2.4 The Spreading Crust Flashcards
What are divergent boundaries also known as? WHY?
Divergent Boundaries
- As such, these spreading ridges are sometimes called constructive plate boundaries: responsible for the generation of new oceanic lithosphere from the partial melting of mantle rocks.
What can divergent boundaries also produce?
Divergent Boundaries
- The ridges form a mountain chain that runs around the planet but is mostly completely submerged.
Many textbooks imply that convection cells are directly associated with divergent boundaries - 4 OPPOSITIONS (OVERVIEW)
- Irregular distribution of convection cells
- Some continents are surrounded by spreading ridges
- Ridges can be subducted
- In addition, when modelled on computers, large total mantle convection cells should be stationary over long periods of time. This is unlike spreading ridges, most of which migrate over the planet’s surface.
Many textbooks imply that convection cells are directly associated with divergent boundaries - REASON 1
Irregular distribution of convection cells
* The regular distribution and width of the convection cells would mean that the spreading reading ridges must be linked, at a regular distance, to an associated subduction zone (representing the downward limb of a convection cell)
* This is not the situation we find - ex: new ocean crust is formed at the East Pacific rise and is eventually subducted in the western Pacific, 1000’s km away
Many textbooks imply that convection cells are directly associated with divergent boundaries - REASON 2
Some continents are surrounded by spreading ridges
* Africa and Antarctica are mostly surrounded by spreading ridges with no associated subduction zone that could represent the downward limb of a convection cell
Many textbooks imply that convection cells are directly associated with divergent boundaries - REASON 3
The East Pacific Rise spreading ridge EXAMPLE
Ridges can literally be subducted as a whole, like under a continent
- The East Pacific Rise spreading ridge used to run all along the western coast of North America, with the Pacific Plate to the west and the Farallon Plate to the East
- Between 30 and 20 million years ago, parts of the East Pacific Rise started to be subducted below North America, splitting the Farallon Plate into what we know today as the Juan De Fuca and Cocos Plates (one on either side - look at pictures if this is confusing)
Many textbooks imply that convection cells are directly associated with divergent boundaries - REASON 4
D: In addition, when modelled on computers, large total mantle convection cells should be stationary over long periods of time. This is unlike spreading ridges, most of which migrate over the planet’s surface (making them not directly correlated)
So, if not via the deep convective flow of hot material from the mantle, how is new material added at spreading ridges?
3 EXPLANATIONS
- Passive upwelling
- Decompression melting and partial melting
- The Fractured Surface
So, if not via the deep convective flow of hot material from the mantle, how is new material added at spreading ridges?
Passive Upwelling
- Spreading ridges are thought to generate a very localized flow in the mantle as the plates move apart under the influence of slab pull/ridge rush
- As the plates separate, a gap opens in the mantle, causing mantle material to rise and fill the gap, called passive upwelling.
So, if not via the deep convective flow of hot material from the mantle, how is new material added at spreading ridges?
Decompression melting and partial melting
- As mantle material moves upwards (passive upwelling), the pressure on the rocks decreases, but temperatures are still high.
- Although the mantle is extremely hot, it is not molten due to the high pressures mantle materials are under.
- However, when the pressure drops due to the passive upwelling at spreading ridges, it allows some melting: decompression melting
- Mantle, called peridotites, are ULTRA -MAFIC in composition due to having iron-rich minerals and very low silica content
- The minerals that makeup peridotites have different melting points.
- Hence, as the mantle rises and the pressure drops, some, but not all, of the minerals melt, forming a magma that is basaltic MAFIC in composition.
The solid material left behind a partially melted mantle is a…
“depleted mantle” called Harzburgite
What happens when a passive margin causes a gap in the mantle?
- The zone of partial melting near the ridge crest forms a triangular shape, spanning approximately 60 km in thickness
- The proportion of magma within the rock volume is roughly 10%. Consequently, when the melt emerges from its original rock and rises, it creates a crust measuring around 6 km thick.
What happens to the cooling magma caused by partial melting?
- The magma produced by the partial melting is hot and rises, forming a magma chamber below the ridge crest
- Some magma cools slowly to form the plutonic form of basalt, gabbro.
What happens to the magma that doesn’t cool (caused by partial melting)?
- The magma that doesn’t cool continues upward in thin vertical sheets called dikes (sometimes spelled dykes), eventually erupting on the ocean floor, forming pillow lavas: lava comes into contact with cold water and rapidly cools, causing the outer surface to solidify almost instantly, forming a glassy crust. Lava continues to flow into this area, inflating the structure into a bulbous pillow shape until it cracks, forming a new pillow. As new pillows form and inflate, they stack on top of the previously formed ones (like a ball pit)
This overall sequence of rocks matches the succession of rocks we find in…
Ophiolite complexes
The Fractured Surface - what can this turn into?
Decompression melting and partial melting
- The area around divergent ridges is under extensional tension, causing the crust to fracture, allowing blocks to subside relative to their neighbours.
- A fracture where movement occurs is called a geological fault. In extensional regimes such as this, the faults that form are called normal faults
- You can recognize a normal fault as the rock above the fault line (called the hanging wall) has moved down relative to the rocks below the fault line (called the footwall).
- These faults are relatively shallow and, as they move, generate shallow focus earthquakes
Where can we study divergent boundaries?
Iceland
Why is iceland visible on land?
- Pangaea began to break apart about 200 million years ago during the late Triassic, but by 143 million years ago,** the Mid-Atlantic Ridge was actively spreading**, creating a young and narrow Atlantic Ocean
- By 56 million years ago, the Atlantic had continued to widen, and a mantle plume had developed below Greenland, causing the eruption of vast amounts of basalt lava to erupt on the eastern side of the continent. Such a large outpouring is called a flood-basalt event
- The plume remained fixed in location while the Atlantic continued to widen.
- By 35 million years ago, the plume had intersected with the Mid-Atlantic ridge and became fixed at that location
- It is thought that the mantle plume added (and continues to add) additional magma to that region of the divergent boundary.
- This, and the associated increase in the volume of volcanic materials, allowed Iceland to become emergent above the surface of the Atlantic between 16-18 million years ago.
To be classified as a transform boundary, the crust…
Transform Boundaries
must move in opposite directions on either side of the fracture
