Large scale topography Flashcards
What does plate et carre mean?
Flat and square
What does the Mercator projection show?
A deformed image of the Earth
Why is the Mercator map of the Earth used?
Because the spherical (orthographic) representation shows only one side
Where is the most deformation on Mercator projection?
In the North and South which is why Antarctica looks so big
When should the Mercator map not be used?
When comparing areas- need to integrate the deformation
What is the equal-area Hammer projection aim?
To reduce the amount of deformation
What is an issue with the equal-area Hammer projection?
It distorts other areas
What should be done when using an equal-area Hammer map?
It should be centred to the location you are looking at to avoid distortion
What is another type of projection?
Interrupted Sinusoidal projection
What has disappeared the most?
The ocean floor
Why is a relatively recent time scale needed?
Due to erosion
What needs to be understood when looking at geomorphology?
Need some background on what happened before
For which period do we have background information for?
Quaternary
Define hypsometry
Percentage of volume of land below or above a certain altitude
How is hypsometry displayed?
Through a histogram
What do the two peaks in the histogram mean?
Deepest point and highest point
Where is the deepest point?
Mayan trench
What is the average elevation of continents?
0.8km high
What is the average elevation of ocean basins?
4 km
What is the average depth of ocean trenches?
7.5-11km
What is the average elevation of mountain islands?
6km in relief
What are continental shelves part of?
Continents
What percentage of the Earth is covered in oceans?
71%
What percentage of the Earth is covered by continents?
29%
What percentage of the Earth is covered by continental shelves?
5%
Why is hypsometry of a catchment useful?
To see if the landscape is active e.g. rivers are affected by uplift, source of deformation
What does a hysometry curve show?
The elevation of something e.g. a river
What is the x axis on a hypsometry curve?
Relative area
What is y a function of in a hypsometry curve?
Y is a function of x
What does the y axis show on a hypsometry curve?
Relative height
What is the knickpoint?
This is usually where there is an over-steeped section of a river
What is hypsometry often used to characterize?
Landscape morphology, fluvial dissection, glacial erosion and tectonic uplift rates
What two profiles can hyspometry curves show?
Convex and concave
Where is the knickpoint on a hypsometry curve?
In between the convex and concave profiles
Where is the convex profile on a hypsometry curve?
It is above the knickpoint
Where is the concave profile on a hypsometry curve?
It is below the knickpoint
What is hypsometry analysis?
Frequency distribution of elevations
What sort of profile does older, more stable landscape have?
Concave
What sort of profile does a youthful and tectonically active landscape have?
Convex
What does a dip in the knickpoint show?
Retreating base level fall
What do rivers do that causes the knickpoint?
Try to smooth out the landscape
How thick is the oceanic crust?
7-10km (relatively thin)
What is oceanic crust mainly made of?
Basalts
What is the density of oceanic crust?
2.9g/cm3
What is the thickness of the continental crust?
25-70 km (thick)
Why is continental crust so thick?
Because of the mountain ranges, higher the peak the thicker the crust
What is the main component of continental crust?
Granite
What is the density of continental crust?
2.7 g/cm3
Which type of crust is being lost the most quickly?
Oceanic
What have seismic waves found?
Discontinuities
Where does the oceanic crust material come from?
Material from the mantle
What is the Mohorovick discontinuity?
This is at the base of crust
Where is the Gutenberg discontinuity?
This separates the upper and lower mantle from the core
Where is the Lehmann-Bullen discontinuity
Separates the inner and outer core
How thick is the mantle?
2885 km
How thick is the outer core?
2270 km
How thick is the inner core?
1216 km
Where does the information about the Earth’s structure come from?
Geochemical
Geophysical
How is geochemical data collected?
Through magma chambers
How is geophysical data collected?
Through seismic waves from earthquakes
What is the geophysical model split into?
Lithosphere Asthenosphere Mesospheric mantle Outer core Inner core
When do seismic waves disappear?
When they go through liquid
What is the mesospheric mantle?
