Image in header: Earth’s tectonic plates on map. Source: https://www.geographypods.com/introducing-tectonics.html
Plate tectonics: what do you need?
Plate tectonics, the accretion of new Earth’s crust and the recycling of old crust in the mantle into so-called ‘lithosphere plates’ all over the planet’s surface, is extremely important for our living planet. Little do we know how unique the system of plate tectonics that so characterises our Earth is. We have only known about this phenomenon even on our own Earth for about 60 years. But some typical features of Earth’s crust and mantle do clearly favour enabling plate tectonics.
A thin lithosphere of ideal strength
The lithosphere plates are relatively ‘rigid’ units – see world map above – that accrete along one side and submerge into the mantle along the other. Each plate consists of crustal material at the top (mainly silicate-rich basalt in oceanic crust) and mantle material at the bottom (mainly Mg-Fe-rich peridotite and perovskite). Both layers together form a lithosphere plate. This material must be strong enough not to break under pressure on the one hand, but soft enough to flex when the slab submerges into the mantle (subduction zones) on the other. Moreover, the plates must not be too thick or too thin. Like so many features on Earth, this again is just ideal.
Long-term heat source
Below the lithospheric plates lies a mantle layer that is slightly plastic like hot chcolate (not melted chocolate). This layer is called asthenosphere and is not fundamentally different in composition as the layer above it. Yet this is the only layer that is so plastic and movable. This is due to the combination of temperature and pressure at a depth of several tens of kilometres. The temperature is above 1600 Kelvin (1230°C), which is not enough for real melting of the rock, but is enough to make the layer plastic enough. This allows the movement of the lithosphere plates, albeit enormously slow : order of magnitude several cm per year maximum. Moreover, the plastic nature of the asthenosphere allows heavier lithosphere plates to ‘sink’ very slowly into the mantle (subduction). This is because the oldest parts of the lithosphere plates have become colder, thicker and heavier with time. They sink into the mantle by their weight, helped to a lesser or greater extent by the pressure of an adjacent lithosphere plate pushing against it.
That our Earth still exhibits plenty of these active plate tectonics after 4.5 billion years is not entirely obvious. This is only possible if there is enough internal heat in the planet. We owe this in part to residual heat left over since Earth’s formation: heat from collisions and frictions. Our planet is cooling slowly, and it owes this to its size and composition, which is again … just right. But an even more important source of internal heat (about 80% today) comes from the decay of radioactive material. For that, you need a Solar System with enough heavy elements. So that too is a prerequisite for the movements of the Earth’s crust to this day.
Liquid water
Furthermore, again we see the importance of all the liquid water with which 71% of the Earth’s surface is thanked. The seawater largely determines the smoothness of the lithosphere plates. By the way, an underexposed plate brings extra water into the mantle. As a result, the mantle rock in those places will have a lower melting temperature, and this gives rise to localised fluid magma. We therefore often see a region of active volcanoes near a subduction zone. Further in the course, we will look at the importance of this for the habitability of our planet. This will mainly be about the cycle of carbon over long periods.
Plate tectonics: why do you need it?
What do we all owe to plate tectonics on Earth? Just a quick rundown:
- Long-term habitable climate (carbon cycle)
- Formation of contintents (land life)
- Hydrothermal springs in the deep sea (where life is thought to have originated)
- Mountains and valleys (habitat variation and carbon cycle)
- Permanent changes of habitats (driving force of biological evolution)
The Earth has managed to achieve unimaginable biodiversity after billions of years, with highly complex life forms and ecotopes. This would not have been possible without the above phenomena. And therefore not without plate tectonics either. We will discuss some of them in the following chapters.