For students studying A level geology. Please do look at other articles within the blog to further your knowledge and understanding.
· Lithospheric plates are made up of crust and the upper mantle. There are no gaps between plates. There are two types of plate – oceanic and continental.
· Oceanic crust is more dense than continental crust. The denser oceanic lithospheric plate sinks beneath the continental lithospheric plate at subduction zones. For example the Nazca Plate descends (subducts) beneath the South American Plate near Chile.
· Oceanic crust is made largely of basalt, dolerite and gabbro rocks. These rocks are usually found in distinct layers with basalt at the surface and then dolerite followed by gabbro underneath the basalt.
· The crystal sizes of basalt are very small as the rock cools very quickly, the dolerite has slightly larger crystals than basalt as it cools less quickly and gabbro at the bottom cools even slower so has still larger crystals.
· Basalt, dolerite and gabbro all contain crystals of augite, olivine and plagioclase feldspar.
· Beneath the gabbro there is a rock called peridotite. This is largely green in colour and a mantle rock. When peridotite reacts with water it can form serpentinite.
· Serpentinite is a very soft rock and can lubricate faults. This may be the cause of lubrication for the detachment fault we are looking at.
· The asthenosphere is a partially molten layer (1 -5% liquid) within the otherwise solid mantle.
· The solid mantle is not static but geologists do not generally believe that mantle convection provides the force to drive plate movement.
· What drives the plates is disputed.
· Many scientists think that the old cold dense oceanic crust descending beneath continental crust pulls the slab of crust causing movement at the ridges.
· Other scientists think that the young, hot, less dense crust at Mid Oceanic Ridges pushes the plates outwards.
There is much research into how and why plates move and scientists are learning all the time as more data is collected.
For more info please see: http://www.geolsoc.org.uk – How do plates work?
Symmetrical and Asymmetrical Spreading and Ocean Core Complexes
Symmetrical and Asymmetrical Spreading
About 10 years ago, scientists discovered that there is a new mode of seafloor spreading operating in some 50% of the Mid-Atlantic Ridge. Whereas the ‘classical’ mode of spreading is symmetric (see diagram), this new mode is highly asymmetric.
In the symmetric mode, as the plates separate, ductile asthenosphere wells up to fill the gap, and partially melts (about 10%). The melt rises and solidifies to form the gabbro plutons, doleritic dykes and basaltic lavas of the crust. The melt may accumulate in one or more small mid-crustal magma chambers on the way up. Once the crust has formed, the stresses pulling the plates apart produce normal faults, with displacements of ~100m, which add some 5-10% of strain. The structures of both plates are very similar, as are their accretion or spreading rates; new material is added equally to both sides.
The newly discovered asymmetric mode is quite different. Such faults may penetrate right through the lithosphere (detachment fault), and accumulate displacements of tens of kilometres over millions of years (see diagram). The hanging wall plate (right-hand one in the diagram) receives less melt than normal and so is thinner. The footwall plate (on the left) is formed by pulling mantle material up. If all of the plate separation is taken up by slip on the detachment fault, then no new material will be added to the hanging wall plate, and spreading will be 100% asymmetric. The resulting seafloor landform of an Ocean Core Complex (OCC) is formed. The one studied by the JC132 at 13oN in the Atlantic Ocean is about the size of Ben Nevis.
(Diagrams created for the Teacher At Sea by Professor Roger Searle – the diagrams have extreme vertical exaggeration)
OCEAN CORE COMPLEXES:
Scientists have several ideas about what may be causing Ocean Core Complexes. They are hoping with the research gained on the JC132 expedition to have a better understanding of the formation of these structures. They are large domed ‘hills’ on the sea floor about the size of Ben Nevis with striations (grooves) clearly visible from top to bottom. Most scientists agree that these grooved surfaces are faults, where lots of small amounts of motion over time has built up the ocean core complex “hills” we see today. Scientists call the faults here “detachment faults”.
This can be seen below:
Below: Is a picture of the Ocean Core Complex we are studying taken on a previous research expedition. (Thank- you to Prof Tim Reston)
The OCC looks very different to the surrounding rock. It is a smooth dome with distinct scratches.
