Ocean crust: What is beneath the seafloor?

Have you ever thought about what happens beneath the oceans? What rocks below the seafloor might look like? And how we know about them?

Well, almost 70% of the Earth`s surface is covered by oceans. This means that ocean crust makes up the biggest part of the whole Earth’s surface, notably more than the continental crust that we live on! Yet, we know very little about ocean crust. All the information we have so far comes from seafloor geological exploration, scientific ocean drilling expeditions, and from studying ophiolites on the continents. Ophiolites are parts of old oceanic crust that were moved on top of continents by tectonic processes, thus making it easy for us to study ocean rocks. Some can be found in Oman, New Caledonia, Cyprus (where we recently had a short course) or in Italy and Corsica where Valentin and Carlotta spent some time in the field.

Fig 1: A profile through the oceanic crust down to the mantle.

Since we, the authors at SeaRocks Blog, are dealing with processes related to formation or alteration of oceanic crust, I would like to give a brief introduction into what ocean crust is and what kind of rocks it consists of. If you are not so familiar with geology this post includes a general introduction, hopefully helping you to understand further posts. In a couple of months, Carlotta is joining a scientific drilling expedition on the Joides Resolution. That will be the perfect moment to write about scientific drilling and how we learn about the structure of the oceanic crust. Therefore, we need a solid background!

Generally, new oceanic crust is created at mid-ocean ridges (MOR) where hot magma is coming up, partially reaching the seafloor as basaltic flows, and cooling down. Over thousands or even millions of years these newly formed rocks get colder and therefore also denser. Mid-ocean ridges are not just the cradle of oceanic crust, they are also divergent plate boundaries. This means that two oceanic plates are drifting apart once the new crustal material is formed at the ridge level. Generally, the amount of magma that wells up from the mantle increases with spreading velocity. Scientist divide mid-ocean ridges in groups according to their rifting speed: slow spreading ridges (< 40 mm/year), intermediate spreading ridges (40 – 80 mm/y) and fast spreading ridges (> 80 mm/y). In figure 1 you see a profile through oceanic crust accreted at a fast spreading ridge.

Fig 2: A typical pillow lava found in the ophiolite in Cyprus. You can the see the roundish, pillow like shape defined by a dark rim (visible top right of the structure (black arrow). In some parts the dark, glassy rim is altered to more whitish colours (white arrow).
Fig 3: An outcrop in Cyprus consisting of pillow basalts only. Notice that the whitish colours are due to alteration that transforms glass and primary minerals to secondary minerals such as clays due to reaction with water.

In the topmost layer you find basaltic lavas. These lavas often show very characteristic pillow shaped structures (see fig.2 and 3), which is why they are also called pillow lavas. The reason for the pillow shape is that lava flowing out on the ridge has a quite high viscosity (like honey) and therefore it flows rather slowly and quenches in contact with cold seawater. Very often these pillow lavas show a black, glassy rim. If a magma cools down very fast it has no time to crystallize and become a rock but instead a glass will be produced. The rim is generally only a few millimeters thick (see fig.2). The inner part of the pillow cools down during a longer time interval hence leading to a massive rock with crystals that are so fine grained that you cannot spot them with the naked eye. You might also see some larger crystals occurring in this fine grained groundmass. Those are mineral grains which already crystallized before the magma erupted on the seafloor. In hand samples they are visible as either greenish (olivine), dark (pyroxene) or white (a feldspar named plagioclase) minerals.

If you go deeper into the ocean crust you will find the so called sheeted dykes. They are straight, vertical structures that correspond to former conduits in which magma rose up to erupt on the seafloor. Sheeted dykes are also fine grained, greyish and massive basaltic rocks which ideally show two dark rims of a few centimetres, one on each side of the structure (figure 4). Those rims attest again for rapid cooling on the conduit walls and in some cases they can be even glassy like the rims of the pillow lavas.

Fig 4: An example of a sheeted dyke which is now lying almost horizontally due to exhumation and rotation of the Cyprus ophiolite. The dark rims are not so easy to spot therefore the black dotted line as guidance.

The lower part of the oceanic crust is made out of gabbros. Gabbro is a rock which consists of pyroxene (dark in hand rock sample) and plagioclase (white) minerals and it can also contain a small amount of olivine (green) crystals. Those rocks are much coarser grained than the pillow lavas and the sheeted dykes: you can actually spot the single mineral crystals with your naked eye. That is because gabbros cool down slowly, therefore the minerals have more time to crystallize and grow and they get much bigger compared to those in the lavas. In the upper part of the whole sequence gabbro shows a random spatial distribution of feldspars and pyroxenes. But if you go deeper into the crust you start to see dark and fair bands, like in figure 5. The dark bands contain a lot of dark minerals like pyroxenes and olivine and the fair bands are dominated by whitish plagioclase. These bands form when already crystallized minerals in a magma settle along the bottom of magma chambers. Very nice examples of such layered gabbros can be found in the Oman ophiolite.

Fig 5: An example of layered gabbros from the Oman ophiolite. Black layers (black arrow) refer to pyroxene enriched zones whereas white layers (white arrow) are more enriched in plagioclase.

With the gabbros, we have reached the end of the sequence through the oceanic crust. Deeper lying rocks belong to the mantle and are named peridotite. They contain mainly green olivine and dark pyroxene. Very important to note is that mantle rocks are the source of the magma which forms the gabbros as well as the sheeted dykes and the lavas.

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