Flashcards in Chris Deck (48):
• Nicolas Steno – Law of Superposition – horizontally deposited layers of rock, oldest at bottom
• James Hutton – Law of Uniformitarianism – Crust has been shaped by continuous and uniform processes – present is the key to the past
o Assumes the processes of today are the same as in the past
o Rate and intensity of processes have changed
• More important are physical laws – are uniform everywhere and always
Earth's composition (outline)?
Radius – 6400 km
• Crust (Basalt to granite)
• Mantle (Mostly periodotitie)
o More homogenous than crust
o Rock, mainly Mg, Fe, silicates, left over after segregation
• Outer Core (iron/nickel)
o Produces magnetic field – protects from solar radiation
• Inner core (iron/nickel)
Breakdown of Crust types and core??
• Silicates (enriched in K, Al, Na)
• Formed since 4 billion years – average age 2-2.5 billion years
• Silicates (enriched in Ca, Al)
o Composition difference to continental make it more dense
• Less SiO2, more calcium and metal oxides
o Similar to mantle as its formed by mantle
• Forming continuously from mantle
• Average age < 200 Myr
• Oceanic crust formed since at lease 1 Gyr, may be earlier
o Younger than continental
• Subductive zones – where you get mantle material into crust
Core: (Takes up almost half of radius)
• Slowly crystallising from inward to outward
• Solid due to immense pressure
• Minimise the potential energy by moving denser material to the centre
• Differentiation is energetically favoured
• Formed early by segregation and sinking of a metal phase
• Released huge amounts of gravitational energy (heat)
• Primordial heat also formed by aggregating
• Magnetic field generated when the liquid in the outer core crystallises onto the inner core, liberating the latent heat of crystallisation of nickel and iron.
Bulk composition = core + mantle (mostly iron)
Ways of determining Earth's composition?
Direct methods of determining Earth’s composition:
o Max about 15km (crust at max is 40-70km)
o Safest place to drill = non-tectonic areas = also where crust is thicker
o Bits of other rocks
• Ophiolites – Cyprus
o Part of oceanic crust that’s been scrapped up
Currently can’t directly measure much of earths composition
• Heat flow
• Comparison with chondritic meteorites
• 2 types of meteorites, rocky and metal
• Originate in the asteroid belt
planet that never formed, or one that disaggregated
• Have ages of 4.5 billion years (mostly)
• May be differentiated
• Fall to Earth
• Rocky = more basaltic
• Meteorites represent building blocks of solar system – show the solar abundances seen in the solar system – with larger elements being less abundant as it takes larger elements to form these
• Accretion hypothesis suggests that Earth should have the same bulk composition as meteorites
• More mass = more likely to attract more mass
Bulk composition of the Earth, meteorites and the Sun should be very similar
• Except for the Earth losing hydrogen and helium as its so light that gravity can’t contain it to Earth
Types of fault?
3 main types:
• Tension – extending faults - Normal
o Reverse – Hanging wall moves up Footwall down, over each other
o Thrust – Hanging wall moves completely over Foot wall
• Shear – Move past each other (San Andreas style) – Strike-Slip
• Earthquakes are assumed to originate from a single point – focus – within 700km of the surface
• Most caused in fact by movement along a fault plane so the focus can extend for several kms
• Point of earth surface vertically above is epicentre
• Angle subtended at the centre of the Earth by the epicentre and the point at which the seismic waves are detected is known as the epicentre angleΔ
• Magnitude is a measure of energy release on a logarithmic scale – one change on the Richter scale is a 30-fold increase in energy release
Types of waves?
