  A balanced ratio of ocean to land is believed to be essential for an
Earth-like biosphere and one may conjecture that plate-tectonics planets should
be similar in geological properties. After all, the volume of continental crust
evolves towards an equilibrium between production and erosion. If the interior
thermal states of Earth-sized exoplanets are similar to the Earth's, one might
expect a similar equilibrium between continental production and erosion to
establish and, hence, a similar land fraction. We will show that this
conjecture is not likely to be true. Positive feedback associated with the
coupled mantle water - continental crust cycle may rather lead to a manifold of
three possible planets, depending on their early history: a land planet, an
ocean planet and a balanced Earth-like planet. In addition, thermal blanketing
of the interior by the continents enhances the sensitivity of continental
growth to its history and, eventually, to initial conditions. Much of the
blanketing effect is however compensated by mantle depletion in radioactive
elements. A model of the long-term carbonate-silicate cycle shows the land and
the ocean planet to differ by about 5 K in average surface temperature. A
larger continental surface fraction results both in higher weathering rates and
enhanced outgassing, partly compensating each other. Still, the land planet is
expected to have a substantially dryer, colder and harsher climate possibly
with extended cold deserts in comparison with the ocean planet and with the
present-day Earth. Using a model of balancing water availability and nutrients
from continental crust weathering, we find the bioproductivity and the biomass
of both the land and ocean planet to be reduced by a third to half of Earth's.
The biosphere on these planets might not be substantial enough to produce a
supply of free oxygen.
