Practice Exam Flashcards
What is Airy’s Model?
Continental crust has a uniform density. Consequently, the highest mountains must be supported by deep crustal roots that reach greater depths within the ductile mantle below.
Mantle convection
The temperature of the Earth increases with depth – a thermal gradient. On geological timescales the mantle is a viscous fluid and therefore convects.
Models for mantle convection
- Whole mantle convection
- Two-layer convection
The Wilson Cycle
A model that describes the opening and closing of ocean basins and the subduction and divergence of tectonic plates during the assembly and disassembly of supercontinents.
Stages: Continental rifting, oceanic divergence, oceanic convergence, continent-continent collision, post-collisional orogeny, and peneplanation.
Key Processes: Seafloor spreading, subduction, mountain building, and erosion.
The Wilson Cycle
Stage A
The cycle begins with a stable continental craton, an old and stable part of the continental crust that has minimal tectonic activity. This craton remains relatively undeformed and inactive.
Example: Cratonic regions like the Canadian Shield or the Australian Shield.
The Wilson Cycle
Stage B
Early Rifting
Description: Mantle upwelling beneath the stable craton leads to the thinning and stretching of the crust, initiating rifting. Faults form, and volcanic activity may occur as the crust begins to break apart.
Example: The East African Rift, where the continental crust is actively rifting.
The Wilson Cycle
Stage C
Full Ocean Basin
Description: Continued rifting eventually forms a mid-ocean ridge, leading to seafloor spreading. The two rifted continental blocks move further apart, creating a mature ocean basin.
Example: The Atlantic Ocean, which is a fully developed ocean basin with passive margins..
The Wilson Cycle
Stage D
Subduction Zone
Description: As the oceanic crust ages, it becomes denser and eventually begins to subduct beneath another plate. This subduction creates an active convergent margin with features like volcanic arcs and ocean trenches.
Example: The Pacific Ocean, where subduction zones exist along its margins.
The Wilson Cycle
Stage E
Closing Remnant Ocean Basin
Description: Continued subduction causes the ocean basin to narrow. The ocean basin is in its final stages as the two continental plates are drawn closer together, leading toward collision.
Example: The Mediterranean Sea, a remnant of the Tethys Ocean, is closing as the African plate converges with the Eurasian plate.
The Wilson Cycle
Stage F
Collision Orogeny
Description: The final closure of the ocean basin brings the two continental plates into collision, forming a collisional orogen (mountain belt). Intense deformation and uplift occur as the plates push against each other.
Example: The Himalayas, which formed from the collision between the Indian and Eurasian plates.
The Wilson Cycle
Stage G
Peneplained Mountain
Description: Over time, the mountain belt formed by the collision undergoes extensive erosion, reducing it to a nearly flat surface or peneplain. This eroded surface eventually stabilizes, returning to a cratonic state.
Example: The Appalachian Mountains, which have been heavily eroded over millions of years.
What drives plate tectonics?
THE SLABS DRIVE PLATE TECTONIC
SLAB PULL ~ 1X10 14 N/M
Mueller and Phillips (1991)
analysed force balance on
oceanic margins
They concluded that you
needed between 7x10 12 N/m to
1x10 13 N/m to initiate subduction on a passive margin.
The only force big enough to
do this is a mature subduction
zone.
FLEXURE RESISTANCE ~ 8X10 12 N/M
SHEAR RESISTANCE (OCEAN TRENCH FAULT) ~ 1X10 12 N/M
SLAB RESISTANCE ~ 8X10 12 N/M
RIDGE PUSH ~ 3X10 12 N/M
BASAL DRAG ~ 1X10 12 N/M
What is the Transition Zone?
Give an example of a minerals journey?
Between 410km and 670km
Olivine transforms first to Wadsleyite
Then to Ringwoodite
It transforms in bridgmanite and ferropericlase in the
lower mantle.
Each phase is denser than the ones above.
Key point – the 670 km transition is endothermic. This
means in happens in the warm mantle before the cold
slabs – could prevent slabs from getting through.
What is the D Layer?