Superimposed upon the picture is a fault scarp from the Andes – similar in scale. You can see the flat valley floor and the detachment fault rising from beneath the surface. The scratches (striations) are on the footwall which is the part that rises. The valley is part of the hanging wall.
(Original image of the 1320 OCC is from Chris MacLeod. The original source of the Andes photo -Glenn Wallace UCSC. Text added by Professor Tim Reston – Birmingham University)
The ship has echo sounders on the bottom of the hull. Generally the images have a resolution of 100m but because we going over the same tracks as we do our research the picture builds up.
Picture 1 – the detachment fault and locations of the following pictures.
Picture 5 – Zoomed in but not as clear data as from autosub. The volcanoes are not clearly depicted.
Below you can compare the clarity of autosub data – the middle diagram on the right with the same area in picture 5 above. The circular features are volcanoes.
Deserts are full of sand, desert sand is mostly quartz. Quartz is very stable at the Earth’s surface, as a result it resistant to weathering and erosion, resistant to being altered. I use the Peter Kay example in class. He talks about hob nobs being the ‘marine’ of biscuits with an ability to be dunked in tea and yet remaining intact. Olivine is a mantle mineral. It is stable in the mantle but at the Earth’s surface is like the ‘Rich Tea’ biscuit. Its ability to be dunked is somewhat limited. Olivine at the Earth’s surface is easily altered, weathered, eroded, reacts to change its composition.
Peridotite is a mantle rock that mainly consists of olivine. The crystals are bright green and usually large (5mm in size) clearly visible.
· At the Mid Atlantic Ridge (MAR) the top layers of basalt (which also contain olivine) have many cracks, fissures and faults.
· Sea water sinks into these cracks and is warmed by the heat of the mantle below (the crust is very thin at the MAR). As the seawater is warmed it starts to rise again to the surface.
· This warmed seawater reacts with the olivine and forms mineral serpentine which makes up the rock of serpentinite.
· Serpentine (asbestos is a form of serpentine) has a soap-like and weak texture, this texture allows the rock to slip. Serpentine feels very smooth and slippery.
· Serpentine has the effect of lubricating the faults. Shallow detachment faults are likely areas for the mineral serpentine to form. The lubricating nature of the mineral probably exacerbates the rate of slippage of the fault. Thus, deeper peridotite is brought nearer to the ocean surface. The peridotite reacts with the warm seawater in the fault, more serpentine is formed and the slippage continues bringing more peridotite to the surface…. so the cycle continues
Serpentine could be very important in the development of Ocean Core Complexes – detachment faults. Serpentine also likely to be very important in subduction zones.
· As serpentine is formed as reaction with olivine and water so it is known as a hydrous mineral.
· The oceanic lithosphere spreads out from the mid oceanic ridge it becoming older, colder, thicker (from sediment). The oldest sea floor is around 250 million years old – think the oldest dinosaurs and you won’t be too far out. Very little of our sea floor pre dates the dinosaurs.
· This old, cold, dense, thick oceanic lithosphere starts to descend beneath the less dense continental lithosphere at subducting plate boundaries.
· The oceanic crust still contains the hydrous minerals such as serpentine.
· When the oceanic crust descends into the Earth to about 100km deep the water from the hydrous minerals is released – due to heat and pressure of the environment.
· The water released starts to rise and causes the overlying mantle to melt, magma is formed. Minerals from the surrounding continental crust can also be subsumed into the melt.
· The chemistry of the magma can be quite complex as it is affected by cooling, heating, sinking and the formation of crystals; this affects the percentage of silica within the magma.
· The resulting magma is andesitic in nature. This is the type of magma found in volcanoes in the Andes and other similar chains of volcanoes ‘behind’ subduction zones such as Indonesia and Japan. The higher silica magma is more viscous. Gases are then trapped and the only way to escape is through a violent explosive eruptions.
Magma at oceanic ridges and in areas like Hawaii are formed of oceanic basalt. This magma is lower in silica content and the resulting eruptions are mainly of runny lava. Gas can escape easily. Violent eruptions are rare.