Volcanism at the Mid Atlantic Ridge
The Eurasian Plate is moving slowly away from the North American Plate – at about the rate your fingernails grow. This is a fairly slow movement compared to other plates. There is no gap between the plates, however the pulling of the lithospheric plates sometimes forms a rift valley.
The axis of slow-spreading Mid Oceanic Ridges are rift valleys – rather like the East African Rift Valley (EARV), though there are no Ocean Core Complexes at the EARV, there are similar features to Ocean Core Complexes on mainland Western USA in the Basin and Range around Death Valley.
Ocean Core Complexes appear to only form at slow spreading ridges and this asymmetrical spreading makes up about 50% of the North Mid Atlantic Ridge. So these detachment faults (Ocean Core Complexes are a type of detachment fault) are not uncommon but relatively little is known about them.
Like the East African Rift Valley there are faults and volcanism throughout the region. Also like the East African Rift Valley there is pulling apart motion – tension which is stretching lithosphere along these plate margins. The mantle is close to the surface, and the rocks can be very hot. I should mention here that the plate margin is not one single line. You cannot put one foot on the N. American Plate and one on the Eurasian Plate – in Iceland the boundary is @20km across. It is the same here at 13o N – the ridge axis is about 10km across and the actual spreading axis is disputed/difficult to locate.
Unlike at the EARV the faults at the Mid Atlantic Ridge (MAR) cannot be easily seen or studied by scientists. Here, at 13oN the ocean floor is approximately 3000m below the sea surface and the challenge of water pressure and salt on instruments makes researching what is actually happening very difficult.
One of the major purposes of this expedition is to map the area in more detail to get a further understanding as to what is actually happening 3000m below the RRS James Cook as we traverse the area in some perverse scribble.
Why do we get volcanoes at the MAR?
The mantle is not a liquid like we get at the surface. The mantle is a solid but it is ductile, like play dough. There are convection currents within the mantle, their role in driving plate tectonics is disputed with the effects of slab pull and ridge push needing to be considered. Perhaps it better to think of the lithosphere as the top of a convection cell – with the MOR being where rock rises to the surface and the trenches being where rock is driven down. (See Dietz 1961 – for more information.)
At Mid Oceanic Ridges a ductile part of the mantle (the asthenosphere) is drawn up. This rock is very hot @1300oC. At a depth of 100km below the surface this hot rock would remain solid. However, at above 100km depth rock this hot starts to melt (crosses the solidus curve – when things cease to be solids). A lack of pressure would mean that your blood would boil in space – a real problem for people designing space suits -it’s the same concept/physics.
Top diagram shows the geotherm at an old plate, the bottom diagram shows the geotherm at Mid Oceanic Ridges. (The geotherm is the relationship between depth and temperature in the Earth.)
The volcanism at slow spreading ridges such as the MAR
Small (1oom diameter) cone or dome like hummocks are common and are thought to come from fissure eruptions (tears in the crust). Often these hummocks will join together to make ridges of hummocks or larger more complex hummock volcanoes.
Smoother shield and flat-topped volcanoes also occur,
Below: Flat-topped volcano with a traditional presentation
Below:The same flat-topped volcano at 13oN illuminated by colour. The colour shows the direction of each slope. Black means the bathymetry is flat.
Along the ridge axis (often the best way of defining the spreading axis) Axial Volcanic Ridges (AVRs) occur. Many volcanoes join together in line that can be tens of kilometres long and a few kilometres wide. There is usually one AVR for each spreading segment. Often the AVR is surrounded by smooth lava flows and flat topped volcanoes but scientists do not really understand how they are related.
Pillow lavas are very common. As lava meets the cold seawater the outside edge of the lava cools very rapidly. The steeper the slope the more elongated the pillow.
Below: Pillow lavas in a haystack formation (slightly further North along the MAR).
There are no volcanoes on the domes of Ocean Core Complexes.
Ongoing research is being carried out to try to understand volcanism at the MAR.
Trifle for 50+
The anchor chains – also HUGE!!