AKA D Double Prime Layer
– Get flattening of velocity & density gradients between 200-250 km above core-lower mantle boundary
- Strong density contrast at CMB = good seismic reflector
–Steep thermal gradient across core-mantle boundary (1500°K)
–Temp. estimates at CMB are 3570 ±200°K to 4000°K
Partial melt in the D layer.
Temp change is the most important
silicate solid mantle with metallic liquid outter core. Seismic waves passing through this layer is a good reflector
- Post - perovskite phase transition: bridgmanite
converts into denser phase. But – only where it’s cold (eg. Slabs) - LLSVPs – Large, low shear-wave velocity provinces (chemically different).
- ULVZ (ultra-low velocity zones) – velocities so
low they have to be partial melt, or infiltrated
molten core metal.
Picture the D Layer. Describe what you see
Physical properties of the Earth’s interior?
Each Layer
Supercontinents Throughout Earth’s History and there ages
Vaalbara: Formed around 3.6 billion years ago from the collision of the Kaapvaal and Pilbara cratons. It was the earliest known supercontinent.
Kenorland: Assembled around 2.7 billion years ago, consisting of parts of present-day North America, Greenland, Scandinavia, and southern Africa.
Columbia (or Nuna): Existed between 2.1 and 1.8 billion years ago, formed from the amalgamation of several smaller continents.
Rodinia: Formed around 1.1 billion years ago and broke apart around 750 million years ago. It was a massive supercontinent that included most of Earth’s landmass.
Pannotia: Assembled around 600 million years ago from the fragments of Rodinia. It was a short-lived supercontinent that existed for about 60 million years.
Pangaea: Formed around 335 million years ago and began breaking apart around 175 million years ago. It was the most recent supercontinent and is well-known for its C-shape.
What are the driving and resisting forces acting on tectonic plates
List nine.
RESISTING:
TF = TRANSFORM FAULT FRICTION
DF, CD = BASAL DRAG (CD ON CONT)
CR = CONTINENTAL RESISTANCE
SR = SLAB RESISTANCE BY MANTLE
DRIVING:
RP = RIDGE PUSH
SP = SLAB PULL
SU = SLAB SUCTION
DF = DRAG FORCES CAN PULL PLATES ALONG
Convergent Boundaries CC and OC Explain how each works
OC: The Oceanic plate is denser and cooler than the continental crust and therefore subducts below it. It has negative density greater than a thin hot crust and will go down preferentially.
CC: In this case both crusts are relatively light and hot from the lithosphere. Buoyancy must be balanced and mountain belts start to form.
Divergent boundaries
OO: Tectonic plates pull apart, causing the mantle to rise and fill the gap. This mantle material is very hot (around 1300 °C) and low in density, making it buoyant. As it cools, it forms new oceanic crust, creating a ridge that sits higher than the surrounding ocean floor. Over time, as the crust cools and becomes denser, it sinks lower, leading to varying ocean depths—2.5 km at the ridge and around 5 km in deeper ocean basins.
CC: As the plates pull apart it forms rift valleys. Normal faults develop on both sides of the rift vally to form on both sides. This process thins and drops the crust, often resulting in significant geological features like the East African Rift Valley (graben).
Transform boundaries
Transform boundaries occur where tectonic plates slide past each other horizontally. They serve to connect segments of divergent boundaries.
The mid-ocean ridges, where plates move in opposite directions on either side. In oceanic settings, transform faults between ridge segments are seismically active. Once past the ridge segments, the transform fault becomes a fracture zone with no relative motion and no seismic activity.
On continents, transform faults, like the San Andreas Fault, can be dramatic, causing significant earthquakes due to accumulated stress. This differential motion highlights the dynamic interactions of plates across various tectonic settings.
What is the Ophiolite Suite?
What are are the layers and depths?
Mean age? Average Age?
It is the Oceanic crusts uniform stratigraphy
- Pelagic Sediments
- Basaltic (upper part of oceanic crust)
2A & 2B = pillow lavas, hyaloclastites
2C = sheeted dykes usually 1-3 m wide - Basaltic, mostly gabbro (lower part of oceanic crust).
