Lecture 6: How Plates Move Flashcards
How do divergent plate boundaries create new lithosphere?
Plates moving away from each other, creating new lithosphere
What does a divergent plate boundary look like on a map?
A double line with two arrows pointing away from the boundary (the directions the plates are moving)
What is the most common type of divergent boundary?
Mid-ocean ridge
What is a continental rift zone?
A divergent plate boundary on land
What features do continental rift zones typically have?
Thinning lithosphere
Normal faults causing shallow earthquakes
Chains of Volcanoes
What can continental rift zones evolve into?
Mid-Ocean Ridges
What can ocean-ocean convergence plate boundaries form?
Island arcs
Example of an ocean-ocean convergence plate boundary formed Island Arc?
The Aleutian arc in the Northern Pacific
What is an island arc?
A chain of islands that is parallel to a subduction zone
Which plate normally subducts at an ocean-ocean convergence plate boundary?
The older plate subducts (colder and denser)
What does a converging plate boundary look like on a map
A single line with arrows converging on either side. The line has triangles (teeth) on the overwriting plate, pointing towards the side being subducted
What is formed at an ocean-continent convergence?
An active continental margin
Give an example of an active continental margin
The Andrean continental margin
Which plate subducts at an ocean-continent convergence?
the oceanic plate always subducts (continental plate is thicker, with lower density rock)
What is formed when an continental plate subducts an oceanic?
A chain of volcanoes along the coast
What is special about a chain of volcanoes formed by an ocean-continent convergence?
They can all be active (all of the volcanoes are underlain by subduction)
Which plate subducts at a continent-continent convergence boundary?
The continental lithosphere is too thick and buoyant to subduct - so neither
What happens at a continent-continent convergence boundary?
The two plates ‘fold up’ - creating a mountain range
Give an example of a landform formed by a convergent continent-continent boundary?
The Himalayas
What general height and depth do fold mountains reach?
A lot higher than sea level, and delve deeply into the mantle
What do we get beneath a converging continent-continent boundary?
Plutonism and possibly volcanism
Why do we get plutonism (and maybe volcanism) at a converging continent-continent boundary?
Because the crust and lithosphere are so thick here, may be deep enough for pressure and temperature to be sufficiently high to melt some of the crustal rock into granite type magma.
Possible that they might reach the surface as eruptions as well
What plates are conservative boundaries nearly always between?
both oceanic plates, or both continental plates
Can there be a transformative plate between a continental plate and an oceanic?
yes, but only very rarely
Where is the most common type of transformative plate and what does it do?
On the seafloor - offsets two segments of a mid-ocean ridge
What does a transformative plate in a MOR cause?
A step profile with fracture zones on either side, causing a scarp on the seafloor
What does a transform fault between a MOR look like on a map?
Two arrows separating two sets of two lines. The arrows represent a transform fault, the two sets of two lines represent mid-ocean ridges
What do the arrows mean on a transformative plate boundary?
The Mid-ocean ridge is creating a new part of the plate (new crust) and pushing it away in both directions - the arrows are determined by the direction that the mid-ocean ridge is pushing in
What does a transformative plate between a MOR and a trench look like?
Same as a normal one with 2 MORs, but with one replaced by a line with teeth pointing away
What does a transformative plate between two trenches look like?
Same as a normal one with 2 MORs, but with the MORs replaced with lines with teeth pointing inwards
What is created at a continental transform fault system?
A complex band of more-or-less parallel faults (because the continental crust is much thicker and more complex than the oceanic)
Does the plate break cleanly at a continent-continent transformative boundary?
No
What is an example of a continent-continent transformative boundary?
The Californian transformative plate boundary between the Pacific and North American plate
What is the dominant fault on the Californian boundary?
The San Andreas fault
What is a triple junction?
Points on the Earth’s surface where 3 plates meet
How many different possible combinations of triple junctions can we have?
16
What three different parts to a triple junction can we have?
Ridge (R), Trench (T) and Fault (F)
What is the most common triple junction?
Ridge, Ridge, Ridge (RRR)
Can we have a triple junction of three transform faults?
No
What are triple junctions essential for?
Discovering how transformations of plates are changing overtime
Are all continental margins plate boundaries?
No
What happens at a passive continental margin?
continental lithosphere is welded on to oceanic lithosphere, with no relative motion between them
What features does a passive continental margin have?
Have no trench (gradual slope), no volcanoes and low seismicity
What is the safest and most stable plate boundary
A passive continental margin
Give an example of a passive continental margin
The edges of the Atlantic Ocean or offshore Northern Britain
When does a passive continental margin form?
Form when you have new crust being created at mid-ocean ridges, but instead of becoming two continents, become 2 passive continental margins
What are all plate movements equivalent to?
rotations about a point on the surface called the Euler pole/pole of rotation
Do plates have the same Euler point?
No, they are all different
What can we think of plate movement as instead?
Rotations around the Euler point
What reveals the direction of relative plate motion and why?
The transform faults of the plate and another
Because they are just sliding past each other (no convergence or divergence) here, this must be their dominant direction
How can we find the Euler pole of a plate?
The Euler pole is:
At the centre of all the circles traced by transform faults
Where all lines perpendicular to the Transform Fault intersect
How can we find relative speed of a plate?
Using magnetic stripes on the seafloor
Process to calculate relative speed of a plate?
Measure the distance (km) between equivalent magnetic reversals of known ages (Ma) on each side of the MOR
We then use Speed=distance/time
Gives us the full spreading rate of the MOR (how fast a point on one plate is moving away from a point on the other)
We can then divide by 2 to give us the half spreading rate (plate speed relative to the MOR)
Is relative spreading speed constant?
No, varies with distance
At what angle from the Euler point is relative spreading speed greatest?
90 Degrees
What can we use instead of cm/y for spreading speed?
Angular speeds in degrees per million years
How can we use hot spots to find absolute rates of plate motion?
If we then plot the ages of rocks in a particular hot spot chain, vs how far away it is from the hot spot, we get a rough straight line graph.
If we average our data, we can calculate the absolute speed (giving the pacific plate an absolute speed of approx. 10cm/y)
How can we find absolute rates of plate motion now?
GPS is one method we use
Daily measurements can be plotted on a graph overtime to show us a plates movement and direction
Do GPS velocity vectors agree with vectors predicted by geological methods?
GPS velocity vectors agree closely (but not exactly) with vectors predicted by geological methods
