Earth and Space Science, Part 12

Question 1: Discuss the geologic time scale.

Answer 1: Geologists use the geologic time scale when discussing Earth’s chronology and the formation of rocks and minerals. Age is calculated in millions of years before the present time. Units of time are often delineated by geologic or paleontologic events. Smaller units of time such as eras are distinguished by the abundance and/or extinction of certain plant and animal life. For example, the extinction of the dinosaurs marks the end of the Mesozoic era and the beginning of the Cenozoic, the present, era. We are in the Holocene epoch. The supereon encompasses the greatest amount of time. It is composed of eons. Eons are divided into eras, eras into periods, and periods into epochs. Layers of rock also correspond to periods of time in geochronology. Current theory holds that the Earth was formed 4.5 billion years ago.

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Question 2: Explain how the geologic time scale was developed.

Answer 2: The first known observations of stratigraphy were made by Aristotle, who lived before the time of Christ. He observed seashells in ancient rock formations and on the beach, and concluded that the fossilized seashells were similar to current seashells. Avicenna, a Persian scholar from the 11th century, also made early advances in the development of stratigraphy with the concept of superposition. Nicolas Steno, a Danish scientist from the 17th century, expounded upon this with the belief that layers of rock are piled on top of each other. In the 18th century, Abraham Werner categorized rocks from four different periods: the Primary, Secondary, Tertiary, and Quaternary periods. This fell out of use when the belief emerged that rock layers containing the same fossils had been deposited at the same time, and were therefore from the same age. British geologists created the names for many of the time divisions in use today. For example, the Devonian period was named after the county of Devon, and the Permian period was named after Perm, Russia.

Question 3: Discuss the composition of Earth’s atmosphere and name its five main layers.

Answer 3: The atmosphere consists of 78 oxygen, and 1 argon. It also includes traces of water vapor, carbon dioxide and other gases, dust particles, and chemicals from Earth. The atmosphere becomes thinner the farther it is from the Earth’s surface. It becomes difficult to breathe at about 3 km above sea level. The atmosphere gradually fades into space. The lowest layer of the atmosphere is called the troposphere. Its thickness varies at the poles and the equator, varying from about 7 to 17 km. This is where most weather occurs. The stratosphere is next, and continues to an elevation of about 51 km. The mesosphere extends from the stratosphere to an elevation of about 81 km. It is the coldest layer and is where meteors tend to ablate. The next layer is the thermosphere. It is where the International Space Station orbits. The exosphere is the outermost layer, extends to 10,000 km, and mainly consists of hydrogen and helium.

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