Remnants of shallow axial magma chambers (feeds the dikes and basalts). - Ultramafic rocks & section of uppermost mantle.
Harzburgite and dunite (residuum of the original mantle)
Mean Thickness ~ 9km
Average Age ~ 180Ma
List all the major tectonic plates
What is Plate Tectonic Theory?
What are the key aspects?
Earth’s outer, cool, rigid shell is divided into fragments called “plates.”
Plates float on a viscous, weaker layer beneath them.
Plate movements include collision, sliding past each other, and breaking apart.
Two types of plates exist, differing in density and composition:
Continental: Thicker, less dense, older.
Oceanic: Thinner, denser, short-lived and recycled into Earth’s interior through subduction.
Topographic nature of Earth’s crust?
Picture the graph. Where is mass distributed in relation to altitude/depth?
Isostasy
Is the rising or settling of a portion of the Earth’s crust that occurs when weight is removed or added in order to maintain equilibrium between buoyancy forces that push the crust upward, and gravity forces that pull the crust downward.
The crust neither floats nor sinks within the ductile asthenosphere beneath.
The state in which pressure from every side are equal.
Lithosphere-Asthenosphere Boundary (LAB)?
It is a temperature defined discontinuity (1300C). That is defined by:
Reduced seismic wave velocities
Higher electric conductivity
Sheared boundary (peridotite xenoliths)
It seperates the chemically depleted lithosphere from more fertile asthenosphere below it.
It is shallower beneath younger crust.
In continents it is highly varied. The older the crust means more deposition and more trapped mantle beneath.
Under Oceanic crust LVZ is approximately 80km.
The LAB depth varies, generally deeper under continents (up to 200 km) and shallower under oceanic crust (around 50-100 km).
What is Pratt’s model?
The Compensation Depth was a horizontal surface beneath mountains. Consequently, the density must vary laterally across a mountain range, with the higher altitudes underlain by rocks of lower density
Orogeny, erosion and shield formation.
Draw the three main steps
Normal Fault
AKA dip-slip fault
Defined by hanging wall moving downwards relative to the footwall due to extensional forces.
45*
Reverse Fault
AKA Dip-slip fault
Hanging wall is forced upward driven by compressional forces.
45*
Strike-Slip Transverse
What are the two types?
Sidewards movement horizontally in the direction of the fault
left-lateral (sinistral)
right-lateral (dextral)
What type of fault is this?
Reverse fault
What type of fault is this?
Strike fault
What type of fault is this?
Normal fault
What type of fault is this?
Oblique
Fault move in two different directions simultaneously, combining both normal or reverse faulting with a strike-slip component.
What type of fault is this?
Thrust - compressional forces
When basalt cools and solidifies at Mid Ocean Ridge spreading centres, the newly formed oceanic crust is magnetised according to the Earth’s magnetic field at that time and location. How have the seafloor magnetic anomalies helped inform Plate Tectonic theory?
Magnetic Stripes and Symmetry: When basalt cools and solidifies at mid-ocean ridges, it records the Earth’s magnetic field at that time. Due to periodic reversals in Earth’s magnetic field, the seafloor shows a pattern of alternating magnetic “stripes” of normal and reversed polarity. These magnetic anomalies are symmetrically distributed on either side of the mid-ocean ridge, indicating that new crust is being continuously created at the ridge and pushed outward, preserving a record of past magnetic reversals.
Proof of Seafloor Spreading: The discovery of these symmetrical magnetic stripes provided direct evidence for seafloor spreading, a central idea in Plate Tectonic theory. It showed that oceanic crust is generated at mid-ocean ridges, moves laterally away from the ridge, and is eventually recycled back into the mantle at subduction zones. This mechanism of seafloor spreading explains how tectonic plates move over time.
Determining Plate Movement Rates: By measuring the width of the magnetic stripes and knowing the timeline of Earth’s magnetic reversals, scientists can calculate the rate of seafloor spreading. These rates match observed plate velocities, reinforcing the concept of moving tectonic plates and quantifying plate movement over geological time.
