relative ages of rocks

law of lateral continuity

Rock layers extend laterally, or out to the sides. They may cover very broad areas, especially if they formed at the bottom of ancient seas. Erosion may have worn away some of the rock, but layers on either side of eroded areas will still match up. Look at the Grand Canyon in Figure 11.8. Its a good example of lateral continuity. You can clearly see the same rock layers on opposite sides of the canyon. The matching rock layers were deposited at the same time, so they are the same age.

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law of original horizontality

Sediments were deposited in ancient seas in horizontal, or flat, layers. If sedimentary rock layers are tilted, they must have moved after they were deposited.

law of crosscutting relationships

Rock layers may have another rock cutting across them, like the igneous rock in Figure 11.9. Which rock is older? To determine this, we use the law of cross-cutting relationships. The cut rock layers are older than the rock that cuts across them.

unconformities

Geologists can learn a lot about Earths history by studying sedimentary rock layers. But in some places, theres a gap in time when no rock layers are present. A gap in the sequence of rock layers is called an unconformity. Look at the rock layers in Figure 11.10. They show a feature called Huttons unconformity. The unconformity was discovered by James Hutton in the 1700s. Hutton saw that the lower rock layers are very old. The upper layers are much younger. There are no layers in between the ancient and recent layers. Hutton thought that the intermediate rock layers eroded away before the more recent rock layers were deposited. Huttons discovery was a very important event in geology! Hutton determined that the rocks were deposited over time. Some were eroded away. Hutton knew that deposition and erosion are very slow. He realized that for both to occur would take an extremely long time. This made him realize that Earth must be much older than people thought. This was a really big discovery! It meant there was enough time for life to evolve gradually.

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laws of stratigraphy

The study of rock strata is called stratigraphy. The laws of stratigraphy can help scientists understand Earths past. The laws of stratigraphy are usually credited to a geologist from Denmark named Nicolas Steno. He lived in the 1600s. The laws are illustrated in Figure 11.6. Refer to the figure as you read about the laws below.

law of superposition

Superposition refers to the position of rock layers and their relative ages. Relative age means age in comparison with other rocks, either younger or older. The relative ages of rocks are important for understanding Earths history. New rock layers are always deposited on top of existing rock layers. Therefore, deeper layers must be older than layers closer to the surface. This is the law of superposition. You can see an example in Figure 11.7.

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divisions of the geologic time scale

The largest blocks of time on the geologic time scale are called eons. Eons are split into eras. Each era is divided into periods. Periods may be further divided into epochs. Geologists may just use early or late. An example is late Jurassic, or early Cretaceous. Figure 11.13 shows you what the geologic time scale looks like.

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using index fossils

Index fossils are commonly used to match rock layers in different places. You can see how this works in Figure

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your place in geologic time

We now live in the Phanerozoic Eon, the Cenozoic Era, the Quaternary Period, and the Holocene Epoch. Phanero- zoic means visible life. During this eon, rocks contain visible fossils. Before the Phanerozoic, life was microscopic. The Cenozoic Era means new life. It encompasses the most recent forms of life on Earth. The Cenozoic is sometimes called the Age of Mammals. Before the Cenozoic came the Mesozoic and Paleozoic. The Mesozoic means middle life. This is the age of reptiles, when dinosaurs ruled the planet. The Paleozoic is old life. Organisms like invertebrates and fish were the most common lifeforms.

life and the geologic time scale

The geologic time scale may include illustrations of how life on Earth has changed. Major events on Earth may also be shown. These include the formation of the major mountains or the extinction of the dinosaurs. Figure 11.14 is a different kind of the geologic time scale. It shows how Earths environment and life forms have changed.

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matching rock layers

When rock layers are in the same place, its easy to give them relative ages. But what if rock layers are far apart? What if they are on different continents? What evidence is used to match rock layers in different places?

widespread rock layers

Some rock layers extend over a very wide area. They may be found on more than one continent or in more than one country. For example, the famous White Cliffs of Dover are on the coast of southeastern England. These distinctive rocks are matched by similar white cliffs in France, Belgium, Holland, Germany, and Denmark (see Figure 11.11). It is important that this chalk layer goes across the English Channel. The rock is so soft that the Channel Tunnel connecting England and France was carved into it!

