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1

Geological Theories

• 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

2

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 Liquid
o Produces magnetic field – protects from solar radiation
• Inner core (iron/nickel)

3

Breakdown of Crust types and core??

Continental Crust:
• Silicates (enriched in K, Al, Na)
• Formed since 4 billion years – average age 2-2.5 billion years

Oceanic Crust:
• 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)

4

Ways of determining Earth's composition?

Direct methods of determining Earth’s composition:
• Drilling
o Max about 15km (crust at max is 40-70km)
o Safest place to drill = non-tectonic areas = also where crust is thicker
• Xenoliths
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
Indirect methods
• Seismology
• Magnetism
• Gravity
• Heat flow
• Comparison with chondritic meteorites
• 2 types of meteorites, rocky and metal

Meteorites:
• 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
• 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

5

Types of fault?

3 main types:
• Tension – extending faults - Normal
• Compression
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

6

Earthquake terms?

• 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

7

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

8

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

9

What is rheological layering and what are the layers?

• Rheological = stress and strain properties of a rock
• Rheological layers
o Lithosphere
• Crust + upper mantle
• Strong, forms tectonic plates
o Asthenosphere
• Remainder of upper mantle
• Weak (caused by temperature)

10

Moho and Lehmann?

Moho
• Seismic velociites increase significantly after about 50km
• Moho discontinuity
• Base of the crust before the upper mantle
Lehmann Discontinuity
• Boundary that divides the inner and outer core

11

Main rheological layer breakdowns?

Lithosphere
• 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

Asthenosphere
• “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

Mantle
• 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

12

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

Convection
• Asthenosphere is hot and therefore weak and can flow
• Allows for convection
• Convection allows for adiabatic gradient within the mantle
Conduction
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

13

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

14

Mantle Phase transitions?

• Increase in seismic velocity at 410km = Olivine to Wadsleyite
• Wadsleyite to Ringwoodite at 660km

15

Processes of partial melting?

Magma formation:
• Geotherm does not usually intersect the solidus – upper mantle periodotite still melts – why?

Decompression Melting:
• 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 –
Fluid-flux melting:
• 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
• Rare
• Geotherm intersects the solidus

16

Earth's elevation trends?

• Bi-modal
• 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

17

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

18

Two gravity anomalies?

Free Air Anomaly
• Free air correction – difference between the observed and theoretical acceleration due to gravity at sea level

Bouger Anomaly
• 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

19

Isostasy and theories of this?

Isostasy
• 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”

Airy’s hypothesis
• High elevations underlain by thick, low density “root”

Pratt’s hypothesis
• Rarer
• 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

20

Continental drift evidence and evolution of theory?

• Evidence
o First came the idea of the jigsaw fit of the continents
Continental Fit:
• 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

21

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

Magnetism preserved:
• 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

Pole Wander
• 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

22

Geomagnetic reversals?

• 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

23

Earth's creation equilibrium?

• Material made at rifts
• Destroyed at subduction zones
• Earth is not growing
• Oceanic lithosphere gets older, away from the MOR

24

Crustal bouyancy?

• 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

25

Whole mantle convection?

• Tomographic images show that some subducting slabs penetrate the transition zone
o Whole mantle convection may be possible

26

Driving mechanisms of plate tectonics:

Ridge Push:
• 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

27

Mid-Ocean Ridges?

• 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
o Rifting

28

Subduction Zones?

• 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 Metamorphism
o Earthquake activity - shallow to deep (>600 km)
• Oceanic-Oceanic
o Forms:
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
• Oceanic-continental
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

29

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

30

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