• Energy from an earthquake is transmitted through the Earth by several types of seismic wave, which propagate by elastic deformation of the rock
• Waves penetrating the interior of the Earth are body waves
• P waves, (longitudinal or compressional waves), correspond to elastic deformation by compression/dilation – particles of the transmitting rock oscillate in the direction of travel – disturbance proceeds as a series of compressions and rarefaction
• S waves (shear or transverse) – elastic deformation by shearing and causing the particles of the rock to oscillate at right angles to the direction of propagation
Rigidiity of a fluid is zero – s waves cannot be transmitted through them
• Velocity equations mean that P velocity is about 1.7 times greater and S veloicty in the same medium – in identical travel paths, P wave arrive before S waves
• Seismic waves which can only travel through a free surface (Earths surface) are known as surface waves – travel at lower velocities than body waves in the same medium – unlike body waves they are dispersive, their different wavelengths travel at different velocities
Uses of measuring seismic waves?
• Measure velocity from travel-time curves
• Can then determine how velocity varies with depth
• P-wave shadow zone – explained by refraction of waves encountering core-mantle boundary
• S-wave shadow zones – suggests outer core is liquid
What is rheological layering and what are the layers?
• Rheological = stress and strain properties of a rock
• Rheological layers
• Crust + upper mantle
• Strong, forms tectonic plates
• Remainder of upper mantle
• Weak (caused by temperature)
Moho and Lehmann?
• Seismic velociites increase significantly after about 50km
• Moho discontinuity
• Base of the crust before the upper mantle
• Boundary that divides the inner and outer core
Main rheological layer breakdowns?
• 70 km thick between oceans
• 125-250 km thick beneath continents
• Thickness correlates with age
o If thicker, has more time to cool and more time for mantle to move up and thicken it
• “Like toffee”
• Seismic wave speeds abruptly decrease after the lithosphere
o Called the low-velocity zone (LVZ) = the asthenosphere
o Velocity of waves through rock affected by density and shear modulus
• Extends < 300 km depth
• Is solid yet behaves like plastic, allowing for convection
o Unknown if convection through the whole thing or between upper and lower
• Division of upper and lower mantle seen by discontinuity at depth of 660km
o 660km is the deepest depths that earthquakes from subduction zones can be traced
• At 660 is the lower Upper Mantle and a seismic discontinuity of the transition zone
• 70% mantle = olivine
• Upper mantle – several transitions
• Lower mantle – fairly uniform -homogenous
• Post-perovskite near Core Mantle Boundary
Double Prime layer
• Above the core-mantle boundary
• 100 km – 300 km thick
• May be graveyard of sub ducted slabs
• Towards its base is a 5 km – 40km zone of ultralow seismic velocities, indicating the presence of partially melted rock
o Perhaps where magmas for hotspots arise
Sources of Earth's heat and convection and conduction?
• Primordial heating (core differentiation, accretion)
• Radioactive decay
• Higher heat flow through the base of the oceanic crust than through the base of the continental crust
• Geothermal gradient average – 25 degree C per km
• Asthenosphere is hot and therefore weak and can flow
• Allows for convection
• Convection allows for adiabatic gradient within the mantle
Lithosphere is therefore part of earth which transfers by conduction as it is strong and rigid
Convection takes place because buoyancy forces are able to overcome viscous resistance
Energy in natural systems? (GFE)?
• All natural systems want to go to their lowest energy state
• Gibbs free energy = measure of the chemical energy of a system
o Melting and crystallisation take place in order to minimise Gibbs free energy
o Controls: temperature and pressure
o Different minerals have different temperature and pressure conditions which favour lower Gibbs free energy – explains why they crystallise and mineralise at different conditions
Mantle Phase transitions?
• Increase in seismic velocity at 410km = Olivine to Wadsleyite
• Wadsleyite to Ringwoodite at 660km
Processes of partial melting?
• Geotherm does not usually intersect the solidus – upper mantle periodotite still melts – why?
• Adiabatic cooling gradient steeper than solidus
• Pressure – increase in pressure actually increases melting temp of rocks – When confining pressure drops enough, decompression melting is triggered – rocks moves to zone of lower pressure and lowers melting temperature– occurs along mid ocean ridges where plates are rifting apart – mainly occurs when hot mantle rock ascends
• Oceanic ridge – as rock pulled apart the mantle moves upwards to fill the rift = moves to an area of lower pressure – undergoes melting without an addition of heat – basaltic magma produced –
• Addition of volatiles (H2O)
• Water acts as a contaminent, makes material less solid
• Wet solidus is lower than dry solidus
• Decreases melting point
• Subduction zones – Increased pressure causes hydrothermal alteration minerals react and liberate water and CO2 which rise into the overlying mantle wedge – volatiles
Raising the geothermal gradient:
• Mantle plumes or abundance of radioactive materials
• Geotherm intersects the solidus
Earth's elevation trends?
