Relative Dating
Decoding Earth's History
Scientists believe that the oldest human ancestor first walked on the Earth’s surface about approximately 195,000 years ago. Recorded history has taken place over the last 5,000 years. During this time, humans have maintained records of existence. Now, through laboratory investigations and observations, scientists have determined that the Earth is an estimated 4.6 billion years old. This record is not complete, but geologists interpret clues found in rocks to discover what ancient environments and life existed on Earth since its formation. In this interactivity, you will explore strategies that geologists use to decode the history of the planet. Click the player button to begin.
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Relative Dating
With 4.6 billion years of Earth's history with which to contend, the process of investigating rocks to discover Earth’s past can be overwhelming. In order to simplify the process, geologists developed a simple method for interpreting the sequence of events recorded in the rock and fossil record. Relative dating is a process of determining the ages of rocks and geologic events in a given sequence. Relative dating involves assigning ages to rock without using numbers or other finite values. In this interactivity, investigate the concept of relative dating including the associated principles. Click the player button to begin.
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Rock Correlation
The White Cliffs of Dover in England can be correlated with other white cliffs in Denmark and Germany.
Superposition and cross-cutting are helpful when rocks are located in an easily observable outcrop. How do geologists correlate rocks layers that are separated by great distances? The image above shows cliffs in the countries of Germany, England, and Denmark. These cliffs are composed of the sedimentary rock chalk. Geologists were able to match the layers of rocks by uncovering subtle clues in each of the outcrops. Rock correlation is a method of matching identical rocks that are separated by large distances on Earth.
When geologists correlate rocks, they look for the following elements:
- Color: The color of a rock can vary greatly or subtly. For example, sandstones can occur in white, red, gray, and green varieties. If a geologist has found a 10 million old red sandstone in a deposit of rocks and discovered bed of sandstone 80 miles away that is also 10 million years old and is the same hue, then that could be a strong indicator that the two rocks were formed from the same sediments at the same time.
- Thickness: The thickness of the outcrop can indicate a common beginning.
- Texture: Sizes or unique textures in the sediments that compose the rock can help distinguish or match deposits of rock.
- Index fossils: If a geologist finds two beds of sandstone with fossils of identical species present within the rock, the geologist may conclude that the two beds of sandstone are actually the same rock.
- Rock Type: Sedimentary rocks should only be matched with the same type of sedimentary rock.
- Position: When rocks are deposited, many different types of rock can stack in sequence. If sandstone is found sandwiched between a gray shale and a white limestone, geologists will note this sequence of rock and look for other similar formations when correlating rock.
Unconformities
One problem with relative dating and rock correlation exists because the deposition process was interrupted throughout Earth’s history. This interruption creates gaps in the rock record called unconformities. Unconformities indicate a time when deposition was stopped while erosion was ongoing. Once geologists find an unconformity, they can determine what precise rock times in the rock record are missing. In this interactivity, you will learn about the three different types of unconformities. Click the player button to begin.
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Relative Dating Review
Now that you have investigated the relative dating, check your knowledge. In this non-graded interactivity, read each question and select the appropriate answer. Click the player button to get started.