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Geologic History
 “Big Bang” about 13.7 billion years ago
 All matter in the universe created made entirely of
Hydrogen and Helium
 ~13 b.y.a. oldest galaxies form and are moving apart
 ~ 5 b.y.a our sun begins to form in the milky way galaxy
 Question you should be asking… If the universe was
entirely H, and He…. Where did all the other elements
come from that we know and love?! (O, C, Fe, Ag, etc.)
 That leaves 8 – 9 billion years between the big bang
and our sun.
 We know that stars, in life and death create new
 We can also see stars forming from the nebula of stars
that have died before them. (think recycling!)
 In our solar system, and on Earth, we have 92 naturally
occurring elements.
 The abundance of these elements indicates that our
sun is definitely not the first, or likely even the second
start that has lived here.
 Since in recycling you never regain 100% of the
ingredients each star that has come since has been
smaller and longer lived.
 After a while our sun would have had enough gravity
and heat to begin explosive fusion, which blew away
most of the gas and ice away from the inner planets,
leaving only rock and dust.
 Farther away from the sun large gas giants were
forming that had the gravity to keep and pull in more
 Farther still, there was mainly gas and ice left to form
 This was referred to as the nebular hypothesis.
 In this model the sun
and planets form in a
process called
 About 4.6 b.y.a all the planets in our solar system were
 But, the solar system was full of asteroids from the size
of a grain of sand to the size of Kentucky.
 The gravity of the early
planets pulled in most of the
left over asteroids over the
next 2 billion years.
 But the question remained: “Where did our big
beautiful moon come from?”
Fission Theory: Moon is a chunk of Earth that broke
off on its own.
Capture Theory: Moon was snagged by Earths gravity
while strolling by.
Co-creation Theory: We are just really close neighbors
that formed at the same time
 Fission: Wrong! It would go against gravity for a blob
of molten rock to go away from the Earth on its own.
 Capture: Wrong! The moon is too big to be pulled into
such a nice orbit if it were passing by. Its geochemistry
is too similar for it to be made somewhere else and
drift here.
 Co-creation theory: Wrong! It would be made out of
exactly the same material as Earth and it isn’t.
 Where did it come from?!
 1985: Bill Kaufmann, Jay Melosh and others, proposed
the Giant Impact Theory.
 About 4.4 b.y.a. a mars sized planet, Theia, smashed
into Earth.
 Presence of Oxygen isotopes is too close to Earth’s
composition to have come from somewhere else.
 The rocks of the moon are nearly chemically identical
to the rocks of Earth’s mantle, which would have been
ejected during impact.
 Moon has a small iron core. The impact would not
have been able to eject much of the iron at Earth’s
 Universe began about 13.7 b.y.a.
 Over the following 8-9 billion years stars have created
the elements that make-up Earth and all the planets.
 Our sun and solar system formed about 5 b.y.a.
 Earth started forming around 4.6 b.y.a.
 Theia struck early earth about 4.4 b.y.a. Creating our