That's a great question. Until we invent a way to
go back in time -- and find someone who is brave
enough to go back 4.6 billion years -- scientists
can only guess. We've made our guess based on what
we know about the Earth and other planets, and
based on how we think our solar system formed.
Scientists always base their ideas on physical
evidence: things we can measure test and observe.
God lies outside this sphere. So science is just
one way of looking at things. I'm going to give
you the scientific explanation.
know the Earth is made up of layers: a brittle,
thin crust on the outside; a thick, soft layer in
the middle (the mantle) and an incredibly hot,
metallic center (the core). We think the
temperature of the inner core (the center of the
Earth) is about the same temperature as the
surface of the sun. Wow!!! Where does that heat
come from? One source is natural nuclear energy.
The amazing pressure at the center of the Earth
from all that overlying rock causes nuclear fusion
reactions, which release heat. The rocks in the
mantle and crust trap the heat and keep it from
escaping. A second source of that incredible
amount of heat at the Earth's center is very old:
heat left over from when the Earth first formed,
about 4.6 billion years ago.
We think the
planets formed from collisions of space dust and
gas that were rotating around the sun. (Where does
space dust come from? Exploding stars! Because we
are made from the same material the Earth is made
of, we are all star dust.) For rocky planets, like
Earth, the dust stuck together and became small
meteors, the small meteors stuck together and
became large meteors, the large meteors crashed
together and became small planets, and the small
planets crashed together and became the planets we
see today. The fact that the plants weigh so much
(gravitational attraction) shapes them into
spheres instead of jumbled chunks of rock. All
that crashing released enormous amounts of heat,
and at some point, the Earth was so hot from these
collisions that ALL the rock melted, and the Earth
was a hot ball of melted rock orbiting the sun.
The heavy material (metals) sank to the center and
the lighter material (silicate minerals, like in
quartz and other gem stones) floated to the top.
Eventually, the Earth cooled off, and a hard,
cold, brittle crust formed.
point, there was no oxygen in the atmosphere, and
no water on the surface. All the water within the
surface rocks had evaporated. Slowly, volcanoes
formed as they do today and when they erupted,
they burped up water vapor from the melted rock in
the mantle (silicate minerals contain water). This
water vapor hit the cold atmosphere and condensed
and came down as rain. After a long time, the
oceans filled up and became salty. Some of this
water also came from collisions with comets, which
are giant ice balls with rocks in the center. When
the comets hit the Earth, the ice melted on impact
and became water vapor, which condensed in the
atmosphere and came down as rain. When did the
oxygen in our atmosphere show up? When life
evolved. (Bacteria capable of photosynthesis
evolved about a billion years ago and produced
some oxygen. About 700 to 500 million years ago,
seaweed, algae and eventually land plants evolved
and made a whole lot of oxygen.)
short answer is: so hot everything melted.