About a billion years ago, the continents emerged relatively
suddenly from an ocean that covered 95 percent of the Earth's
surface, according to a new theory by Eldridge Moores, a
geologist at the University of California, Davis. The
appearance of large masses of dry land would have caused more
extreme weather, changes in ocean currents and the emergence
of proper seasons. In turn, these environmental changes may
have led to rise in atmospheric oxygen that enabled the
explosion of new life forms around 500 million years ago.

Since the 1960s, geologists have accepted that the continents
sit on tectonic plates floating above a layer of hot, partly
molten rock. Between the continental plates, the crust of the
ocean floor is continually created or pushed under and
destroyed as the plates drift around and bump into each
other. The crust of the modern ocean floor is much thinner
than that below the continents.

In the early Earth, the ocean crust and the continental crust
were much closer together in thickness, Moores said. That
means that the difference between the height of the ocean
floor and the height of the continents was much less. The
oceans would have been much shallower, and the water would
therefore have spread much further across the continents --
covering 90 to 95 percent of the planet's surface, instead of
the present 70 percent.

Many geologists agree with this scenario, Moores said. What
is controversial is how quickly the Earth changed from a
planet covered in water with a few mountainous islands to one
with large continental landmasses. According to Moores'
theory, the continents emerged quite suddenly, over about 200
million years, at the same time that the supercontinent
Rodinia was forming.

Moores' theory, published in the January issue of the
Geological Society of America Bulletin, also implies that
over time, the way plate tectonics works has changed.

The period around 800 million years ago is also important for
the evolution of life. Complex, multicellular plants and
animals, including the precursors of all the major groups of
animals found today, appear at the beginning of the Cambrian
era around 550 million years ago. That suggests that
something, perhaps a drastic change in the environment,
triggered a burst of evolutionary activity.

To reach his conclusions, Moores studies rocks called
ophiolites. Ophiolites are remnants of ocean crust that are
preserved in continents when plates move. Unfortunately, many
of the world's most interesting ophiolites are located in
areas that are remote and inaccessible due to geography or
politics, such as central Africa.

Moores' theories also have implications for studying other
planets, such as Mars, where plate tectonics seems to have
started but then stopped. Venus is thought to have active
volcanoes and wrinkled, broken crust, but does not seem to
have tectonic plates in the same way that the Earth does. The
crust of Venus might be more like the early Earth, with all
the crust about the same thickness. A crucial difference
between the Earth and Venus might be the presence of water on
Earth, which could help to "lubricate" the continental
plates, Moores said.

Additional Story Resources
-- Eldridge Moores, Geology, (530) 752-0352,
moores@geology.ucdavis.edu