Inside planet Earth is another planet. Not quite the way Jules Verne or Edgar Rice Burroughs imagined, but …
We live at the boundary between Earth’s solid surface, its liquid water surface, and its atmosphere. The sun pours energy into this boundary, and drives the circulation of water and air. The altitude of the surface varies. This rules our lives. We live at the mercy of weather, worry about climate change and rising sea levels, and (if we are Tibetans or Andean Indians) have genes adapting us to life at high altitude.
There is an equally dramatic landscape 1800 miles deep in the Earth, at the boundary between the silicate mantle and the iron core. The core mantle boundary is as big a transition as the Earth atmosphere boundary. It is not smooth or featureless: We know from measuring the speeds of seismic waves that there is a complex uneven landscape at the boundary. There is a kind of slow weather in the mantle, driven by heat from the core. The core mantle boundary has cool uplands where material falling slowly through the viscous mantle from above has accumulated. It has hot lowlands, from which hotter, less viscous plumes of magma rise, to generate hotspots above. (Two of the major plumes nowadays are under the central Pacific and Africa.) And there may be lakes and seas of liquid minerals at the boundary.
Long-term geological processes – the movement of tectonic plates, the formation and breakup of continents – are driven by this slow weather in the mantle. One theory is that things changed around the end of the Archaean and beginning of the Proterozoic — that the “weather” inside the Earth grew calmer, the lower and upper mantle grew more separate, and the modern pattern of supercontintent formation and breakup began. If this is true, then changes we see at the surface of the Earth, like the rise of an oxygen atmosphere, may owe something to changes where mantle meets core.