Absolute age dating - 3. Geological time scale - 4. Geological maps. It may surprise you to learn that geologists were able to determine much of the history of the Earth and its life without knowing anything about the actual ages of the rocks that they studied. Through use of absolute age dating techniques which were developed during the 20th century; see Section 2 , they were able to later assign dates in years before the preset to important events in Earth's history.
These laws are illustrated in Figure below. If a sedimentary rock is found tilted, the layer was tilted after it was formed. When a valley cuts through sedimentary layers, it is assumed that the rocks on either side of the valley were originally continuous.
Jun 24, You can help us choose the best absolute dating method for each rock. Instructions. We've numbered the rock layers Layer 1 is the oldest. Drag and drop the rock sample from each layer onto the dating method you think will tell us its age. To learn more about the dating methods, select the image from the lab. Most sedimentary rocks are laid down in flat, horizontal layers. These can later tilt and fold due to tectonic activity, and river cuttings can cause gaps among the layers. Geologists are able to 'read' the rock layers using relative and absolute dating techniques. Relative dating arranges geological events - and the rocks they leave behind - in a sequence. The principle of superposition is simple, intuitive, and is the basis for relative age dating. It states that rocks positioned below other rocks are older than the rocks above. The image below shows a sequence of Devonian-aged ( Ma) rocks exposed at the magnificent waterfall at Taughannock Falls State Park in central New York. The rocks near the bottom of the waterfall .
The youngest layers are found at the top of the sequence, and the oldest layers are found at the bottom. Other scientists observed rock layers and formulated other principles.
Geologist William Smith identified the principle of faunal succession, which recognizes that:. Scottish geologist, James Hutton recognized the principle of cross-cutting relationships.
This helps geologists to determine the older and younger of two rock units Figure below. If an igneous dike B cuts a series of metamorphic rocks Awhich is older and which is younger?
Relative Ages of Rocks
In this image, A must have existed first for B to cut across it. The Grand Canyon provides an excellent illustration of the principles above. The many horizontal layers of sedimentary rock illustrate the principle of original horizontality Figure below.
At the Grand Canyon, the Coconino Sandstone appears across canyons. The Coconino is the distinctive white layer; it is a vast expanse of ancient sand dunes. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. The relative age of a rock then is its age in comparison with other rocks.
If you know the relative ages of two rock layers, 1 Do you know which is older and which is younger? In some cases, it is very tricky to determine the sequence of events that leads to a certain formation. Can you figure out what happened in what order in Figure below?
1. Relative age dating
Write it down and then check the following paragraphs. The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. The fault cuts through all three sedimentary rock layers A, B, and C and also the intrusion D. So the fault must be the youngest feature.
Determining the Relative Ages of Rocks. Steno's and Smith's principles are essential for determining the relative ages of rocks and rock layers. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the.
The intrusion D cuts through the three sedimentary rock layers, so it must be younger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is still younger. One of the first scientists to question this assumption and to understand geologic time was James Hutton.
Hutton traveled around Great Britain in the late s, studying sedimentary rocks and their fossils Figure below. Often described as the founder of modern geology, Hutton formulated uniformitarianism : The present is the key to the past. According to uniformitarianism, the same processes that operate on Earth today operated in the past as well.
Relative Dating of Rock Layers
Why is an acceptance of this principle absolutely essential for us to be able to decipher Earth history? The trash at the bottom was thrown out earlier than the trash that lies above it; the trash at the bottom is therefore older and likely smellier! Or, think about a stack of old magazines or newspapers that might be sitting in your home or garage: most likely, the newspapers at the bottom of the pile have dates on them that are older than the newspapers at the top of the pile.
Use superposition to determine which is older: the road or the lava flow? How do you know? The principle of cross-cutting relationships states that a rock unit or other geological feature, such as a fault that is cut by another rock unit or feature must be older than the rock unit or feature that does the cutting.
Imagine cutting a slice of bread from a whole loaf.
When investigating rocks in the field, geologists commonly observe features such as igneous intrusions or faults that cut through other rocks. Because these features are the ones doing the cutting, we know that they are younger than the rocks that they cut into. Have a look at the photographs below, which show the curb of a road in a neighborhood in Hollister, California. You can see that the curb is offset: the bottom half does not line up with the top half. As it turns out, the famous San Andreas fault runs below the curb at this location, which has caused the curb to be broken and displaced.
We know that the curb was originally straight when it was first constructed. The fault cut the curb and is thus younger than the curb itself. A curb in Hollister, California that is offset by the San Andreas fault.
The cartoon below shows an imaginary sequence of rocks and geological events labeled A-I. Using the principles of superposition and cross-cutting relationships, can you reconstruct the geological history of this place, at least based upon the information you have available? An imaginary cross-section, showing a series of rock layers and geological events A-I. A is a fault. B-F are sedimentary rock layers. G and H are both igneous intrusions.
Finally, I is an erosional surface. Based on the principles of superposition and cross-cutting relationships, what are the relative ages of these rocks and events?
Therefore, the youngest layers are found at the top, and the oldest layers are found at the bottom of the sequence. A rock formation or surface that cuts across other rock layers is younger than the rock layers it disturbs.
For example, if an igneous intrusion goes through a series of metamorphic rocks, the intrusion must be younger than the metamorphic rocks that it cuts through Figure This nicely illustrates the principle of original horizontality. The youngest rock layers are at the top of the canyon, while the oldest are at the bottom, which is described by the law of superposition.
Distinctive rock layers, such as the Kaibab Limestone, can be matched across the broad expanse of the canyon. We know these rock layers were once connected, which is described in the rule of lateral continuity. Finally, the Colorado River cuts through all the layers of sedimentary rock to form the canyon. Based on the principle of cross-cutting relationships, the river must be younger than all of the rock layers that it cuts through.
If you know the relative ages of two rock layers, you know which is older and which is younger, but you do not know how old the layers are in years. In some cases, it is very tricky to determine the sequence of events that leads to a certain formation.
Relative age dating of rock layers
Take the example, Figure The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through.
So the fault must be the youngest formation that is seen. The intrusion D cuts through the three sedimentary rock layers, so it must be younger than those layers. The principle of superposition states that the oldest sedimentary rock units are at the bottom, and the youngest are at the top. Based on this, layer C is oldest, followed by B and A.
Start studying Relative Dating of Rock Layers by Principles. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Relative Dating. STUDY. Flashcards. Learn. Write. Spell. Test. PLAY. Match. Gravity. Created by. erika_rose Terms in this set (14) A geologist finds four layers of sedimentary rock. She determines that no geologic events have shifted the layers. She labels the layers A, B, C, and D from the top to the bottom. Give a relative age to a. Sep 30, Relative age dating also means paying attention to crosscutting relationships. Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
So the full sequence of events is as follows:. Steno discovered the rules for determining the relative age of rock beds, but he did not have a good understanding of how long it would take for these rock formations to form. At the time, most Europeans believed that the Earth was around 6, years old, a figure that was based on the amount of time estimated for the events described in the Bible. One of the first to question this time scale was a Scottish geologist named James Hutton According to uniformitarianism, the same processes we see around us today operated in the past as well.
For example, if erosion and deposition occur slowly now, they probably have always occurred slowly. Hutton discovered places where sedimentary rock beds lie on an eroded surface. Hutton reconstructed the sequence of events that led to this formation. For example, consider the famous unconformity at Siccar Point, on the coast of Scotland Figure