Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 36

This represents good evidence that, as far back as 3 billion years ago,
ocean crust was forming, and was also being destroyed by being sub-
ducted to great depths in the mantle. That is a modern-like Earth in
action.
Diamond inclusions a little more than 3 billion years old, though,
are different. They still contain peridotite, but there are none of
eclogite. It is a tiny and seemingly insignificant change in mineral-
ogy—but the implications are momentous. It suggests that while
the ocean crust was forming on a very ancient Earth, deep subduc-
tion was not taking place. Plate tectonics, therefore, in the sense we
understand it, appears to have started up on Earth just over 3 billion
years ago.
The original explanations for this puzzling change in diamond
composition was that early Earth tectonics could have been carried
out via mantle plumes. 33 These are enormous, slow-moving fountains
of mantle material that move vertically up over hundreds (or perhaps
thousands) of kilometres. Where they reach to just below the Earth's
crust, they stimulate increased magma production and outbursts of
volcanism. Mantle plumes today are a controversial topic, but there is
good evidence for them beneath places like Hawaii (the chain of
islands forming as the ocean crust moves slowly over a stationary
plume top) and Iceland.
Strong plume activity in the early Earth could have produced vol-
canic centres that—perhaps—could have seeded the first continental
cores. There also were likely mid-ocean ridges, formed as a result of
the hot mantle convecting upwards. If that was the case, then there
must have been some kind of downwelling mechanism too, to coun-
terbalance it. That has been envisaged as producing a kind of stack-
ing ('imbrication') of crustal slices above the point where the mantle
currents turn downwards, but without deep subduction of that crus-
tal material. Melting associated with those stacked crust piles would