What Can Be Used to Determine the Relative Age of Two Rocks

Lesson Objectives

• Explain Steno's laws of superposition and original horizontality.
• Based on a geological cross-department, identify the oldest and youngest formations.
• Explicate what an unconformity represents.
• Know how to use fossils to correlate rock layers.

Vocabulary

• biozone
• cross-cut relationships
• geologic fourth dimension scale
• key bed
• lateral continuity
• microfossil
• original horizontality
• relative age
• superposition
• unconformity
• uniformitarianism

Introduction

Something that we hope you accept learned from these lessons and from your own life experience is that the laws of nature never change. They are the same today as they were billions of years agone. Water freezes at 0° C at 1 atmosphere pressure; this is always true.

Knowing that natural laws never modify helps scientists empathize Earth's past because information technology allows them to interpret clues about how things happened long ago. Geologists always use present-day processes to translate the past. If you find a fossil of a fish in a dry terrestrial environment did the fish flop effectually on country? Did the stone class in water and and then motility? Since fish do not flop around on state today, the explanation that adheres to the philosophy that natural laws do not change is that the rock moved.

Fossils were Living Organisms

In 1666, a young doctor named Nicholas Steno dissected the caput of an enormous great white shark that had been defenseless by fisherman about Florence, Italy. Steno was struck by the resemblance of the shark's teeth to fossils found in inland mountains and hills (Figure beneath).

Fossil Shark Tooth (left) and Modern Shark Tooth (correct).

Most people at the time did not believe that fossils were once function of living creatures. Authors in that 24-hour interval idea that the fossils of marine animals plant in tall mountains, miles from any ocean could be explained in one of two ways:

• The shells were washed upward during the Biblical flood. (This explanation could not business relationship for the fact that fossils were not only found on mountains, just too inside mountains, in rocks that had been quarried from deep beneath Earth'southward surface.)
• The fossils formed within the rocks equally a result of mysterious forces.

But for Steno, the shut resemblance between fossils and modern organisms was impossible to ignore. Instead of invoking supernatural forces, Steno concluded that fossils were in one case parts of living creatures. He so sought to explain how fossil seashells could exist found in rocks and mountains far from whatever ocean. This led him to the ideas that are discussed below.

Superposition of Rock Layers

Steno proposed that if a stone contained the fossils of marine animals, the rock formed from sediments that were deposited on the seafloor. These rocks were then uplifted to become mountains. Based on these assumptions, Steno fabricated a remarkable serial of conjectures that are at present known as Steno's Laws. These laws are illustrated in Figure below.

(a) Original Horizontality: Sediments are deposited in fairly flat, horizontal layers. If a sedimentary rock is institute tilted, the layer was tilted afterwards it was formed. (b) Lateral continuity: Sediments are deposited in continuous sheets that bridge the torso of water that they are deposited in. When a valley cuts through sedimentary layers, information technology is assumed that the rocks on either side of the valley were originally continuous. (c) Superposition: Sedimentary rocks are deposited one on height of some other. The youngest layers are institute at the top of the sequence, and the oldest layers are plant at the bottom.

Other scientists observed rock layers and formulated other principles. Geologist William Smith (1769-1839) identified the principle of faunal succession, which recognizes that:

• Some fossil types are never found with certain other fossil types (e.g. human ancestors are never found with dinosaurs) significant that fossils in a stone layer represent what lived during the period the rock was deposited.
• Older features are replaced by more modernistic features in fossil organisms as species change through time; e.g. feathered dinosaurs precede birds in the fossil tape.
• Fossil species with features that modify distinctly and quickly can be used to determine the historic period of stone layers quite precisely.

Scottish geologist, James Hutton (1726-1797) recognized the principle of cross-cutting relationships. This helps geologists to determine the older and younger of two rock units (Figure beneath).

If an igneous dike (B) cuts a serial of metamorphic rocks (A), which is older and which is younger? In this prototype, A must accept existed first for B to cut across it.

The Grand Canyon provides an first-class illustration of the principles higher up. The many horizontal layers of sedimentary rock illustrate the principle of original horizontality (Figure below).

• The youngest rock layers are at the top and the oldest are at the lesser, which is described by the law of superposition.
• Distinctive rock layers, such every bit the Coconino Sandstone, are matched beyond the wide expanse of the canyon. These rock layers were one time connected, as stated past the rule of lateral continuity.
• The Colorado River cuts through all the layers of rock to grade the canyon. Based on the principle of cantankerous-cutting relationships, the river must be younger than all of the rock layers that it cuts through.

At the G Canyon, the Coconino Sandstone appears across canyons. The Coconino is the distinctive white layer; it is a vast expanse of ancient sand dunes.

