The Oceanic Crust of the Earth
The deep ocean water column is stratified, with cool (high-density) water masses generally ‘hugging’ the deepest portions of the oceans. The oceans cover about 72% of our planet’s surface, and have a mean depth of ca. 3,600 m. The deepest portions of the oceans are floored by ‘Oceanic Crust’ (OC), composed mainly of the high-density rock, basalt. Thus, the OC covers about 60% of the Earth’s surface.
In contrast to the Continental Crust, which consists of low-density, granitic rock, and reaches thicknesses of up to 200 km, the OC, is much thinner, with an average thickness of only 8 km. Because the upper mantle is partly molten, it has a temperature of about 1,100 – 1,200 °C, and because the overlying OC is so thin, it lets some of the heat from the interior of the Earth leak through, to the ocean floor, where the heat flux interacts with the ocean water column. Over 35 years of scientific ocean drilling (e.g., ODP ‘Ocean Drilling Program’), has shown that the OC is highly mobile and has a very complex structure, depending on where it is located relative to spreading centers and subduction zones. ODP-results have shown that the OC is surprisingly porous, with up to 25% regional porosity, which allows seawater to circulate in and out, and even lets water interact with the upper mantle.
One of the first documentations of high-temperature deep-water anomalies was made during the British ‘Discovery’-expedition to the Red Sea, in 1964. One oceanographic station was located in the middle of the 1,800 km long Red Sea, at a water depth of just over 2,000 m, the ‘Discovery Deep’. When the water sampler reached the decks of Discovery, the scientists were astonished to find hot, high-salinity water of 44 °C, and a pH-value of only 5.2!
What was heating the seawater, here? Why was the seawater so salty (158 ‰, whereas normal seawater has a salinity of 30 ‰)? Could there be heating of the seawater by volcanic eruption, or was seawater circulating into and out of the local crust? It took earth scientists another 30 years, before they finally witnessed how seawater is sucked into the OC and vented out of it after being heated by a magma chamber located near the seafloor surface.
In 1977, the deep-diving submarine ‘Alvin’ dove over the ‘East Pacific Rise’ (EPR), which was known to be a ‘spreading ridge’, where two OC-plates were rifting-apart, and partly exposing a magma chamber. Inside the Alvin were J.B. Corliss, and J.M. Edmond. For the very first time, the immense force of a hydrothermal system, a so-called ‘black smoker’ deep hot vent was visually documented. It billows water blackened by heavy loads of different minerals, some of which are metals. Inside the chimney structure from which the scolding hot water emits, there is supercritical water, a phase of water which is neither gas (vapor), nor a liquid, but something in between. It has a density of 0.3 and a temperature close to 400 °C.
Today, about 50 years after the Discovery-expedition, we have documented about 300 of the estimated 11,000, or so deep-ocean hot vents of the Earth…In addition, we know that there are also other warm and hot vents, which are not ‘black smokers’. Perhaps the most important geo-process on our planet is the so-called serpentinization process, whereby seawater interacts directly with hot (ultramafic) rocks of the upper mantle.
The rocks of the upper mantle consist of magnesium silicates, called ‘peridotite’ and ‘pyroxenites’ (Holm et al., 2015), which contain olivine, (Mg,Fe)2SiO4. In 2001, the ‘Lost City’ vents were discovered, - again, with ‘Alvin’, diving near the Mid Atlantic Ridge, south of the Azores. This time, no black smokers were seen, but up to 60 m high spires of seeping, white carbonate. The temperature of the emitting water was 90 °C, and the pH-value of the water was up to 10 (highly alkaline)! The process producing these warm, highly alkaline fluids turned out to be serpentinization, a reaction between seawater and the mantle magnesium silicates. The reason why this is such an important process, is that it produces large amounts of free hydrogen (H2), which combines with CO and CO2 to produce enormous volumes of methane (CH4) and other hydrocarbons (Holm et al., 2015).
In addition to the thousands of hot and warm vents in the deep ocean, there are also cooler venting systems, associated with the deep sedimentary basins of the world, like those found in river deltas, and collision zones, ‘accretionary prisms’. In these locations, there is active natural production of light and heavy hydrocarbons and seeps of brines and petroleum. The study of these ‘cold vents’ started about 40 years ago, and is still taking place. Judging from the rate of discovery so far, there must be hundreds of thousands of such seeps. All of them interact with the seawater, both chemically and thermally, and therefore also perturbing the local near-seafloor pH-value (Hovland et al., 2012).
We are just about to embark on understanding the interactions between the lower part of the ocean water column and the seafloor, including the Ocean Crust (OC). We know that such interaction is much more dynamic than previously thought, and we have to find out how these processes feed into the rest of the ocean, including its surface waters, and the general and global marine environment.
- Holm, N.G., Oze, C., Mousis, O., Waite, J.H., Guilbert-Leoutre, A., 2015. Serpentinization and the formation of H2 and CH4 on celestial bodies (planets, moons, comets). Astrobiology 15 (7), Doi:10.1089/ast.2014.1188
- Hovland, M., Jensen, S., Fichler, C., 2012. Methane and minor oil macro-seep systems — Their complexity and environmental significance. Marine Geology 332-334, 163-173. Doi:10.1016/j.margeo.2012.02.014