Continental plates move because they are pushed and pulled by convection currents in the Earth's mantle, which is located just beneath the continental and oceanic plates that make up the Earth's thin crust. When plates collide, one is forced down beneath the other into the mantle creating what geologists call a subduction zone. Because subducting slabs are colder and denser than surrounding mantle material, they tend to sink. Van der Voo and his colleagues selected the Lake Baikal area for their study because it is the site of an ancient, well-documented subduction zone and is located in a part of the world with many earthquakes and an extensive network of seismic monitoring stations. Seismologists study the time and trajectory of sound waves traveling through the Earth from earthquake sites to seismic monitoring stations to learn more about the temperature, density and composition of the mantle layer.
Acoustic energy travels through different types of mantle material at different speeds.
The warmer the material, the longer it takes for sound energy to pass through. Using supercomputers to analyze the travel times and paths of thousands of seismic waves, seismologists can create a three-dimensional image, similar to a CAT scan, of the Earth's interior. Seismic tomographic images in Van der Voo's study clearly show the slab of subducted material descending to the bottom of the mantle layer. Whether its visibility is the result of differences in temperature, composition, pressure or a combination of these remains unclear. The fact that the slab is located almost directly below the site of its original subduction indicates that the Siberian continental plate has moved very little in the last 150 million years.