From the 1995 lecture programme:
Volcanoes are among the most dramatic features of the Earth, and a sure sign that the planet is alive. But what we see on land is only a small part of the story; far more is going on out of sight underwater.
In the middle of the oceans about twenty cubic kilometres of liquid rock is added to the sea floor every year to make new plates. This activity poses a major paradox. We know that the outer part of the Earth is solid, because it rings like a bell in earthquakes. How, then, is it able to produce such quantities of molten rock?
Most volcanoes are located near the boundaries between the plates that cover the Earth, and the type of boundary provides a clue to what is going on. With submersible ships we can travel to where plates are created at mid-ocean ridges, where we find a sea floor belching clouds of superheated black water from chimneys — yet this dark, apparently inhospitable world is alive with creatures that never see the sun.
The runny magma that pours from the Earth where plates separate can flow huge distances on land and make spectacular displays of fountains and bubbling cauldrons of liquid rock. By contrast, the magma produced where plates converge is often much stickier and hardly flows at all; instead it erupts in catastrophic explosions. Why should these magmas be so different?
Some volcanoes occur away from plate boundaries, at 'hot spots' that occur above rising currents in the hot creeping rock that circulates beneath the plates. They are a useful reminder that the plates at the surface are only part of the story, and that much is hidden below. Occasionally a rising current below coincides with plate separation at the surface. The Earth can then produce fantastic quantities of lava, enough to bury whole continents to depths of a kilometre or two, with devastating effects for the environment and climate.