The role of serpentine:
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 – quartz is the ‘Hob Nob’ of minerals at the Earth’s surface. 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.
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.
Here you can compare the clarity of autosub data – the middle digram on the right with the same area in picture 5 above.
Autosub returns from its adventures.
Gael wishes his father ‘Happy Birthday’.
Nigel and Bert.