• 2 different types of crust (Oceanic, continental)
• Continental – (average) above sea level (but can be below – North Sea)
• Oceanic – below
• Get highest mountains near deepest trenches
• Continents are less dense and therefore due to isostasy, have higher elevations
Distribution of mass and acceleration due to gravity?
• Gravitational force varies over Earth’s surface
• F= GM1M2/r2
• Change due to latitude and elevation as you get further from core
• Earth is not perfectly circular
• At poles = heavier due to being closer to core
• At equator = closer to axis of rotation – more centrifugal force, less gravity = lighter
• Weigh less on a mountain than at sea level
• Variations predictable because of formula
Acceleration due to gravity:
• Law of Gravitation combined with Second Law of Motion
• g = GM/r2
• g = acceleration due to gravity
• Difference in the observed and predicted value of g = a gravity anomaly
Two gravity anomalies?
Free Air Anomaly
• Free air correction – difference between the observed and theoretical acceleration due to gravity at sea level
• Free air anomaly corrects for elevation but not for mass above or below
• Bouger correction accounts for excess mass
• Negative Bouger Anomaly over the Rocky Mountains in the USA = Less Dense Rockies
Isostasy and theories of this?
• Equilibrium distribution of mass
• Earth’s surface features are in isostatic equilibrium
• Asthenosphere is weak and therefore allows an area where lithospheric blocks can “float”
• High elevations underlain by thick, low density “root”
• High elevations underlain by low density material
• Low elevations underlain by high density material
Both assume that the lithosphere is uniform and is weak but we know this is untrue and the lithosphere is strong due to earthquakes
Continental drift evidence and evolution of theory?
o First came the idea of the jigsaw fit of the continents
• Present shorelines make rough fit
o Shorelines regular eroded
• Closer fit is made using the continental shelves
o Erosion and deposition since split explains slight discrepancies
o Alfred Wegener amassed evidence for continental drift:
• “Fit” of the continents
• Location of glaciations
• Paleoclimatic evidence
• Fossil evidence
• Rock type and structural similarities
• Later came the concept of “Pangea”
• Craton – old, stable parts of the lithosphere which are used to timeline Earth = Ancient Precambrian terrains
• Concluded that if the continents were joined they must have moved apart
Magnetism and pole wander in terms of continental drift?
• When crust is formed, magnetite is active and lines up with the Earth’s magnetic field
• In Igneous rocks in the basalt (high in Fe)
• Lava cools
• When cools past the Curie point (ca. 580 degrees C) the Fe-minerals align to the magnetic field
How the measurements are made:
• Samples drilled out of rock are spatially orientated
• Observed magnetism in ancient lava flows to determine the magnetic poles from the past were different
• Paleomagnetic pole moved through time
• Rocks of different age sampled in one location to determine polar wander path through time
• Poles not moving continents are – proof
• Earth’s magnetic field reverses polarity
• Reverses the poles
• Over geological history the poles have reversed thousands of time
• Can be periods of normal, reversed or mixed polarity
• Last polar reversal took place nearly 800,000 years ago
• Overdue a reversal
Link to marine magnetism:
• Correspond with width of anomalies in the sea floor
• Positive anomalies are due to presence of rocks with RMS of normal polarity
• Negative anomalies for presence of reversed polarity rocks
Seafloor spreading and plate motion:
• Can calculate spreading rates using distance from ridge and time from age which corresponds to polarity of rocks
Earth's creation equilibrium?
