6. How did Earth's atmosphere develop?
When Earth was first
formed it was a solid ball of molten rock with no atmosphere.
As Earth began to cool and the crust and mantle began to harden, material from the inner regions would sometimes erupt through the surface bringing hot gases. Scientists theorize (no-one really knows because no one was there!) that these gases were methane, ammonia, carbon dioxide and water (steam). The water eventually condensed to form oceans but almost certainly there was at first no oxygen or nitrogen in Earth's atmosphere. It is believed that the atmosphere (air) that we now breathe was formed only after the development of the first living things, blue-green algae (now generally classified as blue-green bacteria), that happened about two billion years ago. These simple organisms used carbon dioxide during photosynthesis and gave off oxygen as a biproduct of the chemical process.
7. Where did the Moon come from?
The best, simple answer to this question is that we dont know. There are several theories: a) perhaps the Moon was torn out of Earth's crust and mantle at some time in the distant past (billions of years ago); b) perhaps the Moon was created out of planetisimals (see the Introduction and Day 1) and somehow got caught by Earth's gravitational influence and went into orbit around Earth as well as around the Sun; c) perhaps a giant meteorite hit Earth soon after it was formed and the material thrown out into space as a result of the impact coalesced into a small mass that became the Moon.
8. How can we tell the age of the rocks in Earth's crust?
There are several methods: a) We can make educated guesses - in general, the deeper the rock, the older it is. Using this generalization geologists have created a relative time scale that describes what is older or younger but does not give a specific ages in years except in the most general terms; b) It is also possible by comparing the fossils in two different rock layers to tell that they are about the same age if the same types of fossils are found in each; c) Radiometric dating such as the carbon-14 method can provide quite accurate specific ages of the remains of living things.
9. How old is Earth?
Earth's oldest rocks are believed to be about 3.8 or 3.9 billion years old. Because rocks formed quite slowly from the Earth's original molten mass, clearly Earth is older than its oldest rocks. Scientists believe that Earth is about 4.6 billion years old. (Note, in this and other explanations the US billion is the number used. In the US one billion is 1000 million.)
10. What is Plate Tectonics?
This is a relatively modern and widely accepted theory concerning the structure of Earth's crust. Plate tectonics proposes that the Earth's crust is made up of six major
plates and several smaller ones. The plates consist of the crust including a thinnish layer of the upper mantle. These plates have moved throughout geologic time converting the very early land mass known as Pangaea into the present-day positions of continents. The North American plate extends from the middle of the Atlantic ocean to the West Coast of the United States ending at California's San Andreas Fault - an active earthquake zone. The Pacific plate is entirely under the Pacific Ocean and has no continental areas. Plate margins, where the six main plates roughly coincide, are generally zones of seismic or volcanic activity.
11. How have Earth's
mountains been formed?
Mountains are formed in several different ways. For example the Appalachian Mountains on the east coast of the United States are fold mountains that were pushed up as ridges on Earth's crust when the Eurasian and American tectonic plates collided. These mountains are perhaps several hundred million years old.
The Sierra Nevada mountains of California were formed through a fault-block process in which large chunks of rock sank leavother chunks standing.
The Black Hills of South Dakota are dome mountains that were pushed up by a hot spot in the mantle of that region.
The Rocky Mountains of the Western United States are thought to be quite young, perhaps only about 65 million years old. They are being studied by geologists to determine the method or methods by which they were formed.