a snowball earth would destroy all metazoans as ecosystems and habitats froze over, although microbial life could easily adapt and survive to those conditions and be available to evolve in future warm epochs. So even multiple ice-earth phases would allow life to survive, but only in its simplest, unicellular form.

The arising of complex life forms (the Cambrian Explosion) may not have been a particularly rare event in earth's history, maybe that was just the last time it happened, since we haven't had another snowball earth since. And since no fossil evidence of any previous metazoan ages survives, could it be there weren't any?

Or, then again, maybe pre-Cambrian metazoan fossils have all been subducted into the mantle, the evidence destroyed. Maybe they have evolved multiple times in earth's history and we have no way of knowing it!

Either way, I guess there still may be a few surprises left to be uncovered and explained in geology. Any guesses or speculations? Tell me, as a geologist yourself, what big puzzles remain to be resolved in your discipline?

Astronomy has Dark Matter and Energy, and galactic evolution has never been fully worked out. We still don't know if life or intelligence is present in the cosmos (other than earth). Biology had its last big breakthrough in 1959, with DNA. Physics had its last big paradigm changes with Relativity and Quantum Theory, about a century ago. The latter led to the last big revolution in Chemistry, the nature of the chemical bond.

Is there anything in geology comparable to say, plate tectonics or the dinosaur-killer asteroid, that is still not fully understood, or waiting to be proven or discovered? Remember, both were 20th century discoveries. What kind of major revolutions can we expect in geology in the next century or so? What major problems can we expect to solve?