• Material made at rifts
• Destroyed at subduction zones
• Earth is not growing
• Oceanic lithosphere gets older, away from the MOR
• Arise due to variable crustal thicknesses
• Buoyancy forces resist convergent plate tectonic forces
• Mountains collapse under their own weight
• Mountain belts are soft underneath – low root at high pressure and temp
Whole mantle convection?
• Tomographic images show that some subducting slabs penetrate the transition zone
o Whole mantle convection may be possible
Driving mechanisms of plate tectonics:
• Newly formed plates at oceanic ridges are warm and are therefore at a higher elevation
• Gravity pushes the ocean floor toward the trench
• This forces the cooler lithosphere away from the ridge
Mantle convection (Mantle drag):
• Core’s energy heats the mantle
• The mantle rises towards the Earth’s surface which is cooler
• The mantle transfers its heat to the surface, mainly at mid-ocean ridges, becomes denser and sinks
• These motions break the lithosphere into plates and move them around the surface of the planet
Slab Pull (Edge forces):
• Older, colder plates sink at subduction zones as they are more dense than the underlying mantle
• They pull the rest of the warmer plate behind
• Faster moving plates are ones with more of their edges being slab pulled suggesting that slab pull is the main driving force
• Plates separating and new oceanic lithosphere forming
• New crust has lower density, so isostatically stands high
• Crust bulges from magmatism and is extended, thinned and fractured
• Magma from partial melting of the magma
• Decompression melting to produce basalts
• Cooling by seawater – causes metamorphism
• Pelagic organisms decay here bringing constant sediment – mainly carbonates
• Thickens away from ridge
• Some magma rises to the surface and is extruded as lava flows
• Mid-Atlantic ridge
o Rugged topography
o Shallow earthquakes
• Older oceanic crust must be destroyed so that Earth’s surface area remains the same
• During subduction the subducting plate moves into the asthenosphere, is heated and is incorporated into the mantle
• Characterized by:
o Deformation - folding and faulting
o Andesitic volcanism (except at continental collisions)
o Mountain building
o Earthquake activity - shallow to deep (>600 km)
o Oceanic trench
o Accretionary wedge (Depends on angle of subduction + amount of sediment)
o Forearc basin
o Backarc basin
o A volcanic island arc is formed if volcanoes emerge as islands
o Colder, older subducts
o Oceanic will subduct = more dense
o West South America – Andés
o Magma generated by subduction
• Rises into continental crust to form large igneous bodies
• Or erupts to form a volcanic arc of andesitic volcanoes
Partial melting at subduction zones?
• Increases pressure due to subducting slab causes metamorphism and release of water
o Water rises into overriding plate or mantle wedge causing partial melting
• At ocean-subducting convergent boundaries, wet basalt and sediment is heated and compressed as it subducts, resulting in dehydration and sometimes partial melting (sediment melting is probably common, basalt melting is rare).
• The escaping fluid transfers into the mantle above and causes melting to produce basalts and andesites
Whats an orogenic belt?
Orogenic belts are long, commonly arcuate tracts of highly deformed rock that develop during the creation of mountain ranges on the continents.
• Show where old inactive plate margins were
Continental collision zone?
o When oceanic crust has completely subducted the two continents collide, resulting in mountain building – Himalayas
o Plates are welded together
• An interior mountain belt forms
• Consists of
o Deformed sedimentary rocks
o Metamorphic rocks
o Igneous intrusions
o Fragments of oceanic crust
• At continent-continent convergent boundaries (collision zones) the continental crust is thickened (up to 75 km thick).
• Temperatures in the thickened lithosphere can exceed 700°C, sometimes up to 1000°C.
• Temperatures high enough to melt water-rich sediments or even mantle rocks.
• Continent-continent collisions mainly result in intrusive granites.
• The Himalayan–Tibetan belt and the European Alps represent orogens that form by continent–continent collision following the closure of a major ocean basin (i.e. Himalaya-type)
• Where the continental lithosphere is relatively cool and strong, orogens tend to be comparatively narrow, ranging between 100 and 400km wide. The Southern Alps of New Zealand and the southern Andes exhibit these characteristics
• Conversely, where the continental lithosphere is relatively hot and weak, strain tends to delocalize and is distributed across zones that can be over a thousand kilometers wide. The central Andes and the Himalayan–Tibetan orogen (Section 10.4) display these latter characteristics.
