The Earth's crust is made of two layers called sima
and sial. Sima is the lower and denser layer. It is
made mostly of basalt. Sima is 5 to 10 km thick and
covers the entire surface of the Earth. Sial is 20-70
km thick and covers only 30% of the Earth's surface;
this is the elevated part of the Earth's surface called
continents. The remaining 70% of the Earth's surface
is covered with oceans. 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%).

None of the existing theories explain how the sial
was selectively scooped up from 70% of the Earth's
surface and deposited on the Moon. A glancing collision
with a ball-shaped moon could not have scraped off sial
from 70% of the Earth's surface. Even a shovel-shaped
moon could not have scraped off so much sial.

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). This means that the sial is made of
hard and rather cool rock.

If a geological process removed the sial from 70% of
the Earth's surface, it must have taken place when
the sial was still hot and liquid because sial is not
a pile of rocks, but a solid, rather uniform layer
of bedrock. This hypothetical process had to lift
liquid sial up to 70 km against the force of gravity.
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It is very difficult to imagine any geological or
astronomical event that could selectively scoop up
sial from 70% of the Earth's surface. I believe that
the most probable event was a sequence of three
collisions:

1. HYPOTHESIS: About 4.5 billion years ago the Kuiper
belt object that is now 2003EL61 collided obliquely with
another, unnamed, large Kuiper belt object. The oblique
impact caused 2003EL61 to spin rapidly and it transformed
its shape from a ball to american football. The
probability that the impact was oblique is low, on the
order of 0.01, because the 2003EL61 is the only large
object in the solar system that spins rapidly and has
american football shape.

2. FACT: The absence of planet in the place where
Ceres asteroid is now is the only exception of the
Titius-Bode Law. HYPOTHESIS: 4.5 billion years ago
there was a bigger asteroid in the place where Ceres
is now. Let us call it Theia and let us call the
unnamed large Kuiper belt object Orpheus. Ceres has
rocky core overlain with icy mantle. Theia had
the same composition as Ceres but it was larger.
Orpheus was made mostly of water ice. It was not
broken into small pieces by the impact with 2003EL61
because the impact was oblique. The impact hurled
Orpheus into a collision path with Theia. When Orpheus
hit Theia, the impact moved Theia toward Jupiter and
melted most of the water ice. The average distance
between 2003EL61 and Ceres is on the order of 5000 Gm
(35 AU). Diameter of Orpheus was probably on the order
of 1000 km. Diameter of Theia is unknown; let us assume
that it was 2000 km. The probability that Orpheus hit
Theia is on the order of 10^-13.

(/ 1.0 (expt (/ (* 5000.0 1000000000) 2000000) 2)) =
1.6e-013

3. HYPOTHESIS: The enormous gravity of Jupiter hurled
Theia toward the Earth. As Theia was moving toward
the Earth, its mantle of liquid water was vaporized by
the sunlight, creating watery atmosphere. Theia became
giant comet. Its rocky core collided with the Moon thus
creating a new, hot Moon. A few hours later Theia's
watery atmosphere collided with the Earth. It ablated
some of the Earth's sima and all sial except the back
30% of the Earth's surface. The original crust was made
of 20 km thick sial layer on top of 10 km thick sima
layer. Some of the original sial layer near the back
of the Earth was not ablated by the collision, but it
was pushed by the collision towards the back of the
Earth where it piled up and formed the very thick sial
layer that is now known as the continents. Dust
particles made from the sial, the Moon, and the rocky
core of Theia were suspended in the huge atmosphere
that enveloped the Earth and the new Moon. Some of
the atmosphere was captured by the new Moon. Very
large quantity of the dust fell on the Moon and the
Earth over a period of several thousand years. Soon
after the collision the Moon and the Earth were hot,
so the dust melted as soon as it fell. The Moon was
cooled quickly by the contact with the atmosphere, so
a few hundred years later it was so cool that the dust
falling on the Moon did not melt. When the collision
separated Theia's rocky core from its atmosphere, the
atmosphere quickly expanded due to the heat generated
by the collision and due to reduced gravity (no core).
The expansion reduced density of the atmosphere before
the collision with the Earth. Theia was quickly loosing
its volatile atmosphere after the collision with Orpheus
because much of its elliptic orbit was close to the sun.
If it was loosing its atmosphere at the rate of 0.1
meter per day, it had to collide with the Earth in about
10,000 years. The probability that the collision between
Earth and Theia took place within 10,000 years since the
collision between Theia and Orpheus is on the order of
10^-9.

(/ 1.0 (expt (/ (* 816620000 1000.0) 1000000) 2)) =
1.49955e-012

The probability that all these events occurred is
on the order of (0.01)*(10^-13)*(10^-9) = 10^-24.

One percent of stars in our galaxy, called Milky Way,
has Earth-like planets which have liquid water and
thus seem capable of supporting life. This means
that the probability that the Earth has oceans and
continents is on the order of 10^-26.

There are about 100 billion (10^11) stars in our
galaxy and about 7*10^22 stars in the entire visible
universe. The probability that another planet in
the entire visible universe has oceans and continents
is on the order of (10^-26)*(7*10^22) = 7*10^-4;
about one event in one thousand.
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FACT: Moon is deficient in iron and nickel. Moon's
Fe/Si ratio is equal to 0.22 as a whole (crust +
mantle + core). This is the lowest known Fe/Si ratio
of any object in the solar system. Comet gas tails
contain high concentrations of ionized carbon monoxide
gas. Nickel carbonyl and iron pentacarbonyl form upon
treatment of the metals with carbon monoxide. Both
carbonyls are volatile liquids at room temperature.
Nickel carbonyl forms by the direct combination
of carbon monoxide and nickel metal at room
temperature. Nickel carbonyl decomposes back to Ni
and CO upon contact with hot surfaces.

