Types of Unconformities 40

The Rock Record<br />

TEACHING TRANSPARENCY<br />

<strong>Types</strong> <strong>of</strong> <strong>Unconformities</strong> <strong>40</strong><br />

<strong>Types</strong> <strong>of</strong> <strong>Unconformities</strong><br />

Type Example Description<br />

Nonconformity<br />

Angular<br />

unconformity<br />

Disconformity<br />

Unstratified igneous or metamorphic rock<br />

may be uplifted to Earth’s surface by crustal<br />

movements. Once the rock is exposed, it<br />

erodes. Sediments may then be deposited on<br />

the eroded surface. The boundary between<br />

the new sedimentary rock and the igneous<br />

or metamorphic rock is a nonconformity. The<br />

boundary represents an unknown period <strong>of</strong><br />

time during which the older rock was eroded.<br />

An angular unconformity forms when rock<br />

deposited in horizontal layers is folded or tilted<br />

and then eroded. When erosion stops, a new<br />

horizontal layer is deposited on top <strong>of</strong> a tilted<br />

layer. When the bedding planes <strong>of</strong> the older<br />

rock layers are not parallel to those <strong>of</strong> the<br />

younger rock layers deposited above them, an<br />

angular unconformity results.<br />

Sometimes, layers <strong>of</strong> sediments are uplifted<br />

without folding or tilting and are eroded.<br />

Eventually, the area subsides and deposition<br />

resumes. The layers on either side <strong>of</strong> the<br />

boundary are nearly horizontal. Although<br />

the rock layers look as if they were deposited<br />

continuously, a large time gap exists where<br />

the upper and lower layers meet. This gap is<br />

known as a disconformity.<br />

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Name Class Date<br />

Transparency Worksheet<br />

<strong>Types</strong> <strong>of</strong> <strong>Unconformities</strong><br />

1. How do noncomformities differ from angular conformities and<br />

disconformities?<br />

2. Why do unconformities represent a break in the geologic record?<br />

3. In a nonconformity, what types <strong>of</strong> rock underlie the sedimentary rock?<br />

4. Which type <strong>of</strong> unconformity do you think is the most difficult to detect?<br />

Explain your answer.<br />

5. The geologic processes <strong>of</strong> uplifting and folding would most likely precede<br />

which type <strong>of</strong> unconformity?<br />

Copyright © by Holt, Rinehart and Winston. All rights reserved.<br />

Holt Earth Science<br />

The Rock Record

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ANSWER KEY<br />

2. the turbine<br />

3. a generator<br />

4. Answers may vary. Sample answer: The<br />

turbine spins rapidly to generate electricity.<br />

Fish would have to pass through<br />

it to get past the dam.<br />

5. Answers may vary. Sample answer:<br />

Yes, but a smaller amount <strong>of</strong> electricity<br />

would be generated.<br />

38 Wind Power in the United States<br />

1. WY and AK<br />

2. CA and WA<br />

3. TX<br />

4. the Rocky Mountains<br />

5. TX<br />

6. The comparison means that these<br />

states could be a major source <strong>of</strong> wind<br />

energy, much as Saudi Arabia is a<br />

major source <strong>of</strong> oil.<br />

39 Law <strong>of</strong> Superposition<br />

1. sedimentary rock<br />

2. that the rock layer is underformed<br />

3. Sedimentary layers will always be<br />

deposited on top <strong>of</strong> metamorphic rock<br />

or large igneous rock masses. Igneous<br />

rock from volcanic eruptions can be<br />

deposited as part <strong>of</strong> the rock sequence.<br />

4. Layer B is older than layer C, because<br />

the law <strong>of</strong> superposition dictates that<br />

an underformed rock layer is older<br />

than the layers above it.<br />

<strong>40</strong> <strong>Types</strong> <strong>of</strong> <strong>Unconformities</strong><br />

1. Nonconformities involve layers <strong>of</strong><br />

sedimentary rock that form on top <strong>of</strong><br />

unstratified rock. Angular unconformities<br />

and disconformities both involve<br />

sedimentary rock layers that form on<br />

top <strong>of</strong> stratified rock.<br />

2. <strong>Unconformities</strong> indicate that a rock<br />

layer was exposed to erosion for a<br />

period <strong>of</strong> time before the overlying sedimentary<br />

layers were deposited. This<br />

erosional period creates a break in the<br />

geologic record.<br />

3. metamorphic or igneous rock<br />

4. a disconformity, because all <strong>of</strong> the rock<br />

layers are stratified and horizontal<br />

5. an angular unconformity<br />

41 Crosscutting Relationships<br />

1. the fault, because the law <strong>of</strong> crosscutting<br />

relationships states that the fault is<br />

younger than the rock it cuts<br />

2. Faulting must have occurred most<br />

recently because it cuts through all the<br />

layers <strong>of</strong> rock and the intrusion.<br />

3. The igneous intrusion is younger<br />

because it cuts through layer C.<br />

4. No. Either could have formed after<br />

layer C was deposited but before faulting<br />

occurred.<br />

42 Radioactive Decay and Half-Life<br />

1. In beta decay, a neutron gives <strong>of</strong>f a<br />

beta particle. In alpha decay, an alpha<br />

particle consisting <strong>of</strong> two protons and<br />

two neutrons is emitted.<br />

2. The mass <strong>of</strong> the atom decreases<br />

because the atom loses two protons<br />

and two neutrons that make up the<br />

alpha particle.<br />

3. 4<br />

4. one-half<br />

5. after one half-life<br />

43 Geologic Map <strong>of</strong> Bedrock in Ohio<br />

1. the Permian, Pennsylavanian,<br />

Mississippian, devonian, Silurian, and<br />

Ordovician<br />

2. The youngest bedrock is in the southeastern<br />

part <strong>of</strong> the state; the oldest bedrock<br />

is in the western part <strong>of</strong> the state.<br />

3. In traveling east to west, the bedrock<br />

would generally become older.<br />

4. The bedrock layers are tilted upwards.<br />

The more horizontal surface <strong>of</strong> earth<br />

cuts across the angled layers and thus<br />

exposes them. If the layers were horizontal,<br />

only the top, or youngest layer<br />

wuld be exposed.<br />

5. because Mississippian rock was formed<br />

in the Mississippian Period just after the<br />

rock formed in the Devonian Period<br />

6. To find early reptile fossils you’d look<br />

in Pennsylvanian rock, represented by<br />

the color blue, which occurs in a large<br />

stripe extending to the southwest from<br />

eastern Ohio.<br />

44 The Geologic Time Scale<br />

1. More recent rocks have been altered or<br />

eroded less than older rocks have and<br />

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Holt Earth Science 12 Answer Key