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dividing geologic time

Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic time scale. So, many of the names of the time periods are from places in Europe. The Jurassic Period is named for the Jura Mountains in France and Switzerland, for example.

key beds

Like index fossils, key beds are used to match rock layers. A key bed is a thin layer of rock. The rock must be unique and widespread. For example, a key bed from around the time that the dinosaurs went extinct is very important. A thin layer of clay was deposited over much of Earths surface. The clay has large amount of the element iridium. Iridium is rare on Earth but common in asteroids. This unusual clay layer has been used to match rock up layers all over the world. It also led to the hypothesis that a giant asteroid struck Earth and caused the dinosaurs to go extinct.

putting events in order

To create the geologic time scale, geologists correlated rock layers. Stenos laws were used to determine the relative ages of rocks. Older rocks are at the bottom and younger rocks are at the top. The early geologic time scale could only show the order of events. The discovery of radioactivity in the late 1800s changed that. Scientists could determine the exact age of some rocks in years. They assigned dates to the time scale divisions. For example, the Jurassic began about 200 million years ago. It lasted for about 55 million years.

the geologic time scale

Earth formed 4.5 billion years ago. Geologists divide this time span into smaller periods. Many of the divisions mark major events in life history.

instructional diagrams

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This diagram represents the cross-cutting relationships of rocks. Layer 1, as shown, is the oldest layer because it is the layer that is the deepest. This is the law of superposition. In the diagram below, dike is the youngest rock layer. This is figured by the law of cross-cutting relationships. The layers are always older than the rock that cuts across them. In the diagram below, dike cuts through all four layers. Therefore, layer 1 is the oldest, layer 2 is the second oldest, layer 3 is the third oldest, layer 4 is the fourth oldest, and dike is the youngest layer of rock.

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The study of rock strata is called stratigraphy. This Diagram is all about the Laws of Stratigraphy. The laws of stratigraphy can help scientists understand Earths past. The relative ages of rocks are important for understanding Earths history. The diagram refers to the position of rock layers and their relative ages, which is called Superposition. New rock layers are always deposited on top of existing rock layers. Therefore, deeper layers must be older than layers closer to the surface. A is the area covered by Law of Cross-Cutting relationships, B is the unconformities, C is the law of Original Horizontality, D is the Law of Conti-unity, E is the law of Superposition. Some rock layers extend over a very wide area. They may be found on more than one continent or in more than one country.

questions

Earths geologic processes have changed over time.

a. true

-->  b. false

A rocks age compared to the ages of other rocks is called its

a. absolute age

b. confirmed age

c. nominal age

-->  d. none of the above

Extinction occurs when a species completely dies out.

-->  a. true

b. false

The Law of Superposition states that

a. younger rocks are found below older rocks

-->  b. older rocks are found below younger rocks

c. a rock that cuts across other rocks must be younger than the rock it cuts across

d. none of the above

Layers of sedimentary rock are called strata.

-->  a. true

b. false

The rock layers at the Grand Canyon

-->  a. are the same on opposite sides of the river.

b. were formed in different ways on each side of the river.

c. are younger than the Colorado River in that region.

d. none of these.

A good key bed must be

-->  a. found over a large area

b. similar to the rock units it is found with

c. a volcanic ash

d. all of these

The relative age of a rock is its approximate age in years.

a. true

-->  b. false

Rock layers on opposite sides of the Grand Canyon show lateral continuity.

-->  a. true

b. false

A good index fossil

a. is found in a local area

-->  b. is distinctive

c. existed for a long period of time

d. all of these

Key beds are rock layers that have unconformities.

a. true

-->  b. false

More than one type of index fossil provides stronger evidence that rock layers are the same age.

-->  a. true

b. false

The Cretaceous Period ended when the first dinosaurs appeared.

a. true

-->  b. false

The earliest geologic time scale showed how many years ago each era began.

a. true

-->  b. false

Fish were common organisms during the Paleozoic Era.

-->  a. true

b. false

Fossil B is younger than Fossil A, but the rock layer containing Fossil B is beneath the rock layer

-->  a. true

b. false

To help decipher the geologic history of a region, create a geologic time scale using the rock units you

a. true

-->  b. false

James Hutton thought Earth was old because he saw how slowly geological processes work now.

-->  a. true

b. false

Cross-cutting relationships help geologists to determine the older and younger of two rock units.

