Earth's Geologic History

Students apply what they know about the processes that change Earth’s surface to analyze evidence in rock strata to help them figure out an explanation of past changes and establish the relative ages of different events based on clues in rock layers and index fossils. 

• Geologists use what they know about how rocks are constantly being shaped and re- formed in the rock cycle to reconstruct a geologic timeline about how Earth has changed since it first formed 4.6 billion years ago. 
• Fossils are the remains of ancient animals and plants, the traces or impressions of living things from past geologic ages, or the traces of their activities.
• Fossils can tell scientists a lot about Earth’s past, including what the environment was like and how it changed.
• Sedimentary rock holds fossils because its layers built up over time. As sediment was deposited, it sometimes trapped and preserved remains of living things.
• Geologists have to piece together what they know about Earth processes and how the rock strata formed to help them correlate rock layers that don’t match up perfectly.
  
  

• What is the difference between relative age and numeric age?
• Why is relative age useful? What are some limitations of relative age?
• How can geologic columns establish the relative ages of different time periods in Earth’s history?
• Why are fossils found in sedimentary rock?
• How can fossils tell scientists about the environment in which the organism lived?
• How can index fossils help scientists organize Earth’s history?
• How can geologists use the geologic time scale to help them determine the age of new discoveries?
  
  

Misconception: Earth’s surface is static and unchanging.

Fact: Earth’s surface is constantly changing through a variety of processes, including plate tectonic movement, weathering, and erosion.

Fossil : the remains of ancient animals and plants, the traces or impressions of living things from past geologic ages, or the traces of their activities

Fossil Record : includes all of the fossils that have ever been found; scientists use it to understand Earth’s history

Geologic Column : the rock layers and the fossils found in them

Index Fossil : certain fossils known to exist in a particular time and place that may be used to determine the relative age of rocks or other fossils

Numeric Age : a precise number (in years, minutes, or other unit of time) that represents how much time has passed

Relative Age : tells scientists whether something is younger or older than something else

Whale Fossils on a Mountain Top

In 1987, a team of scientists hiked through the Andes Mountains, more than 1,524 meters (5,000 feet) above sea level. They were on a mission: to find fossils of whales and other marine organisms to bring back to a museum to study. Fossils are the remains of ancient animals and plants, the traces or impressions of living things from past geologic ages, or the traces of their activities. Sedimentary rock often holds fossils because its layers built up over time. As sediment was deposited, it often trapped and preserved remains of living things. These remains include whole plants and animals, as well as traces of organisms such as footprints.

Fossils can tell scientists a lot about Earth’s past, including what the environment was like and how it changed. For example, the whale fossils told scientists that the land was once covered by an ocean because whales can only survive in the ocean. The marine fossils on top of the mountain also told scientists about a major change to Earth’s surface. They are evidence that the mountain used to be much lower because it was once covered in water. At some point, two tectonic plates collided, pushing the land upward and forming the mountains.

Geologic Detective Work

Geologists are like detectives. They look for clues in the layers of rock that tell them how Earth has changed since it first formed 4.6 billion years ago. For example, the geologists who traveled to the Andes learned a lot of information from the fossils they found.

They gathered fossils from a 305-meter (1,000-foot) thick rock section of one of the mountains. Within their rock sample, they found fossils of both marine and land animals in different layers. They found oyster beds and sand dollars, which were both evidence of a marine environment. Right above these layers were fossils of land organisms.

Understanding layers is an important part of this geologic detective work. The rock layers and the fossils found in them are called the geologic column. Geologists refer to the rock strata when talking about the layered arrangement of rocks in the geologic column. Scientists know that when they look at rock strata, the bottom layer is generally the oldest layer. The top layer is the newest, most recent layer. This is because of the process of sedimentation. Sediment formed from weathering and erosion slowly accumulates in layers in oceans, lakes, and valleys over time. Each new layer is deposited on top of an older layer. This process also results in layers that are horizontal. In other words, the layers are parallel (or mostly parallel) to Earth’s surface.

The layers of rock and fossils in the geologic column told the geologists in the Andes that the land was once covered by an ocean. It then transitioned to a land environment, which scientists believe happened as tectonic plates pushed the land upward, causing the ocean to disappear.

In this lesson students analyze and correlate sets of simulated rock strata to figure out how evidence in rock layers can be used to organize and figure out Earth’s history. Students do this by conducting an investigation in which they reconstruct the sequence of rock layers with and without index fossils formed over time by looking for similarities in the rock composition and layer sequences at different locations across each of the columns.