Global Correlation of Magnetic Patterns: Similar magnetic patterns are observed at mid-ocean ridges worldwide, confirming that the process of seafloor spreading and plate tectonics operates on a global scale. This coherence between different ocean basins provides consistent evidence that plate tectonics is a planetary-scale phenomenon.
How does Orogeny differ from an Orogen?
??
Orogeny
Definition: Orogeny refers to the process of mountain building that occurs due to tectonic forces, particularly through the collision and convergence of tectonic plates. This process involves intense deformation, folding, faulting, metamorphism, and often magmatism, which reshape the Earth’s crust.
Process: Orogeny includes various geological activities such as subduction, continental collision, crustal thickening, and uplift. These processes can create mountain belts and are driven by compressional tectonic forces.
Examples of Orogenies: Specific orogenies are named after the regions where they occurred and the geological period when they happened, such as the Himalayan Orogeny (responsible for the Himalayas), Alleghenian Orogeny (Appalachians), and Laramide Orogeny (Rockies).
- Orogen
Definition: An orogen is the physical product or result of an orogeny. It is a mountain belt or range that has been formed through the orogenic process. Orogens are typically elongated regions of deformed rock that show evidence of past tectonic activity and mountain building.
Structure: An orogen includes geological features like fold-and-thrust belts, metamorphic cores, fault zones, and, in some cases, volcanic arcs. The rocks in an orogen are usually highly deformed and may show a history of intense compression and metamorphism.
Examples of Orogens: The Himalayan Orogen, Appalachian Orogen, and Alpine Orogen are examples of mountain belts formed by orogenic events.
In this unit, we discussed Earth’s “heat engine” and heat transfer within the planet. What are the two main sources of heat within the Earth today?
??
Radioactive Decay: Decay of isotopes like uranium-238, thorium-232, and potassium-40 in the crust and mantle generates ongoing heat, fueling mantle convection and plate tectonics.
Residual Primordial Heat: Heat from Earth’s formation and differentiation, particularly from early accretion and core formation, still remains, contributing to mantle dynamics and volcanic activity.
Describe stagnant lid and mobile lid mantle convection, and identify which mode occurs within planet Earth today.
For mobile lid convection, cold surficial material continuously recirculates into the mantle.
In mobile lid convection, the lithosphere is broken into separate, moving tectonic plates that are able to subduct and recycle into the mantle. This type of convection drives plate tectonics, where cold, dense sections of the lithosphere sink into the mantle at subduction zones, while new lithosphere forms at mid-ocean ridges. The continuous motion and recycling of the lithosphere make it “mobile.” Mobile lid convection facilitates heat transfer, mantle mixing, and dynamic geological processes at the surface, such as earthquakes, volcanic activity, and mountain building.
Stagnant lid convection results is a thick, stable viscous crust (no PT!).
stagnant lid convection, the outer shell of the planet (the lithosphere) is thick, rigid, and stable, with little to no large-scale horizontal movement or recycling of surface material into the mantle. The lithosphere effectively acts as an insulating “lid” that prevents material from actively recirculating back into the mantle. Instead, convection is confined beneath this stagnant lid, within the underlying mantle, where heat and material circulate in a closed system. This form of convection leads to limited surface geological activity and is typical of planets or moons without active plate tectonics, such as Venus and Mars.
Approximate depth of the Moho beneath continental AND oceanic crust.
The crust has some variation in the MOHO depth. Average 35km. 30 to 50 km. Can reach 70km in areas of thick crust e.g Himalayas.
Oceanic crust has a more consistent thickness. Average MOHO depth of 7km. Varies from 5 to 10 km.
What is the base of the athenosphere?
How do seismic waves behave in it?
LAB:
100km below continents
60km below cold oceanic crus
Temperature and Ductility:
1300C isotherm
The base of the asthenosphere is defined by a temperature threshold below which mantle rocks become more rigid and less ductile, transitioning into the solid, more stable part of the mantle.
Seismic Properties:
Low seismic velocity zone
Seismic waves speed up below the asthenosphere due to the increased rigidity of the mantle rocks, which is often used to detect this boundary.
Plate boundaries have synonymous terms. What do these correspond to?