HYPOTHESIS: When Theia became a comet, it produced
lots of carbon monoxide. When Theia's rocky core
collided with the Moon, a chemical reaction between
iron, nickel and carbon monoxide produced carbonyls.
The carbonyl vapors were suspended in the atmosphere
surrounding the Earth and the Moon until the
atmosphere condensed on the Earth.

FACT: A troilite-rich nickel-iron particle found on the
Moon has surface erosion that, according to the authors
of the following article, is due to passage through a
cloud of hot gas and particulate matter:
Science 5 February 1971: Vol.171. no.3970, pp.479-480
Lunar Metallic Particle ("Mini-Moon"): An
Interpretation. David S. McKay, James L. Carter,
and William R. Greenwood.
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Australian astronomer, Nick Hoffman claims that the
Earth is a unique planet because it has continents
(http://www.spacedaily.com/news/life-01x1.html).
He has not explained why there are no continents
on other planets.

I believe that the missing Earth's sial provides the
explanation and I agree with Hoffman that life cannot
evolve into a technological civilization on a planet
that is devoid of continents.

In the absence of continents there would have been no
advanced forms of life on Earth because the entire
surface of the Earth would have been covered with
oceans and the only source of minerals for the marine
life would have been hydrothermal vents. The vents
cannot support great abundance and diversity of life,
which is necessary for speedy evolution of life.
Marine life of our planet is confined to places that
have abundance of iron, nitrates, phosphates and
silicates. Nearly all of these minerals are
transported from continents by rivers and winds.

If the planet has no continents, it has no land animals
that can make fire, smelt metals, and create
technological civilization. If the planet has no
continents, but it has an ocean, a giant asteroid impact
may create islands, but these islands are eroded by rain
and wind. There is no plate tectonics to counter the
erosion and it takes about one billion years of plant
evolution to produce roots that can prevent soil erosion.

The missing sial leaves empty space between tectonic
plates and thus makes plate tectonics possible on
the Earth. Venus is good example of an Earth-like
planet that does not have plate tectonics. Without
plate tectonics to dissipate heat from its mantle,
Venus undergoes a cyclical process in which mantle
temperatures rise for a few hundred million years
until they reach a critical level that weakens the
crust. Then, over a period of a few million years,
subduction occurs on enormous scale, completely
recycling the crust. The subduction would have killed
all higher forms of life if they had been present on
Venus.

We are lucky to have massive Moon. Earth's obliquity
(the angle between the Earth's equator and the plane
of its orbit) is 23.5 degrees. If the massive Moon
had not existed, the Earth's obliquity would have
varied wildly between 0 and 80 degrees. Such
variation would have caused extreme climatic changes.

We are lucky to have plenty of water. If we had had much
less water, all our flora and fauna would have perished
during a snowball period. Terrestrial life barely survived
during the snowball periods under two kilometers thick
layer of ice (http://en.wikipedia.org/wiki/Snowball_earth).
Evolutionary rates were incredibly slow then.

We are lucky to have plenty of heavy elements (called
metals). According to Wikipedia:
(http://en.wikipedia.org/wiki/Metallicity)
"These youngest stars, including the Sun, therefore have
the highest metal content, and are known as "Population I"
stars. Across the Milky Way, metallicity is higher in the
galactic centre and decreases as one moves outwards."

We are lucky to be far away from the galactic center
and its high concentration of dangerous, exploding stars.

We probably survived cataclysms and close calls that
left no evidence that we can study.

My estimates are not precise but they do not have to be
precise to convey important truth: we are the only
civilization in the visible universe, so SETI is a waste
of time. There is another proof that planets having
continents are extremely ra if they had been common,
extraterrestrial civilizations would have colonized our
galaxy and our planet billions of years ago.
__________________________________________________ ______

My explanation/understanding of plate tectonics of all
terrestrial planets of Earth size except the Earth:

Terrestrial (Earth-like) planets are made of high density
minerals covered with low density sial. The sial abounds in
silicates, so its physical properties are similar to the
properties of ceramics. Ceramics are brittle. Their thermal
conductivity and coefficient of thermal expansion are low.
The dense interior of the terrestrial planets abounds in metals,
so its physical properties are similar to the properties of
metals. Metals are ductile. Their thermal conductivity and
coefficient of thermal expansion are high.

When a terrestrial (Earth-like) planet is young and hot,
its sial surface is liquid. When the planet cools, its sial
solidifies. Sial does not conduct heat well, so it traps the
heat that is generated in the interior by the radioactive decay.
The entire planet warms up and it expands because all its
minerals have positive coefficient of thermal expansion.
The metallic interior expands more than the ceramic sial,
because its coefficient of thermal expansion is higher.
Great tension builds up in the ceramic sial until it shatters
like a glass pane. We call this event an earthquake. Liquid
magma and volcanic ash escape through the cracked sial into
the atmosphere. We call this event a volcanic eruption. When
the magma cools and solidifies, it seals the cracks in the sial
and the next cycle begins.

If the entire surface of a terrestrial planet is covered with
sial, enormous tension builds up in the sial over millions of
years. When the sial shatters, the earthquakes and volcanic
eruptions are enormous. Volcanic ash absorbs sunlight and thus
cools the atmosphere so much that all land animals freeze to
death.