The lower mantle
Where can seismic waves disappear?
Where they try to get across the outer core
What state is the outer core?
Liquid
What state is the inner core?
Solid
What is the lithosphere?
The crust and uppermost mantle
What is a lithospheric property?
It is rigid and behaves elastically
What does the rigid elasticity in the litmosphere mean in terms of hazards?
Earthquakes can occur
What is the asthenosphere?
The region of the upper mantle of the Earth
What is the asthenosphere like?
It is highly viscous, mechanically weak and deformed like plastic
What is the mesosphere?
The region of the lower mantle
What temperature is the material that is dragged down by subduction?
Cold
At what temperature does basalt melt?`
1300 degrees
What happens to the properties of basalt and quartz upon melting?
They change
Become more inelastic (plastic ) so will flow rather than break
At what temperature does quartz melt?
1100 degrees
When did Wegner publish his first paper?
1912
What did Wegner propose?
In the past, the continents were not separated but formed one supercontinent
What was the supercontinent called?
Pangea 225 Ma
Why was Wegner’s theory originally dismissed?
He failed to explain continental drift over time
Where can fossil evidence be found for plate tectonics?
Gondwana region of central India
South America and Africa
What fossil can be found on all continents?
Glossopteris
How do mountains give evidence for continental drift?
Tertiary mountain belts along plate boundaries
Why are permo-carboniferous glaciations evidence for continental drift that is hard to spot?
Difficult to find information for past glaciation
What did Alexander DuToit show?
Continuity, not only in terms of shape but also in the type and age of the bedrock of Africa and South America
What did Wegner think moved the continents?
The tide
What is DuToit’s evidence?
Lineations on both sides of the two continents (South America and Africa)
In which country is there evidence of glaciation?
India
Who came up with the convection currents theory?
Arthur Holmes
What limited plate tectonic theory in the past?
The thought that the Earth was flat
What is island or swell in convection current theory?
Magma rising at the mid-oceanic ridge
What is there a variation of?
The rate of extension of plates
How long is the lifespan of a mid-ocean ridge?
200-300Ma
How often is the ocean floor virtually swept away?
300-400 Ma
In what way are continents passed around the mantle?
Passive
What is an implication of passive continental crust movement?
The leading edges of the continental crust become very deformed
At what rate is the mantle convecting?
1cm a year
What does the localisation of earthquakes and volcanoes show?
They are not randomly distributed- mainly linked to plate boundary deformation
What is a result of boundary deformation?
Magma rises to form a volcano
Earthquakes
Why is there more evidence for plate tectonics from earthquakes?
The slab can be seen going down because the depth of the earthquake can be found
Where are deeper earthquakes?
Away from mid-oceanic ridges
Why are deeper earthquakes not found near mid-oceanic ridges?
Because the crust is thinner
Briefly explain how plate tectonics work
Magma rises up and the asthenosphere raises at the oceanic ridge, the plates spread apart and one day become subducted at a trench
What happens to material over 1100 C?
It is molten
What happens if the material is molten?
There is no earthquake
What are earthquakes like at ridges?
Shallow
What happens at oceanic ridges?
Rising of the asthenosphere and partial melting
What happens at the trench?
Magma rises up to form a volcano, pushes the mid-oceanic ridge at the other side.
Why are earthquakes found at trenches?
Subduction causes friction and melting
Where are sediment accretion wedges found?
At subduction zones
What can happen at accretion wedges?
Earthquakes and deformation
What is a transform fault?
On the ocean floor
Two plates slide past each other
What is a strike slip fault?
Continental plates
Two plates slide past each other
What are the 3 types of plate boundary?
Divergence
Convergence
Conservative
What happens at divergence plate boundaries?
Plates move away from each other
At which plate boundary would you find mid-oceanic ridges?
Divergent
What are convergence boundaries?
This is where the plates move towards each other
What are conservative plate boundaries?
These occur when plates move laterally from each other
At which plate boundary would you find transform faults?
