Question 1

Geologists establish the age relationship of rocks from different locations through the process of __________.

Accepted Answer

Geologists establish the age relationship of rocks from different locations through the process of correlation.

Question 2

Layers of sedimentary rock are __________ if they were deposited without detectable interruption.

Accepted Answer

Conformable layers of sedimentary rock imply that there were no breaks in depositional processes or erosion between the layers. They represent continuous and uninterrupted deposition.

Question 3

To be useful, an index fossil is produced by an organism that:

Accepted Answer

An index fossil must be produced by an organism that existed for a relatively short time. This is because it can be used to date and correlate rock layers that contain it to other rock layers in different locations. If the organism existed for a long time, it would not be as useful for dating specific rock layers.

Question 4

Relative age measurement refers to:

Accepted Answer

Relative age measurement refers to determining the sequence in which events occurred. It does not involve measuring age in years or relying solely on a specific type of dating method, such as carbon-14 or potassium-argon dating.

Question 5

In a disconformity:

Accepted Answer

In a disconformity, the sedimentary layers above and below the unconformity are parallel. This means that there is a gap in the rock record, but the layers that formed before and after the gap are parallel to each other. This type of unconformity is difficult to spot because there is no obvious angular discordance between the layers.

Question 6

Imagine that you found two sandstone beds about 200 kilometers apart. The rock in one region was similar to the rock in the other, but one bed was several million years older than the other. From this information, you would conclude that:

Accepted Answer

The two beds being similar in rock type but differing in age suggests they are lithologically correlated, meaning they share similar physical and chemical characteristics but were not necessarily formed at the same time or contain the same fossils.

Question 7

A magmatic intrusion cutting through sedimentary layers:

Accepted Answer

A magmatic intrusion represents a younger geological event that has cut through existing sedimentary layers. Therefore, it must be younger than these sedimentary layers.

Question 8

Precambrian refers to all the time before the start of the __________ Eon.

Accepted Answer

The Precambrian encompasses all of geological time prior to the start of the Phanerozoic Eon, covering the Hadean, Archean, and Proterozoic eons.

Question 9

The half-life of potassium-40 is 1.3 billion years. If a rock that formed 2.6 billion years ago originally contained 5 grams of potassium-40, about __________ grams of potassium-40 would be left in that rock today.

Accepted Answer

After 2.6 billion years, which is two half-lives of potassium-40 (1.3 billion years each), the amount of potassium-40 would have halved twice. Starting with 5 grams, after the first half-life, it would be 2.5 grams, and after the second half-life, it would be 1.25 grams.

Question 10

Geologic evidence for the cause of the extinction event at the end of the Cretaceous includes:

Accepted Answer

An iridium-rich layer found in sedimentary rocks is the most widely accepted evidence for the cause of the extinction event at the end of the Cretaceous. This layer is thought to have formed as a result of an asteroid impact, which released large amounts of iridium into the atmosphere and caused widespread environmental changes.

Question 11

In a __________, the sedimentary layers above and below the unconformity are parallel.

Accepted Answer

In a disconformity, the sedimentary layers above and below the unconformity are parallel. A nonconformity is characterized by the presence of igneous or metamorphic rocks in contact with sedimentary rocks, while an angular unconformity has tilted or folded layers below the unconformity surface. A correlative unconformity is a type of disconformity where the missing layers on one side of the unconformity can be matched with equivalent layers on the other side.

Question 12

Scientists have noted that mass extinctions have coincided with:

Accepted Answer

Scientists have observed that mass extinctions have coincided with both unusually high rates of volcanic activity and meteorite impacts. These events cause major disruptions to ecosystems, leading to widespread extinctions. The slow step-wise evolution is not necessarily linked to mass extinctions.

Question 13

Fish, amphibians, and reptiles all evolved during the __________ Era/Eon.

Accepted Answer

Fish, amphibians, and reptiles all first appeared during the Paleozoic Era, which is known for the significant increase in the diversity of life forms on Earth.

Question 14

An unconformity in which tectonic activity tilted older sedimentary rock layers before younger sediment accumulated is:

Accepted Answer

An angular unconformity is defined as an unconformity in which older, tilted rock layers are overlain by younger, undeformed rock layers. This type of unconformity is commonly caused by tectonic activity that tilts or folds the older rocks before younger sediments are deposited on top of them.

Question 15

The principle of faunal succession states that:

Accepted Answer

The principle of faunal succession is the observation that fossil organisms have succeeded one another through time in a definite and recognizable order. This is an important concept in determining the relative ages of rocks and the geologic time scale.

Question 16

The earliest fossils have been found from the __________ Eon.

Accepted Answer

The earliest fossils have been found from the Archaean Eon, which lasted from 4.0 to 2.5 billion years ago.

Question 17

Scientists have determined the Earth's age to be about __________ years.

Accepted Answer

Scientists have determined the age of the Earth to be approximately 4.6 billion years based on various dating methods including radiometric dating of rocks and meteorites.

Question 18

The principle of original horizontality is based on the fact that:

Accepted Answer

The principle of original horizontality states that sedimentary rocks are usually deposited in horizontal layers due to the effects of gravity. This principle is based on observation and can help geologists interpret the relative ages of rocks in a sequence.

Question 19

Humans evolved during the __________ Era/Eon.

Accepted Answer

Humans evolved during the Cenozoic Era, which is the current geological era and follows the Mesozoic Era. It began about 66 million years ago after the Cretaceous-Paleogene extinction event.

Question 20

The smallest unit of geologic time is a/an __________.

Accepted Answer

The smallest unit of geologic time is an epoch. An epoch is a subdivision of a period, which is a subdivision of an era, which is a subdivision of an eon. Therefore, the correct answer is B) epoch.

Quiz 4: Geologic Time: a Story in the Rocks