Convergent
Divergent
Transform
Convergent Boundary
Synonyms: Destructive boundary, subduction zone, collision zone
Divergent Boundary
Synonyms: Constructive boundary, spreading center, mid-ocean ridge
Transform Boundary
Synonyms: Conservative boundary, strike-slip fault
In terms of composition, is an island arc basalt the same as an oceanic island basalt?
No, island arc basalts and oceanic island basalts are not the same in terms of composition. While both are basaltic in nature, they have distinct geochemical signatures due to their different origins.
Island Arc Basalts:
Origin: Formed at convergent plate boundaries, where oceanic crust subducts beneath another tectonic plate.
Composition: Enriched in incompatible elements like potassium (K), rubidium (Rb), and cesium (Cs), as well as large ion lithophile elements (LILEs) like barium (Ba) and strontium (Sr). They often exhibit a depleted pattern in high field strength elements (HFSEs) like niobium (Nb) and tantalum (Ta).
Oceanic Island Basalts (OIBs):
Origin: Formed at intraplate settings, often associated with mantle plumes or hotspots.
Composition: Characterized by enrichment in incompatible trace elements and isotopes, including those of helium (He) and neon (Ne), suggesting a deep mantle source. They often show distinct isotopic signatures compared to mid-ocean ridge basalts (MORBs).
In summary, while both island arc basalts and oceanic island basalts are basaltic, their distinct geochemical compositions reflect their different origins and the processes involved in their formation.
The temperature at the lithosphere-asthenosphere boundary is approximately
1300
The base of a tectonic plate is defined by which boundary?
Moho
Transition Zone
Brittle-ductile transition
Lithosphere-asthenosphere boundary (LAB)
Lithosphere-asthenosphere boundary (LAB)
An important way to illustrate the spatial relationships of seismicity and tectonics is through the use of focal mechanisms (“beach balls”), which can tell you whether an individual earthquake is a thrust event (compressional), a normal event (extensional), or strike-slip. Connect each beach ball with its corresponding block diagram of a fault.
Explain why there is a negative correlation between the length of the arc-trench gap and the dip of the downgoing slab at subduction zones.
The cross-sectional diagram below shows the trace of the downgoing slab for a number of modern subduction zones with the slab profiles normalised to the location of the volcanic arc (Volcanic Front). With the aid of this diagram, explain why there is greater seismic activity and a much higher seismic hazard (large magnitude earthquakes and tsunamis) for the Chile and Peru margins compared to the other margins.
Identify which is Airy’s and Pratt’s model of isostasy.
A. Pratt
B. Airy
Describe stagnant lid and mobile lid mantle convection, and identify which mode occurs within planet Earth today.
Stagnant lid mantle convection is when the outer layer of a planet (the lithosphere) is rigid and does not move, while convection happens only in the mantle below. This mode creates a stable, unmoving outer “lid.”
Mobile lid mantle convection is when the outer layer is broken into plates that can move and interact. Convection in the mantle drives these plates, causing tectonic activity like earthquakes, volcanoes, and continental drift.
On Earth today, mobile lid convection occurs, as seen in our tectonic plate movement and active plate boundaries.
Indicate which subducted slab geometry is indicative of trench advance versus trench retreat.
What causes the slab pull force and how does this differ from the slab anchor force?
What defines a tectonic plate?
Crust & Lithosphere
Explain this image
Lithosphere-asthenosphere boundary (LAB) –Can be a Low Velocity Zone (LVZ) with variable depth & lateral persistence
*200-300 km depth beneath cratons *Top of LVZ is ~80 km beneath oceans
*May be non-existent or not detectable beneath most cratons
Explain seismic changes throughout the Earth
P wave
Primary: Compressional or longitudinal motion parallel to the direction of wave propagation. The waves slow down and bend at the core boundary towards the center of the Earth.
Shadow zone 105-140 degrees.
Passes through solids, liquids and gases.
S wave
Secondary: shear or transverse motion 90 degrees to direction of wave propagation pass through solids only.
No S waves beyond 105 degrees because of the core
Mohorovici discontinuity AKA The Moho
Boundary between the Earth’s crust and the mantle. It is characterised by a sudden increase in seismic wave velocities, which indicates a transition from the less dense materials of the crust to the denser materials of the mantle.