-->  a. true

b. false

In the process of relative dating, scientists determine the exact age of a fossil or rock.

a. true

-->  b. false

whether a rock is older or younger than other rocks

a. stratigraphy

b. superposition

-->  c. relative age

d. lateral continuity

e. original horizontality

f. cross-cutting relationships

g. unconformity

law stating that rock layers are deposited in horizontal layers

a. stratigraphy

b. superposition

c. relative age

d. lateral continuity

-->  e. original horizontality

f. cross-cutting relationships

g. unconformity

law stating that rock layers closer to the surface are younger than deeper rock layers

a. stratigraphy

-->  b. superposition

c. relative age

d. lateral continuity

e. original horizontality

f. cross-cutting relationships

g. unconformity

law stating that rock layers are older than any rocks that cut across them

a. stratigraphy

b. superposition

c. relative age

d. lateral continuity

e. original horizontality

-->  f. cross-cutting relationships

g. unconformity

gap in a sequence of rock layers

a. stratigraphy

b. superposition

c. relative age

d. lateral continuity

e. original horizontality

f. cross-cutting relationships

-->  g. unconformity

law stating that matching nearby rock layers are the same age

a. stratigraphy

b. superposition

c. relative age

-->  d. lateral continuity

e. original horizontality

f. cross-cutting relationships

g. unconformity

study of rock layers

-->  a. stratigraphy

b. superposition

c. relative age

d. lateral continuity

e. original horizontality

f. cross-cutting relationships

g. unconformity

If sedimentary rock layers are tilted, they must have

a. formed at an angle.

-->  b. moved after they formed.

c. been cross-cut by igneous rock.

d. formed from deposits on a mountainside.

A key bed of clay from around the time the dinosaurs went extinct led to the hypothesis that the extinction was caused by a

a. large flood.

b. huge volcano.

-->  c. giant asteroid.

d. none of the above

Evidence shows that Earth is about

a. 1.9 million years old.

b. 2.8 million years old.

c. 3.8 billion years old.

-->  d. 4.5 billion years ol

Eons of the geologic time scale are divided first into

a. years.

b. periods.

-->  c. eras.

d. epochs.

The Cenozoic Era is called the age of

a. dinosaurs.

-->  b. mammals.

c. reptiles.

d. life.

What does the term paleozoic mean?

a. fossil life

b. ancient rock

c. rock strata

-->  d. old life

Many of the divisions of the geologic time scale mark major events in the history of

-->  a. life.

b. science.

c. astronomy.

d. Earth science.

diagram questions

question_image

How many layers of land are there in the picture?

-->  a. 4

b. 2

c. 3

d. 1

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How many parts have been marked in the diagram?

a. 3

-->  b. 2

c. 4

d. 1

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What is a bank or mound of earth that is built to control water and especially to protect an area from flooding?

a. Lake

b. Fault

-->  c. Dike

d. Soil

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What is the age of the rocks on the dike compared to the surrounding rocks?

a. Older

b. Same

c. Almost the same

-->  d. Younger

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How many layers of earth are shown?

a. Six

b. Three

c. One

-->  d. Four

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How many letters are depicted in the diagram?

-->  a. 6

b. 4

c. 7

d. 5

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What is shown in this picture?

a. Volcano Eruption

-->  b. Rock Layers

c. Earth

d. Rain Process

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Where is labelled Law of Superposition?

a. D

b. E

-->  c. A

d. B

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Which letter corresponds to the law of superposition?

-->  a. A

b. C

c. D

d. B

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Which alphabet in the below diagram illustrates the Law of Superposition?

a. B

b. D

-->  c. A

d. C

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Which letter represents a dike?

a. E

-->  b. F

c. C

d. K

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The darkest colored layer is above which layer?

a. A

b. C

-->  c. B

d. D

question_image

How many rock layers are in the diagram?

a. 2

b. 4

-->  c. 3

d. 1

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Which level dips into another level?

a. b

-->  b. c

-->  c. a

d. d

question_image

How many ages of rock are presented in the diagram?

a. 1

b. 2

c. 3

-->  d. 4

question_image

Which are the newest rock layers according to the Law of Superposition?

a. C

b. D

c. B

-->  d. A

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Which layer lies between B and D?

a. D

b. B

c. A

-->  d. C

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Where is G in relationship to B?

a. To the right

b. Next to

-->  c. Above/Below

d. Outside

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What happens when M moves downward fully?

a. C will move to the right

b. B will move downwards also

-->  c. it will cause G to rise

d. G will shrink

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Which law states that the older the rock, the deeper the layer?

-->  a. Law of superposition

b. Law of stratigraphy

c. Law of Reaction

d. Law of motion

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On what color is labeled the law of superposition?

a. Blue

b. Green

c. Yellow

-->  d. Red

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Which is the oldest layer in the diagram?

a. L

-->  b. E

c. A

d. G

question_image

How many labels are shown in the diagram below?

a. 11

-->  b. 12

c. 13

d. 10

question_image

This Law pertains to the study of Rock Strata?

-->  a. Stratigraphy

b. Sedimentation

c. Superimposition

d. Superposition