Growth of Continents
• Grow by accretions of arcs at subduction zones
o Collision and welding
Plate convergence velocities
• Distance from point = higher velocity
o Like a wheel or spinner ride
• Himalayas 50mm/yr
Formation of the Himalayas?
• The Himalayan–Tibetan orogen was created mainly by the collision between India and Eurasia over the past 70–50Myr
• The India–Eurasia collision was brought about by the rifting of India from Africa and East Antarctica during the Mesozoic (Section 11.5.5) and by its migra- tion northward as the intervening oceanic lithosphere was subducted beneath the Eurasian Plate.
• Magnetic anomalies in the Indian Ocean and paleomagnetic measurements from the Ninety-East Ridge and the Indian subcontinent record the northerly drift of the Indian plate and allow the reconstruction of its paleo- latitude
• the reconstruction of its paleo- latitude (Fig. 10.14). The data show a rapid decrease in the relative velocity between the Indian and Eurasian plates at 55–50Ma. This time interval commonly is interpreted to indicate the beginning of the India– Eurasia collision – could also be slowing of mid ocean ridge spreading south of India
• Before the main collision, Eurasia was collided with micro-continents. The collision and accretion of these terranes is marked by a series of suture zones
• Zones of concentrated thrust fault- ing occur along both the northern, southern, and eastern margins of the Tibetan Plateau. Within the Himalaya, thrust faulting is prevalent. South of the Himalaya (Fig. 10.18), intra-plate earthquakes and other geophysical evidence indicate that the Indian plate flexes and slides beneath the Himalaya, where it lurches northward during large earthquakes (Bilham et al., 2001). The overall pattern of the deformation is similar to that which occurs at ocean–continent convergence zones where an oceanic plate flexes downward into a subduction zone. North of the Himalaya, normal fault- ing and east–west extension dominate southern and central Tibet. Strike-slip faulting dominates a region some 1500 km wide north of the Himalaya and extend- ing eastward into Indo-China
Subsidence and rifts? Rift trends?
Continental rifts are regions of extensional deformation where the entire thickness of the lithosphere has deformed under the influence of deviatoric tension.
• Where the lithosphere is thick, cool, and strong, rifts tend to form narrow zones of localized strain less than 100 km wide The Baikal Rift, the East African Rift system, and the Rhine Graben are examples of this type of rift
• Where the lithosphere is thin, hot, and weak, rifts tend to form wide zones where strain is delocalized and distributed across zones several hundreds of kilometers wide. Examples of this type of rift include the Basin and Range Province and the Aegean Sea.
• Both varieties of rift may be associated with volcanic activity
• Asymmetric rift basins flanked by normal faults. Continental rifts are associated with the formation of sedimentary basins that are bounded by normal faults. Most tectonically active rift basins show an asymmetric half graben morphology where the majority of the strain is accommodated along border faults that bound the deep side of the basins
• Beneath the axis of most continental rifts earthquakes generally are confined to the uppermost 12–15 km of the crust, defining a seismogenic layer that is thin relative to other regions of the continents. Away from the rift axis, earthquakes may occur to depths of 30 km or more. These patterns imply that rifting and thinning locally weaken the crust and affect its mechanical behavior
Rift to Ocean formation process?
o Silica rich
o Lithosphere fractured along normal faults and rift valley created
o Asthenosphere rises
o Upwarping of magma under continental crust
o Magma forms igneous rocks from sediment fallen from high lands
o Rift extends and deepens, creates a Linear sea
o Mid-ocean ridge formed with rift in centre of ocean
o Basaltic rich
East Africa rift?
• Cause = “Swell push” due to elevation of East Africa above hot mantle upwelling
• Mostly extensional focal mechanisms
• P-wave velocities suggest crustal thinning and rise of hot mantle beneath rift axis
• Decompression melting
• The asthenosphere is anomalously hot leading to uplifts and pervasive volcanism result.
