Section 21.1 Study Guide Section 21.1 Main Ideas Geologists have separated Earth’s history into divisions based upon the fossil record. The divisions of the geologic time scale, in descending order and decreasing length of time spans, are eons, eras, periods, and epochs.

Section 21.2 Study Guide Section 21.2 Main Ideas The principles of uniformitarianism, original horizontality, superposition, and cross-cutting relationships are used to interpret Earth’s rock record and, thus, to describe the planet’s history. Unconformities caused by weathering and erosion or by periods of nondeposition mark missing layers in the rock record.

Section 21.3 Study Guide Section 21.3 Main Ideas Absolute-age dating measures the actual age of an object such as a mineral, rock, or fossil. Radioactive decay is the emission of particles from a radioactive atom. The decay rate can be used to determine the age of a rock or fossil. The time it takes a radioactive element to decay to 50 percent of its original mass is known as its half-life. Tree rings and varves can also determine the dates of events and changes in the environment. Volcanic ash and meteorite-impact debris create key beds that mark the time of the event.

Section 21.4 Study Guide Section 21.4 Main Ideas The remains and evidence of plants and animals that once lived on Earth are called fossils. Fossils preserved in the rock record provide information about past environmental conditions, evolutionary changes in life-forms, and help geologists to correlate rock layers from one area to another.

Multiple Choice 1. Which of the following is not an eon? Chapter Assessment Multiple Choice 1. Which of the following is not an eon? a. Phanerozoic c. Cenozoic b. Proterozoic d. Archean The Cenozoic Era is the most recent period in the Phanerozoic Eon. The Cenozoic was preceded by the Mesozoic and Paleozoic Eras which form the rest of the Phanerozoic Eon. The Phanerozoic Eon was preceded by the Proterozoic and Archean Eons.

Multiple Choice 2. A gap in the rock record is known as a(n) ____. Chapter Assessment Multiple Choice 2. A gap in the rock record is known as a(n) ____. a. unconformity c. superposition b. correlation d. inclusion Unconformities can be classified as disconformities or nonconformities. They are the result of deposition on top of an active erosional surface.

Chapter Assessment Multiple Choice 3. Approximately how old is a wood sample that has 3.2 percent of its C-14 remaining? a. 11 000 years c. 24 000 years b. 16 000 years d. 28 000 years After five half-lives of 5730 years (28 650 years total), around 3.2 percent of the C-14 would remain.

Chapter Assessment Multiple Choice 4. A change in populations as a result of environmental change is known as ____. a. permineralization c. altered hard parts b. fossilization d. evolution Fossils provide evidence of the past existence of a wide variety of life-forms, most of which have become extinct. The fossil record also provides evidence that populations have undergone change through time in response to changes in their environments.

Chapter Assessment Multiple Choice 5. On the geologic time scale, which division is defined by the life forms that were abundant or became extinct? a. period c. eon b. era d. epoch The order of the divisions of geologic time classification beginning with the largest is eon, era, period, and epoch.

Short Answer 6. What does the principle of superposition state? Chapter Assessment Short Answer 6. What does the principle of superposition state? The principle of superposition states that in an undisturbed rock sequence, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath.

Chapter Assessment Short Answer 7. What does the principle of cross-cutting relationships state? The principle of cross-cutting relationships states that an intrusion or a fault is younger than the rock it cuts across.

Chapter Assessment True or False 8. Identify whether the following statements are true or false. ______ Fallout from a major volcanic eruption can form a key bed. ______ Homo sapiens evolved in the past .01 M.Y.B.P. ______ Varves are direct evidence of past life-forms. ______ Dendrochronology is limited to the age of the oldest living tree. ______ The first fish appeared in the Paleozoic Era. ______ All organisms contain carbon. true false

Image Bank Chapter 21 Images

Image Bank Chapter 21 Images

Image Bank Chapter 21 Images