How do continents move?

280 million years ago, the surface of the Earth looked different than it does today. Instead of the 6 continents we have today, there was only one, undivided land. This supercontinent existed for nearly a 100 million years and its break-up resulted in today’s look of the Earth. The question is what process initiated and keeps up the movement of the continents. In order to answer this, question it is essential to get to know the interior of the Earth.

The interior structure of the Earth was gradually discovered in the 20th century by the intense study of travel times of seismic waves caused by earthquakes. Two main seismic waves propagate inside the Earth: longitudinal P-wave and transversal S-wave. Based on the travel times and surface distribution of seismic waves four parts can be distinguished in the Earth: the outer, thin part is the crust. There are two types of crust: oceanic and continental, they differ from each other in thickness and composition. Oceanic crust is created at mid-oceanic ridges, where the upwelling mantle originated magma reaches the surface. Subduction of oceanic plate can occur at plate boundaries if the plate is old, in consequence, colder and it has a higher density. Subduction can happen at oceanic-continental and oceanic-oceanic plate boundary as well. The Andes and Japan was created by this process. If the whole oceanic plate is subducted, then continent-continent collision can happen. This process created the Alpine-Himalayan chain, which marks the place of the suture zone between plates. However continents can break up as well, which is currently going on at the East-African rift system. The break-up and assembly of continents is called Wilson-cycle. This quasi-periodic process repeats itself in every 4-500 million years and creates a supercontinent like Pangea.

The interior structure of the Earth

Further inside the Earth is the mantle. It is characterised by viscoelastic behaviour, meaning that mantle material reacts elastically if affected instantaneously. Elastic wave propagation can be considered an instantaneous process, by studying seismic wave travel time results in the conclusion that the Earth’s mantle is solid. However, the mantle behaves viscously if studied on more than a hundred years scale. The movement of the plates are controlled by three forces: ridge push, trench suction, basal traction. Ridge push occurs Due to the relatively higher elevation of the mid-oceanic ridges, the plates get gravitationally instable and slide down.

The Pangea supercontinent with today's

national borders

Trench suction occurs at oceanic trenches, where the subducting plate part drags the rest of the plate with itself. Basal traction only accelerates plate motion if thermal convection exists, the direction of convection is the same as plate motion and the velocity of mantle flows is higher than plate motion. The velocity of thermal convection and plate motion is of the same order with cm/year motion. The fastest moving plate is the Nasca plate with 6-7 cm/year velocity. Calculations Trench suction is the dominant force in controlling plate motion.

Mantle flows are caused by thermal convection in the mantle. If we heat water in a cooking pot, thermal convection will occur. At the base of the pot the volume of the water will increase, its density decrease therefore it will rise up. At the top of the pot, the water will cool down, sink down and the cycle will start again. In the case of mantle convection heat is provided by the outer core. The ascending part of thermal convection are mantle plumes, which reaching the top of the mantle spread in all directions and its horizontal component may help plate motion. The descending part of mantle convection are subducted plates, with the final destination being the core-mantle boundary. The plumes starting point is the same as the end point of subducted slabs, so there is a full cycle.

Theoretical drawing of mantle convection. The ascending part of thermal convection are the mantle plumes, while the descending parts are the subducted oceanic plates

[endif]-- The plate motion on the top of the Earth is strongly connected with mantle flows and the interior of the Earth. The dominant force in plate motion is trench suction, which results in the subduction of slabs. Continents assemble to form a supercontinent from time-to-time and the next such occasion will be in 200-250 million years’ time. ![endif]--![endif]--![endif]--