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. But, before that, they relied upon a different approach to first determine the sequence of important events in Earth's past: relative age dating.
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.
Dating Fossils - How Are Fossils Dated?
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? Second, we observe that rock layer H which is an igneous intrusion cuts into rock layers B-F.
It is therefore younger than B-F. Third, we observe that the fault A cuts across and displaces rock layers B-F.
May 18, Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called stratigraphy (layers of rock are called strata). Relative dating does not provide actual numerical dates for the rocks. 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. Relative dating is qualitative. This technique helps determine the relative age of the remains. It is less specific than absolute dating. Relative dating is comparatively less expensive and time-efficient. It works best for sedimentary rocks having layered arrangement of sediments. The following are the major methods of relative dating.
Throughout the history of life, different organisms have appeared, flourished and become extinct. Many of these organisms have left their remains as fossils in sedimentary rocks.
Geologists have studied the order in which fossils appeared and disappeared through time and rocks. This study is called biostratigraphy. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
This matching process is called correlation, which has been an important process in constructing geological timescales. Some fossils, called index fossils, are particularly useful in correlating rocks. For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. For example, ammonites lived in the Mesozoic era.
Relative Dating BrainPOP
If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic. These are called relative and absolute dating techniques.
Absolute dating, also called numerical dating, arranges the historical remains in order of their ages.
Whereas, relative dating arranges them in the geological order of their formation. The relative dating techniques are very effective when it comes to radioactive isotope or radiocarbon dating. However, not all fossils or remains contain such elements. Relative techniques are of great help in such types of sediments. Stratigraphy: The oldest dating method which studies the successive placement of layers.
What is relative age dating of rocks
It is based on the concept that the lowest layer is the oldest and the topmost layer is the youngest. Biostratigraphy: An extended version of stratigraphy where the faunal deposits are used to establish dating. Faunal deposits include remains and fossils of dead animals. Cross dating: This method compares the age of remains or fossils found in a layer with the ones found in other layers.
The comparison helps establish the relative age of these remains. Fluorine dating: Bones from fossils absorb fluorine from the groundwater. The amount of fluorine absorbed indicates how long the fossil has been buried in the sediments.
Radiometric dating: This technique solely depends on the traces of radioactive isotopes found in fossils. The rate of decay of these elements helps determine their age, and in turn the age of the rocks.
Amino acid dating: Physical structure of living beings depends on the protein content in their bodies.
The changes in this content help determine the relative age of these fossils. Dendrochronology: Each tree has growth rings in its trunk. This technique dates the time period during which these rings were formed.
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 . Relative age dating places rocks and events in chronological order but does not provide information about absolute age. What is the principle of original horizontality? Sedimentary rocks are close to horizontal when deposited.
Thermoluminescence: It determines the period during which certain object was last subjected to heat.