Continents 30-70 km think
Oceanic crust 5-10 km think
The Earth’s crust is composed of lighter rocks like granite and basalt, while the mantle below consists of denser rocks such as peridotite
Lithosphere
The crust and the solid uppermost portion of the mantle.
Contributing to tectonic activity, earthquakes, formation of mountains and ocean basins ect.
up to 75 km beneath ocean basins
up to 125 km beneath continents
Asthenosphere
Low viscosity. 1-2% molten. 400km depth
A semi-fluid, ductile layer of the Earth’s upper mantle. It is solid rock that is so hot that it behaves like a viscous fluid over long periods. This plasticity allows it to flow slowly.
The temperature causes partial melting of rocks, reducing strength and allows for the flow. The pressure is not high enough to prevent this flow.
Explain metamorphic core complexes?
Draw it
Metamorphic core complexes form in tectonic settings that undergo significant extension. Through low-angle faulting and tectonic exhumation, high-grade metamorphic basement rocks are uplifted and juxtaposed against an unmetamorphosed or lightly metamorphosed cover. These complexes showcase a combination of ductile and brittle deformation due to the differing conditions at depth and in the upper crust, illustrating a complex interplay of geological processes that reveal insights into the history of crustal deformation and extension.
List five mantle source components
- Depleted Mantle (DM)
- HIMU
- Enriched Mantle (EM1, EM2)
- Primitive Mantle (PM)
- FOZO
Breakdown of chlorite in the subducted slab plays a crucial role in the formation of arc magmas, which leads to the characteristic positioning of volcanic arcs about 100-110 km above the downgoing slab
HNW = hottest, shallowest nose of mantle wedge
SWI = Slab-mantle wedge interface
Chlorite Stability and Breakdown:
In subduction zones, chlorite, a hydrous mineral, is stable within the specific pressure-temperature (P-T) range of about 2–3.6 GPa and 800–820°C, which corresponds to conditions near the base of the upper mantle wedge. As the subducting oceanic slab reaches these depths and temperatures, chlorite becomes unstable and undergoes breakdown.
Water Release During Chlorite Breakdown:
When chlorite breaks down, it releases structural water into the surrounding mantle wedge. This released water significantly lowers the melting point of the mantle material above the slab. The presence of water allows melting to occur at much lower temperatures than it would under dry conditions, a process known as hydration-induced flux melting.
Melting in the Overlying Mantle Wedge:
The water released by the breakdown of chlorite causes partial melting of the mantle wedge. This melting occurs directly above the point in the slab where chlorite decomposes, typically at depths of 100-110 km above the subducting slab. The melts generated in this hydrated mantle wedge are then buoyant and begin to ascend toward the Earth’s surface, eventually forming volcanic arcs.
Positioning of Arc Volcanoes Above the Benioff Zone:
The consistent position of volcanic arcs 100-110 km above the Benioff Zone (the zone of earthquake activity marking the descending slab) is due to this specific P-T condition for chlorite breakdown. This depth defines where water is released in significant amounts to trigger mantle melting, controlling the location of arc volcanism in relation to the subducting slab.
The material is moving from the white bit in the middle, to the black sections at the side, so it is extensional (a stretching earthquake)
The possible fault planes dip towards the middle of the circle so this would be either a SW dipping plane or a NE dipping plane. There’s no way to tell which plane actually generated the earthquake without other external information.
The material is moving from the white sections at the side, to the coloured section (in this case red) in the middle, so it is compressional (a squishing earthquake). These are the kind of beach balls which you’ll see with big megathrust earthquakes.
We know that thrusts like to dip at a shallow angle, in some cases very shallow (<5°), so the shallow plane is a definite possibility. We also know that a vertical thrust would be geometrically unusual, so the very steep plane is pretty unlikely. If you see a beach ball like this in a subduction zone setting, its generally a pretty safe bet that the earthquake has occurred on the shallow subduction zone megathrust (with implications for tsunami hazards etc.)
Is this NW-SE fault or SW-NE fault?