Conservative
Give an example of a transform fault
San Andreas Fault
At which plate boundary would you find oceanic trenches?
Convergence
Give an example of an oceanic trench
Peru-Chile trench
Give an example of a mid-ocean ridge
Mid-Atlantic ridge
Why can South America and Africa still fit together?
Because they have not been deformed
In what direction to the mid-oceanic ridges are transform faults?
They are perpendicular to the mid-oceanic ridges
Why are transform faults needed at mid-oceanic ridges?
Because of the shape of the Earth
What is a translation on a plane on a sphere?
A translation on a plane is a rotation on a sphere
What is a spreading ridge?
An extension which is attached by a transform fault because of the shape of the Earth
What do all plates have?
A rotation pole
Why is more heat coming up at Iceland?
Because of thermal abmormalities as a result of mantle plumes, these are additional to convection currents
What does convergence cause?
Compression
What do earthquakes do to the accretion of sediment?
They deform it
Can mid-oceanic ridges be subducted?
Yes
Why do deep earthquakes not cause much damage?
Energy diminshes with distance
Why is deformation of subduction complex?
The angle of subduction is complex- dip angle
Slab geometry
Back-arc deformations
Resistance
Trench motions (rollback, advanced or fixed)
Why is subduction critical?
Because of the issues associated with the hazards
Give an example of a place with back-arc compression
The Andes
What is back arc spreading?
As the plate is subducted and melts, some magma rises which leads to an extension of the crust
What is slab roll-back?
Older and colder oceanic crust is subducted at a steep angle and can roll back into the mantle, taking some of the overlying plate with it
What is a good example of continental collision?
India into Asia
What is the Wilson cycle?
A way of organising plate tectonics
Talk through the Wilson cycle
Starts with the destruction of crust then developing an ocean floor, initiating subduction which causes two continents to collide together
What is the main flaw in plate tectonics?
There is deformation elsewhere
Where does the heat come from?
The convection cells
How are convection cells structured?
In two layers
Which layer of convection cell do subduction slabs reach?
They can reach the top layer or the bottom layer depending on whether they are superficial or not
Where do mantle plumes occur?
From the outer core and the mantle
What do mantle plumes cause on the crust?
Hot spots
What are hot spots important for?
Volcanism
What can the trace of volcanism show?
The motion of the plate
What is the Siberian Trap?
A large region of volcanic rock, it is over a large hot spot
What do we know about the Siberian Trap?
There is large thermal anomaly- not just because of asthenosphere melt because the composition of the deposits is not the same as what is found at mid-oceanic ridges.
What are there traces of in the Siberian Trap?
Material from deeper parts of the Earth’s crust
What is the largest flood basalt on Earth?
Otong Java
Why are igneous provinces useful?
Because of climate impacts, there is a correlation with extension and extinction of species on Earth
Explain what happened at the Deccan Traps
65 Ma the volcanism ended and 75% of the species were gone
Explain what happened at the Siberian Trap
251 Ma volcanism ended and 90% of the species were gone
What is useful about basalt?
Easy to date
Which trap was bigger: Siberian or Deccan?
Siberian was 10x bigger
What does the age of volcanism from hot spots show?
The direction and rate of motion
Are hotspots permanent?
No but they can be active for a very long time
What happens to plate motion and direction?
It changes overtime
When does the Hawaiian chain change direction from the Emperor Seamount chain?
42.4 Ma
What is free air anomaly?
The gravity anomaly of the mass resulting from the volcanism
What can happen at the surface of the mantle plume?`
It can spread at the surface
What can spreading of the mantle plume explain?
Initiation of crust subduction because it weakens the crust
What can the spreading of the mantle plume follow?
Mid-oceanic ridges or faults
Where do large plumes come from?
The lowermost mantle layer
What is the lower most mantle called?
D2nd
Are smaller mantle plumes more long term or short term?
They are more short term
What do the plate tectonics help to cool?
They help to cool the mantle
What do plumes help to cool?
The Earth’s core
