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Chp 17: Section 3: The Earth Takes Shape - Student Copy

Terms to Know: 1. crust 2. mantle 3. core
Many different sciences have contributed to our understanding of Earth’s origin. Much remains to be discovered through computer models and study of other planets. The following evidence has shaped the current scientific theories about the formation of earth and its atmosphere:
* the oldest rocks on Earth are nearly _________________________
* some of the oldest rocks are _________________________, so we know that oceans must have existed early in the history of Earth
* _________________________ when the Earth was forming. We know this from the study of how hydrogen fusion reactions work
* The earth must have had a __________________________________ early in its life, or it would have been too cold to have liquid oceans.
* the oldest fossils of primitive life are __________________, blue-green algae colonies that lived between 3.7 and 3.4 billion years ago. Simple life-forms probably appeared on Earth before this time (between 4.6 and 3.7 billion years ago)
* __________________________________to get energy from sunlight to produce oxygen. Evidence from the oxidation of minerals in the rock record indicates that oxygen started to appear in significant concentrations in Earth’s atmosphere between 2.5 and 2.0 billion years ago
*The evolution of certain ______________________________ in Earth’s early atmosphere. Although nitrogen easily combines with surface rocks, the activity of bacteria kept releasing the element back into the atmosphere. Ironically, if the early Earth had contained a lot of oxygen in its atmosphere, the chemical processes that gave rise to life probably would not have occurred! High levels of oxygen would have prevented organic molecules from combining.
* Water that formed the oceans came from the early _________________ and was _________________________ during the differentiation process. As the Earth heated and differentiated, water that was chemically bound in minerals was carried to the surface of the planet along with magma. Water vapor was then released to form the early atmosphere. Even at current rates of volcanism, the water released from lava flows would be more than enough to fill the earth’s oceans in 500 million years.
As scientists now understand it, the Earth was formed from the _________ _______________________. The addition of planetesimals kept the Earth growing until it reached its present size. This happened within the first 10 million years of the collapse of the solar nebula. As planetesimals fell to Earth, the energy of their motion was _________________________. This heat made the Earth warmer. Once the earth reached a certain size, the interior could not cool off as fast as its temperature rose, and the rocky material began to melt. But why is the Earth spherical in shape, while most asteroids and comets are not? This is because the Earth has enough _____ _________________________ and melted rocks during its formation. The force of gravity pulled this material toward the center, forming a sphere. Asteroids are not massive enough for their interiors to be crushed or melted.

It’s a fact: Earth is growing _________________________ every year. Microscopic dust constantly filters through the atmosphere from space and lands on our planet. Movies and films have popularized the notion that large meteors threaten earth and could cause colossal destruction. Although several tons of meteoroid material enters earth’s atmosphere each day, most pieces have a mass of only a few milligrams. Only the largest meteors reach Earth’s surface and become meteorites. Meteor showers that are detectable only by radar occur regularly.