The best way to know which is right – is to look at a map!
Andean margins
Andean-Rocky Mtn-type orogenesis generally related to “flat slab” subduction
What are the three steps to make this occur?
- Positive slab buoyancy (eg, aseismic ridges, oceanic plateau [impactor] or young age of subductingplate < 10 Ma)
- Trenchward motion of overriding plate (trench roll forward)
- Mantle wedge suction force (lowered viscosity in mantle wedge due to hydration)
What is the definition for a mantle wedge?
Draw it
Mantle wedge = triangular section of mantle in the hanging wall of subduction zones. It is the mantle source region for the magmatic arc, being fluxed by fluids delivered by dehydration of the descending slab.
What are the largest known LIPs?
Oceanic plateaus
Example - Ontong Java Plateau OJP
Areal extent of ~2 Mkm2
*larger than Alaska
*comparable to western Europe
Maximum plateau thickness of 30–35 km
Igneous volume up to 70-80 Mkm3
Split apart by seafloor spreading
– The OJ, Manihiki & Hikurangi plateaus
= one mega oceanic plateau
Lip Types?
LIPs major feature of Earth history since Pangea assembly
Picture the map.
Composition and Rheology of the Earth’s Layers
Cratons
- Cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time.
- They are generally tabular bodies with great areal extent (millions of square kilometres) and lithospheric thickness (200-400 km).
The Hadean - Earth’s initial config
Zircons
Zircons indicate that
* there must have been some felsic crust in the Hadean
* There was water in the crust and at the Earth’s surface
* The Hadean crust was stable for 100s millions of years from Hf isotopes
Hf isotopes in early Archean zircons suggests that there must be Hadean mafic crust that melted in the Archean
The Hadean - Earth’s initial config
Nd
142Nd anomalies suggest long term stable Hadean felsic crust and reciprocal depleted mantle must have existed in the Hadean
That no definitive Hadean lithologies are preserved and that 142Nd anomalies disappear in rocks younger that 3Ga indicates that Hadean crust must have been efficiently mixed back into the mantle.
Rifting process inherently asymmetric
–Produces contrasting conjugate margins
Upper plate margins
* Are relatively unextended
* Have large volumes of igneous underplate
* Have passive margin mountains and permanent topography
Lower plate margins
* Are highly extended with rift systems, marginal plateaux
* Have subdued topography and subsided since rifting
* Have thick sedimentary cov
Continental Margins
Passive Rifting
–Direct application of opposing forces to the lithosphere to create extension
–Lithospheric failure by tensional stresses
–Mantle upwelling as a consequence of lithospheric thinning (ie. passive response)
–NB: igneous activity a secondary process
Continental Margins
Active Rifting
–Associated with active (hot) mantle upwelling (eg, mantle plume) and impinging on lithosphere
–Upwelling/plume triggers decompression melting, conductive heating and heat transfer–+/-thermal erosion (thinning) of lithosphere
–Dynamic uplift /thermal buoyancy creates higher gravitational potential for overlying lithosphere
–Tensional stresses & rifting in response to gravitational forces
–NB: Igneous activity onset is pre-to early syn-rift
Draw Fore-arc Basin
Name key features
- Up to ~166 ±60 km wide region
- Dominated by volcanogenic sediment fill
- Can have eroded accretionary wedge material
Draw the three types of ophiolite obduction
A. Overthrusting of oceanic lithosphere onto passive-margin or arc rocks during continental collision
B. Splitting off & overthrusting of the upper section of the subducting slab, or
C. Underthrusting of oceanic lithosphere into an accretionary prism.
Draw an young slab retreating
Draw an old slab advancing
Draw Back-arc Basin
Name key features
Draw a Continental BA
List key features
Broad regions (up to 1000 km) mobile belts characterised by:
Thin, hot, weak lithosphere
High surface heat flow +-elevated topography
Long history of deformation (extensional & contractional)
Rigid microplates/terranes decoupled from deformation
Earth’s Layers are minerals
Upper Mantle
– Includes lithospheric & asthenospheric mantle
– Extends from Moho to 660 km discontinuity
