densities

• they can flow almost as fluidly as water

• debris flows descending steep channels commonly go up to speeds that surpass 10meters per second

Correlation between frequency of mass movements and magnitude

• Small scale movement that affect a small area occur more frequently

• large scale movements whicha ffect large area occurs less frequently

• however, also depends on physical and human factors influencing stability of the slope

Rifting

Splitting apart of a tectonic plate into 2+ tectonic plates separated by divergent plate boundaries