Determining the Relative Ages of Rocks

Steno's and Smith's principles are essential for determining the relative ages of rocks and stone layers. In the procedure of relative dating, scientists do non determine the exact age of a fossil or stone only look at a sequence of rocks to endeavour to decipher the times that an event occurred relative to the other events represented in that sequence. The relative age of a stone then is its historic period in comparison with other rocks. If you know the relative ages of 2 rock layers, (1) Do yous know which is older and which is younger? (ii) Do you lot know how old the layers are in years?

An interactive website on relative ages and geologic time is constitute hither: http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/gtpage1.html

In some cases, it is very tricky to determine the sequence of events that leads to a certain formation. Can you figure out what happened in what gild in (Figure below)? Write it down and and so check the post-obit paragraphs.

A geologic cross section: Sedimentary rocks (A-C), igneous intrusion (D), fault (E).

The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. The fault cuts through all three sedimentary rock layers (A, B, and C) and also the intrusion (D). So the fault must exist the youngest feature. The intrusion (D) cuts through the iii sedimentary rock layers, and then information technology must be younger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is still younger.

The full sequence of events is:

1. Layer C formed.

2. Layer B formed.

3. Layer A formed.

four. Afterwards layers A-B-C were nowadays, intrusion D cutting beyond all three.

v. Mistake Due east formed, shifting rocks A through C and intrusion D.

six. Weathering and erosion created a layer of soil on top of layer A.

Earth'south Age

During Steno's time, virtually Europeans believed that the Earth was around half-dozen,000 years old, a figure that was based on the corporeality of time estimated for the events described in the Bible. One of the first scientists to question this supposition and to empathize geologic time was James Hutton. Hutton traveled around Great Britain in the late 1700s, studying sedimentary rocks and their fossils (Figure below).

A cartoon by James Hutton. "Theory of the Earth," 1795.

Often described every bit the founder of modern geology, Hutton formulated uniformitarianism: The present is the key to the past. According to uniformitarianism, the same processes that operate on World today operated in the by likewise. Why is an acceptance of this principle absolutely essential for us to exist able to decipher Earth history?

Hutton questioned the age of the Earth when he looked at rock sequences like the one below. On his travels, he discovered places where sedimentary rock beds prevarication on an eroded surface. At this gap in stone layers, or unconformity, some rocks were eroded abroad. For case, consider the famous unconformity at Siccar Betoken, on the coast of Scotland (Figure below).

Hutton'southward Unconformity on the Coast of Scotland. Tin you detect the unconformity? What are the geological events that you can notice in this image? (Hint: There are nine.)

i. A series of sedimentary beds was deposited on an ocean floor.

two. The sediments hardened into sedimentary rock.

3. The sedimentary rocks are uplifted and tilted, exposing them above bounding main level.

4. The tilted beds were eroded to form an irregular surface.

5. A sea covered the eroded sedimentary rock layers.

6. New sedimentary layers were deposited.

7. The new layers hardened into sedimentary rock.

8. The whole rock sequence was tilted.

9. Uplift occurred, exposing the new sedimentary rocks above the ocean surface.

Since he thought that the same processes at work on Globe today worked at the aforementioned charge per unit in the by, he had to business relationship for all of these events and the unknown amount of missing time represented past the unconformity, Hutton realized that this rock sequence solitary represented a groovy deal of fourth dimension. He concluded that Earth's age should not be measured in thousands of years, but in millions of years.

Matching Upwardly Rock Layers

Superposition and cross-cutting are helpful when rocks are touching one another and lateral continuity helps match upwardly rock layers that are nearby, but how do geologists correlate rock layers that are separated by greater distances? At that place are three kinds of clues:

one. Distinctive rock formations may be recognizable across big regions (Effigy below).

The famous White Cliffs of Dover in southwest England can be matched to similar white cliffs in Denmark and Germany.

2. Ii separated rock units with the same index fossil are of very like age. What traits do you think an index fossil should take? To become an index fossil the organism must take (i) been widespread so that it is useful for identifying stone layers over large areas and (2) existed for a relatively cursory menstruum of fourth dimension so that the approximate age of the rock layer is immediately known.

Many fossils may qualify every bit alphabetize fossils (Effigy below). Ammonites, trilobites, and graptolites are often used as index fossils.

Several examples of index fossils are shown here. Mucrospirifer mucronatus is an index fossil that indicates that a rock was laid downward from 416 to 359 one thousand thousand years ago.

Microfossils, which are fossils of microscopic organisms, are besides useful index fossils. Fossils of animals that drifted in the upper layers of the ocean are particularly useful every bit index fossils, since they may exist distributed over very big areas.

A biostratigraphic unit of measurement, or biozone, is a geological rock layer that is defined past a single index fossil or a fossil aggregation. A biozone tin can besides be used to place rock layers across distances.

