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Unconformities in Geology: Definition & Types

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  • 0:00 Unconformity Defined
  • 0:55 Types
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor
Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Expert Contributor
Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

We all know someone who just doesn't conform to our expectations. Believe it or not, rocks can do that, too. Learn about unconformable rocks in this lesson.

Unconformity Defined

An unconformity is the contact between sedimentary rocks that are significantly different in age or between sedimentary rocks and older, eroded igneous or metamorphic rocks. Unconformities represent gaps in the geologic record, periods of time that are not represented by any rocks.

Unconformities happen for two reasons: sediment deposition stopped for a considerable time and/or existing rocks were eroded prior to being covered by younger sediment. There is no single time span represented by an unconformity. It depends on how long erosion occurred or for how long deposition ceased.

Some unconformities are easier to identify than others. For example, the contact between a very old granite and a younger sandstone is pretty obvious. On the other hand, figuring out whether two limestone beds are significantly different in age might require more investigation.

Types of Unconformities

The type of unconformity (in other words, what we call it) is based upon what rock types are involved, whether it is the result of erosion or no deposition, and whether older sedimentary rock layers were tilted prior to being eroded. Let's look at some different types in more detail.

Angular Unconformity

An angular unconformity is the result of erosion of tilted layers of sedimentary rock. The erosion surface is buried under younger, horizontal layers of sedimentary rock. Hutton's Unconformity at Siccar Point, Scotland, is probably the most famous one, where tilted beds of eroded sandstone are covered by horizontal beds of younger sandstone.

Disconformity

A disconformity also involves erosion of sedimentary rocks. But here, the older rock layers were not tilted before they were eroded. In the least obvious case, the disconformity (the erosion surface) is parallel to the layering of both rock layers. Those can be difficult to identify and usually require dating of the rocks using fossils or some other method in order to know with certainty that the two rock layers are substantially different in age.

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Additional Activities

Modeling Unconformities

Making models of scientific phenomenon is helpful to understand the phenomenon. Understanding a phenomenon involves knowing what caused it and how to recognize it in nature. Models are much smaller in size compared to the real thing they represent. We're going to make a few unconformity models using everyday materials.

  1. Gather 2 or 3 medium sized, unused cardboard boxes.
  2. Cut out 6 strips of cardboard about 3 inches wide and 10 inches long. These dimensions are a suggestion; you can make your model as large or as small as you like.
  3. Color or paint the cardboard strips earth-tone colors, such as mustard, grey-green, maroon, etc. If you don't have those colors, do your best to make it look earth-tone like. Don't forget to paint/color at least one of the long edges of each cardboard strip. This will represent the finished side of your unconformity.
  4. Stack 3 of the strips on top of each other and bend them into an arc shape.
  5. With the strips stacked and bent into an arc shape, glue the strips together.
  6. Gather 4 newspaper pages and paint them a reddish color.
  7. Crumble up the rest of the reddish newspaper and put it inside the bowl of the arc. Make sure you have filled the bowl with the newspaper and glue it to the arc.
  8. Carefully cut away a sliver of the arc. The reddish-colored newspaper should be exposed at the top with the sides of the arc still visible. You will need to paint the edges of the cardboard strips that are now exposed. Discard the top piece of the arc you cut off.
  9. With the arc bowl side down, lay the remaining cardboard strips on top of the flat part of the arc and glue them in place.


Follow-Up Questions

  1. What does the red twisted newspaper and crumbled up newspaper represent?
  2. What category of rocks do the cardboard strips represent?
  3. What type of unconformity is this?
  4. Rank the rocks in your model from oldest to youngest. For example, say the oldest rock is the flat rocks at the top, etc.
  5. Write a geologic history of your unconformity. State what happened first and sequentially go through the different events that led to the final representation of the unconformity.

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