North American rift?
• The Basin and Range example thus shows that continental lithosphere may be highly extended without rupturing to form a new ocean basin. This pattern is characteristic of rifts that form in relatively thin, hot, and weak continental lithosphere.
Look at diagram in Week 10 of Chris Notes
What is Rayleigh number?
The Rayleigh number describes the conditions required for convection to take place; Ra is 103 convection is possible
• In the numerator: α is the thermal expansion coefficient: the more a fluid expands, the more its density is lowered on heating and the more it will want to rise, i.e. large α favours convection. ρ is fluid density, which controls the weight of the fluid. The greater the weight, the more the fluid will want to sink, i.e. large ρ favours convection. d is the height of the fluid in the convecting region, the taller the column the more it rises for a given expansion coefficient, i.e. large d favours convection. ΔT is the temperature gradient, stronger thermal gradients lead to more vigorous convection.
• In the denominator: γ is the fluid viscosity, very viscous fluids will resist convection, i.e. large γ will inhibit convection. κ is thermal diffusivity, which describes the efficiency of conduction. If the fluid diffuses [or conducts] heat very well it will lose heat that way and will not want to convect, i.e. large κ will inhibit convection.
• So – large values in the numerator and small values in the denominator lead to large Ra.
Partial melting and fractional crystalisation in phase diagrams:
• Peridotie makes up most of upper mantle
• Increase temp the number of crystals increases and the amount of melt decreases
• Oceanic crust most likely to be melted by partial melting of upper mantle
- In equilibrium crystallization or melting in a closed system, the final composition of the system will be identical to the initial composition of the system.
Fractional cystallisation occurs if the crystals are removed from contact with the cooling magma, i.e. their compositions are fixed and do not change with further cooling
Leads to shift in final composition
Adjustment of Earth's crust in response to stress which is applied to it
(pcxhc)-(ho-po)/pM = r (difference in mantle uplift)
Can use to find z (difference in continental and oceanic floor depth)
The Lake Bonneville uplift data suggest that the lithosphere flexes, rather than sinks, in response to loads (e.g. glacial lakes, mountain belts): diving board analogy
Suggests that the lithosphere does have strength and can, up to a limit, support its own weight - also know this from earthquakes
Bouger and Free Air anomalies?
Once the Free Air and Bouguer corrections have been made, the Bouguer anomaly should contain information about the subsurface density alone.
• The effect of latitude and elevation should have been removed. A map of the Bouguer anomaly gives a good impression of subsurface density.
• Low (negative) values of Bouguer anomaly indicate lower density beneath the measurement point.
• High (positive) values of Bouguer anomaly indicate higher density beneath the measurement point.
• The free-air correction accounts solely for variation in the distance of the observation point from the centre of the Earth
• Positive anomaly means a mass excess such as a descending slab
• Negative anomaly means a mass deficiency such as a trench
Back arc behaviour?
In shallow subduction, there is strong frictional “coupling” between the overlying and downgoing plates, resulting in large earthquakes and compression of the back-arc. Back-arc extension occurs where there is “roll back” of the downgoing plate. The negative buoyancy of the subducting slab causes it to collapse downwards (become steeper). Collapse causes the hinge to migrate backwards relative to the trench, allowing the over-riding plate to extend
Plates act a rigid stress guides
Deformation only occurs at boundaries
Euler poles - motion of rigid body on surface of a sphere, may be represented as rotation about a chosen rotation pole
The cyclical opening and closing of the ocean basins
Mountain belts are preferential sites of rifting as
- they have excess G.P.E
- Are not as strong as other areas
- Full of faults which can be reactivated
Structure of oceanic crust (ophiolites)
Extrusive basalts (pillow)
Describe earthquake mechanisms at subduction zones?
Normal faulting near surface as slab beds
Thrusting deeper down where plates collide
Within slab extension at intermediate depths reflecting the slab pull + compression at depth