Re: Why is 70% of Earth's sial missing?
- From: Andrew Nowicki <andrew@xxxxxxxxxx>
- Date: Tue, 03 Apr 2007 00:18:16 +0200
Greg Neill wrote:
Sial is the top layer and would be expected to be the source
of much of the material ejected by the mars-sized impactor
that created the Moon. The 70% of the Earth's surface
covered by ocean is largely areas produced by spreading
zones where magma is welling up.
So what's the problem?
1. When a ball-shaped moon collides with a ball-shaped
Earth, the moon cannot scrape 70% of the Earth's surface.
Even a shovel-shaped moon cannot scrape 70% of the Earth's
surface.
2. The oceanic crust is made of sima, not sial.
_________________________________________________________
John Curtis wrote:
Fresh sima (magnesium-iron silicate) and sial
(aluminum silicate, clay) are produced at midocean volcanoes:
http://www.asu.edu/clas/csss/csss/News/life.html
Unlike sima, sial (aluminum silicate, clay) does not
precipitate at the midocean volcanoe (residual negative
charge prevents clumping), but is propelled toward the
continent by the waves.
Sial is the material that continents are made of. There is
little sial on the ocean floor. You are saying that sial
bedrock is somehow moved by the waves that are hundreds of
meters above the sial bedrock. How is that possible?
On shore it settles out as clay, feldspar or anorthosite
(lunar highlands). When Earth's oceans evaporate, clay
and mud, which line portions of the seafloor, will create
the appearance of sial also dominating Earth's crust.
Feldspar and anorthosite are NOT sedimentary rocks. They
crystallized from magma long time ago, in the Archean and
Proterozoic eons. Clays make up minuscule part of the
Earth's crust, so they are not worth mentioning. Sial is
sometimes called granitic layer of the crust, because
it is made mostly of granite, which is made mostly of
silica (SiO2 content = 72%).
_________________________________________________________
Jeff Root wrote:
Hi Jeff! I was a member of the Minnesota Space Frontier
Society long time ago. Then I wrote the Earth-to-Orbit
Transportation Bibliography; it is posted at:
http://www.islandone.org/LEOBiblio
First, for anyone else reading this, "sima" refers to rock
rich in (si)lica and (ma)gnesium, while "sial" refers to rock
rich in (si)lica and (al)uminum. Moon rock more closely
resembles the lighter sial than the heavier sima.
True. The Moon's crust is very similar to the sial, but
it has more iron than the sial.
I have no idea how Earth's crust was arranged before the
impact which formed the Moon. Maybe it was already
separated into sima and sial, maybe it wasn't. Maybe there
were continents and oceans, maybe there weren't.
The interior of the Earth is very hot now because it is
heated by the radioactive decay. Most of the Earth's crust
is a solid bedrock made from cooled, solidified magma
(igneous rock) rather than something resembling the moon dust.
This fact indicates that the Earth was even hotter in the past
and its surface was liquid. Another good reason for the hot,
liquid surface of the early Earth is that it was made from
smaller parts that collided at high velocity. When a rock from
space falls on the modern Earth, its minimum velocity is the
escape velocity of the Earth (= 11.2 km/s = 6.96 miles per
second). When rocks smash against each other at such velocity
they do not melt; they become plasma.
I believe that the early Earth was so hot that the
lightweight, liquid sial floated on top and solidified as
a layer of uniform thickness. According to Wikipedia:
(http://en.wikipedia.org/wiki/Crust_%28geology%29)
"The Earth has likely always had some form of basaltic oceanic
crust, but there is evidence it has also had continental style
crust for as long as 3.8 to 3.9 billion years."
The "continental style crust" means sial made mostly of
granite. This means that sial is very old.
In any case, the material that got scooped up all came from one
part of the Earth-- not likely the area where the Pacific Ocean
is now, but I can't say it wasn't. The material was heated to
the point that much or most of it vaporized. The gases that
were heavy enough not to be blown away by solar wind were
incorporated throughout the body of the Moon.
True.
It seems pretty likely that the rock closest to the surface
was preferentially scooped up. That would naturally be the
lighter sial.
On the other hand, just because 70% of Earth's surface lacks
continents made of sial is no indicator that any sial is missing.
Any mantle rock which is exposed at the surface cools to
become part of the crust. That is mostly sima, exposed at
mid-ocean ridges where seafloor spreading takes place. The
spreading pushes the lighter sial around, causing the sial to
pile up into continents. Piling up and erosion are in dynamic
balance, so the continents never get very much higher than
they are now, or very much lower.
Good try, but...
The temperature of Earth increases about 36 degrees Fahrenheit
(20 degrees Celsius) for every kilometer (about 0.62 miles) you
go down. Near the center, its thought to be at least 7,000
degrees Fahrenheit (3,870 Celsius). Sial is made mostly of
granite, which is a hard rock made mostly of silica, which is
the main ingredient of glass.
This means that the top 10 kilometers of the sial is made of
hard and rather cool rock. It is too hard and too brittle to
deform under pressure. It is difficult to imagine what could
generate the enormous force needed to break and pile up the
granite into a pile up to 70 km deep. Furthermore, the sial
part of the crust does not look like a messy pile of rocks,
but like a solid, rather uniform layer of bedrock.
.
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