3. A key bed can be used like an index fossil since a key bed is a distinctive layer of stone that can exist recognized across a large surface area. A volcanic ash unit could exist a good key bed. One famous key bed is the clay layer at the purlieus betwixt the Cretaceous Catamenia and the 3rd Period, the time that the dinosaurs went extinct (Figure below). This thin dirt contains a high concentration of iridium, an element that is rare on World but mutual in asteroids. In 1980, the father-son team of Luis and Walter Alvarez proposed that a huge asteroid struck Earth 66 million years ago and caused the mass extinction.

The white dirt is a central bed that marks the Cretaceous-Tertiary Purlieus.

The Geologic Time Scale

To be able to hash out Globe history, scientists needed some way to refer to the time periods in which events happened and organisms lived. With the data they collected from fossil testify and using Steno's principles, they created a listing of rock layers from oldest to youngest. Then they divided Earth's history into blocks of time with each cake separated by important events, such as the disappearance of a species of fossil from the rock record. Since many of the scientists who kickoff assigned names to times in Earth'south history were from Europe, they named the blocks of fourth dimension from towns or other local places where the stone layers that represented that time were found.

From these blocks of time the scientists created the geologic time scale (Figure below). In the geologic time scale the youngest ages are on the peak and the oldest on the lesser. Why do you lot think that the more contempo time periods are divided more than finely? Do you recollect the divisions in the Figure beneath are proportional to the corporeality of fourth dimension each fourth dimension period represented in World history?

The geologic fourth dimension scale is based on relative ages. No actual ages were placed on the original fourth dimension scale.

In what eon, era, period and epoch do we now live? We live in the Holocene (sometimes called Recent) epoch, Quaternary period, Cenozoic era, and Phanerozoic eon.

Lesson Summary

• Nicholas Steno formulated the principles in the 17th century that permit scientists to determine the relative ages of rocks. Steno stated that sedimentary rocks are formed in continuous, horizontal layers, with younger layers on pinnacle of older layers.
• William Smith and James Hutton later discovered the principles of cross-cutting relationships and faunal succession.
• Hutton also realized the vast amounts of time that would be needed to create an unconformity and concluded that Earth was much older than people at the time idea.
• The guiding philosophy of Hutton and geologists who came after him is: The present is the key to the past.
• To correlate rock layers that are separated by a large distance look for sedimentary rock formations that are extensive and recognizable, index fossils, and central beds.
• Changes of fossils over time led to the evolution of the geologic time scale, which illustrates the relative order in which events on Earth have happened.

Review Questions

1. A 15th century farmer finds a rock that looks exactly like a clamshell. What did he probable conclude nearly how the fossil got at that place?
2. Which of Steno's Laws is illustrated by each of the images in Effigy below?
   
3. What is the sequence of rock units in Figure below, from oldest to youngest?
   
4. What kind of geological germination is shown in the outcrop in Figure below, and what sequence of events does it represent?
   
5. The 3 outcrops in Figure beneath are very far autonomously. Based on what you see, which fossil is an index fossil, and why?
   
6. Why didn't the early geologic time scale include the number of years agone that events happened?
7. Dinosaurs went extinct about 66 million years ago. Which period of geologic fourth dimension was the last in which dinosaurs lived?
8. Suppose that while yous're hiking in the mountains of Utah, yous observe a fossil of an animal that lived on the ocean floor. You learn that the fossil is from the Mississippian period. What was the environment like during the Mississippian in Utah?
9. Why are sedimentary rocks more useful than metamorphic or igneous rocks in establishing the relative ages of rock?
10. Which is likely to be more frequently found in rocks: fossils of very old sea creatures or very one-time land creatures?

Further Reading / Supplemental Links

• A US Geological Survey paper on Rocks, Fossils and Fourth dimension: http://pubs.usgs.gov/gip/fossils/contents.html
• Endeavor to guess the mystery fossils in these pictures and run across if you're right. In that location are more than in the athenaeum: http://www.ucmp.berkeley.edu/exhibits/mysteryfossil/mysteryfossil.php
• An interactive "Virtual Museum of Fossils" http://fossils.valdosta.edu/.
• The fossil record in N America http://world wide web.paleoportal.org/.
• An excerpt of the book "The Seashell on the Mountaintop" is found here http://alan-cutler.com/excerpt.html
• Determining the ages of rocks and fossils: http://world wide web.ucmp.berkeley.edu/fosrec/McKinney.html

Points to Consider

• How did preconceived ideas in Steno's time make people blind to the reality of what fossils represent?
• How did Steno explain the presence of marine fossils in loftier mountains?
• Why was Hutton's recognition of unconformities so meaning?
• Can the relative ages of two stone layers that are very far apart be determined?
• Tin the same principles used to study Globe's history also be used to study the history of other planets?

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Source: https://courses.lumenlearning.com/sanjac-earthscience/chapter/relative-ages-of-rocks/

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