Relative age dating and cross cutting relationships
Use superposition to determine which is older: the road or the lava flow? 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.
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.
We know that the curb was originally straight when it was first constructed. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.
The fault cut the curb and is thus younger than the curb itself. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License. Based on the principles of superposition and cross-cutting relationships, what are the relative ages of these rocks and events? Finally, we note an erosional surface, I, at the top of the sequence (and immediately below the corn field) that cuts both A and G. Putting this all together, we can determine the relative ages of these rock layers and geological events: Given the information available, we cannot resolve whether H is older than A (or, vice versa).
Microscopic cross-cutting relations are those that require study by magnification or other close scrutiny.
For example, penetration of a fossil shell by the drilling action of a boring organism is an example of such a relation.
Megascopic cross-cutting relations are features like igneous dikes, as mentioned above, which would be seen on an outcrop or in a limited geographic area.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.Cross-cutting relationships can also be used in conjunction with radiometric age dating to effect an age bracket for geological materials that cannot be directly dated by radiometric techniques.For example, if a layer of sediment containing a fossil of interest is bounded on the top and bottom by unconformities, where the lower unconformity truncates dike A and the upper unconformity truncates dike B (which penetrates the layer in question), this method can be used.