The law or principle of cross-cutting relationships states that a rock body, geological process or feature that disrupts, penetrates, deforms, or cuts another is younger. The disrupted, cut, or deformed geological feature or rock body is older.
This stratigraphy principle was first proposed by Nicholas Steno (1638–1686) in 1669. James Hutton (1726–1797) later formulated the idea, and Sir Charles Lyell (1797–1875) expanded it to its current form.
Geologists use this law or principle of stratigraphy and others, like superposition, to determine the relative age of geological features or rock bodies. It is also important in determining the chronological order of events.
It’s a straightforward law
The cross-cutting relationships law is straightforward and self-explanatory. A geological process, feature, or rock body will only disrupt, penetrate, cut, or deform an already existing feature or rock body. This makes it younger than what it disrupts, cuts, penetrates, or deforms.
Common examples illustrating cross-cutting relationships are folds, faults, rocks, or minerals cutting or deforming other rock bodies or geological features.
The folds, faults, minerals, or rock bodies that deform, cut, penetrate, or disrupt other geological features or rock bodies are younger, while the disrupted features or rock bodies are older.
The other example is where an unconformity exists. Features beneath the unconformity are older, and those above are younger.
The magnitudes of cross-cutting relationships can be microscopic, megascopic, or cartographic. Microscopic relationships need a microscope, such as seeing fossilized, small shell-cutting sedimentary strata.
Megascopic relationships, on the other hand, are large enough to see with an unaided eye, while cartographic ones cover a large, mappable area.
Cross-cutting relationships types
Cross-cutting relationships in geology can be structural, intrusional, stratigraphic, or sedimentological. Others are paleontological and geomorphological.
Here is a quick wrap-up of what each of these relationships means.
1. Intrusional
Intrusional relationships involve magma intruding through geological features or rock bodies. The intruding magma may create a crack, fill cracks, and deform geological features or rock bodies.
It is also possible for magma to intrude without causing deformation or fracture. This can happen if the magma melts or chemically reacts with a surrounding rock before it solidifies.
When magma cuts through an existing rock body or geological feature, it forms concordant intrusions like dikes and batholiths.
On the other hand, a discordant intrusion may occur when magma runs parallel between existing rock beddings or rock layers. Laccoliths, lopoliths, and sills are common examples of discordant intrusions.
In both discordant and concordant intrusions, any geological feature, rock body, or layer intruded is older than the magma that intrudes or runs parallel through it.
Structures formed when magma solidifies, and bakes margins are two ways to note intrusion features. Baked margins are associated with contact metamorphism, which occurs when hot magma interacts with colder country rock.
2. Structural relationships
Structural relationships occur when tectonic activities cause faults or joints that cut existing rock structures or geological features. Joints cause little to no displacement of rock bodies, and faults significantly displace rock bodies, forming slips, throws, or heaves.
Tectonic events like faulting, folding, or cracking are younger in structural relationships than the geological structure or rock bodies deformed, disrupted, rearranged, or cut.
3. Stratigraphic
Stratigraphic relationships occur in places where erosion removes or cuts existing rock bodies, layers, or geological features, creating unconformities. The rocks emplaced or features formed above eroded rock layers or bodies are younger than the event that caused erosion.
4. Sedimentological
The sedimentological relationships reveal the sequence of events. Anything that cuts a sedimentary feature, structure, or strata is younger than the feature, structure, or strata it cuts.
For instance, sediments may fill a crack or erosional channel and then later lithify, forming a rock. The formed rock is younger than the erosional channel or filled crack.
Minerals can also precipitate in existing gaps filled with ground or surface water. The precipitated minerals that fill the gaps are younger than the gaps or the rocks with the gaps.
5. Paleontological
Paleontological relationships study how living organisms interact, disrupt, or truncate rock layers or strata to determine the order of events. For instance, a sedimentary stratum with an animal burrow indicates the stratum existed at the time burrowing occurred.
6. Geomorphological
Geomorphological relationships pertain to studying how geological landforms evolve and determining the sequence of events. For instance, rocks are older than a river flowing over them and excavated rocks after an asteroid impact are older than the impact event.
Exceptions to the law of cross-cutting relationships
Applying the law of cross-cutting relationships in relative dating requires care to avoid misinterpretations. The feature that appears to intrude on or disrupt another may be older, not younger.
For example, sediments deposited around an outcrop, or a remaining unconformity element can lithify and form a sedimentary rock. The outcrop can appear to cut across or disrupt the sedimentary rock and is assumed to be younger. However, the truth is that the outcrop is older, not younger.
Principle of cross-cutting relationships diagrams
You may have a question testing your ability to determine the sequence of events. Solving such a question requires you to apply the laws of superposition and cross-cutting relationships.
Superposition will tell you which layer in a stratigraphic section is older or younger. The lower is older than the upper, so long as there haven’t been any tectonic activities.
The principle of cross-cutting relationships, on the other hand, gives the sequence of events, i.e., what happened first or last.
It is important to note that the process, geological feature, or rock that disrupts, cuts, or deforms another is the younger of the two. Also, only existing geological features or rock bodies are disrupted, cut, or penetrated.
The relative ages from oldest to youngest are A, B, C, D, E, and F. Folding A was the first to occur, followed by intrusion B, which cut through A. Angular unconformity C is the next event, followed by dike D that cuts through A, B, and C. Strata E then forms before faulting F cuts through A, B, C, D, and E.
Significance
The law or principle of cross-cutting relationships is crucial in relative dating because, by studying the various relationships, one can determine what precedes the other or the sequence.
This principle can also determine the age bracket of a fossil or rock body, which radiometric dating techniques alone can’t. For example, you can know the age bracket of a fossil in unconformity if it has dikes cutting across the upper and lower strata. You will need to radiometrically date the dikes.