It appears that we do not need to
subduct continental material in order to rework it so that it may emerge as continental
rock. Instead the lighter rocks held in
the molten mass differentiate upward against the denser material allowing
surface concentration. As stated, it is
able to ooze upward and I am sure heavier material simply gets eroded out of
the way as the recycling continues and the lighter continental rocks at worst
are accumulated and left behind.
The truly interesting problem is
the origination of water. The crust is
presently saturated with water as deep as we can go. A lot of speculation is possible, but either
Earth was formed with enough water in place that then oozed out as the crust formed
and densified or it arrived from cosmic sources. I am most comfortable with a cosmic
explanation except the general lack of confirmation is troubling. Even one fire ball bringing torrential rainfall
anytime in human history would satisfy me.
The reality is zip direct
evidence. We also have Venus, obviously
missing water. Of course, I have
conjectured that Venus is a recent event, so that gets me off the hook. It does not let anyone else of the hook
unless they wish to agree with me.
I also do not like the dryness of
Mars although there are good arguments in favor of water loss into space that
need to be properly confirmed.
What I really do not want to
accept is that Earth simply got really lucky and captured a rare water rich
comet for which we have no evidence.
A new theory on the formation of the oldest continents
by Staff Writers
The earth's structure can be compared to an orange: its crust is the
peel supported by the earth's heavy mantle. That peel is made up of a
continental crust 30 to 40 kilometers thick. It is much lighter than the
thinner oceaniccrust and
protrudes from the earth's mantle because of its lower density, like an iceberg
in the sea.
"According to the current theory, the first continental crusts
were formed when tectonic plates would collide, submerging oceanic crusts into
the earth's mantle, where they would partially melt at a depth of approximately
100 kilometers. That molten rock then ascended to the earth's surface and
formed the first continents," says adjunct professor Dr. Thorsten Nagel of
the Steinmann Institute of Geosciences at the University of Bonn Greenland .
Following trace elements
The composition of the continental crust corresponds to a semiliquid version of the oceanic crust melted by 10 to 30 percent of its original state. Unfortunately, the concentrations of the main chemical components in the re-solidified rock do not provide much information about what depth the fusion occurred at.
"In order to find that out, you have to know what minerals the
remaining 70 to 90 percent of the oceanic crust consisted of," explains
Prof. Dr. Carsten Munker of the Institute of Geology and Mineralogy at the
University of Cologne. Researchers from Bonn and
Cologne
"Trace elements provide geologists with a window to the origin of
continental crust," says Prof. Munker. "With their help, we can
identify minerals in the residual rock that were deposited in the depths by the
molten rock."
Before the magma separated from the bedrock, the semifluid rock and the
leftover solid minerals actively exchanged trace elements.
"Different minerals have characteristic ways of separating when
trace elements are smelted. In other words, the concentration of trace elements
in the molten rock provide a fingerprint of the residual bedrock,"
explains Dr. Elis Hoffmann from Bonn
The oceanic crust did not have to descend
Using computers, the scientists simulated the composition of bedrock and molten rock that would emerge from partially melting the oceanic crust at various depths and temperatures.
They then compared the data calculated for the molten rock with the
actual concentration of trace elements in the oldest continental rocks.
"Our results paint a surprising picture," Dr. Nagel reports.
"The oceanic crust did not have to descend to a depth of 100 kilometers to
create the molten rock that makes up the rocks of the first continents."
According to the calculations, a depth of 30 to 40 kilometers is much more probable.
The primeval oceanic crust could have 'oozed' continents
...it could definitely have had the power to do so in the Archean eon.
Four billion years ago, the gradually cooling earth was
still significantly warmer than it is today. The oceanic crust could have
simply 'oozed' continents at the same time that other geological processes were
occurring, like volcanism, orogeny, and the influx of water.
"We think it is unlikely that the contents were formed into
subduction zones. Whether or not tectonic plates
of the primordial earth had such zones of subsidence is still a matter of
debate," says the geologist from Bonn
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