"

7.2 Stratigraphic Principles and Missing Time

Rock Layers at Quebrada de las Conchos - Salta Argentina.jpg
Figure 7.2.1 Rock layers at Quebrada de las Conchos, Salta Argentina. Image by rodoluca, Wikimedia Commons, CC BY-SA 3.0
Folded Himalayan Rock Layers near Gushal in India
Figure 7.2.2 Folded Himalayan Rock Layers near Gushal in India. Image by Kiran Jonnalagadda. CC BY-SA 2.0.

When geologists see a stack of rock layers, it is like a book of pages telling a story of the Earth’s history. However, sometimes, the Earth’s layers are not neatly stacked in order, but all twisted and folded. How can geologists figure out the order of the “pages” of Geologic history, or determine the order in which each layer of rock was deposited on our planet?

Stratigraphic Principles

We can rely on the principles of Relative Dating. Relative dating is a method that determines the order in which a sequence of rocks was deposited, from oldest to youngest. Even the most lopsided and twisted set of rocks can be interpreted using these principles, which is why so many of them are scientific “Laws” – they have been proven and are scientifically irrefutable.

The principles you are about to review were first developed in the 17th century by Nicholas Steno, a Danish Scientist who paid special attention to stratigraphy, or the ordering of rock layers. These are called Steno’s laws.

Look at the image describing each one of these laws, and think back to what you have already read about them in the assigned text. You will then determine the definition of each law in the exercise below.

The Law of Superposition

Horizontal rock layers where the oldest rocks are at the bottom and the youngest are at the top.
Figure 7.2.3 Law of Superposition. Image by Charlene Estrada, Public Domain.

The Law of Original Horizontality

Law of Original Horizontality where slanted or folded rocks were once laid down horizontally
Figure 7.2.4 Law of Original Horizontality. Image by Charlene Estrada, Public Domain

The Law of Inclusions

Law of Inclusions, where if a rock has pieces of another rock, that other rock had to have been deposited first.
Figure 7.2.5 Law of Inclusions. Image by Charlene Estrada. Public Domain.

Cross-cutting Relationships

Cross-cutting relationships, where if there is an intruding body in the rock, that intruding body must be younger than the rock it's intruding upon.
Figure 7.2.6 Cross-cutting relationships. Image by Charlene Estrada, Public Domain.

Law of Lateral Continuity

Law of Lateral Continuity, where horizontal rock layers can be continuous, but may be broken up by erosion, unconformities, or other events.
Figure 7.2.7 Law of Lateral Continuity. Image by Charlene Estrada, Public Domain.

Unconformities

Unconformities are sections of “missing time” in the rock record. To use our book analogy, if the layers of rock are pages, then unconformities are sections in the book where it is obvious that someone ripped out a large section of pages leaving scientists to wonder what happened during that missing part of the story. Sometimes it is obvious where in the rock record an unconformity has occurred because it can represent a huge section of missing time between two rock layers. Other times, it only represents a tiny amount of missing time, and can be easily overlooked.

Unconformity, where there are two different types of rock in contact with one another with an uneven line of erosion between them.
Figure 7.2.8 Unconformity at Sonoita, Arizona. Image by Mike Conway, AZGS, CC BY.

How does an unconformity form, anyway? Remember weathering and erosion? Erosion is the movement of weathered material by wind and water away from its original source. Sometimes a section of rock erodes so efficiently that there is almost nothing left of it. Eventually, a new rock will get deposited, and that old section is all but lost. Geologists have to look carefully for signs of erosion in between the rock layers to tell if there are unconformities.

There are four types of unconformities. Look at the images of them below and your notes from the assigned reading. Then answer the activity!

Nonconformity

Nonconformity, where there is contact between a non-sedimentary rock and a sedimentary rock
Figure 7.2.9 Nonconformity. Image by Charlene Estrada, Public Domain.

Angular Unconformity

Angular Unconformity, where rock layers come into contact with other layers that are tilted or folded at different angles.
Figure 7.2.10 Angular Unconformity. Image by Charlene Estrada, Public Domain.

Disconformity

Disconformity, where there is missing time or erosion between two units of sedimentary rocks.
Figure 7.2.11 Disconformity. Image by Charlene Estrada, Public Domain.

Paraconformity

Paraconformity, where sedimentation stops suddenly, and then resumes millions of years later.
Figure 7.2.12 Paraconformity. Image by Charlene Estrada, Public Domain.

NOTE: Make sure that you scroll through all three question slides and click the “Finish” button to report your score, or else you won’t get points!

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Exploring Physical Geology Lab Online Copyright © 2022 by Charlene Estrada is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book