What features must the iridium layer at the time of the dinosaur extinction need to be a key bed?

Lesson Objectives

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

Vocabulary

biozone
cross-cutting relationships
geologic time 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 alter. They are the same today as they were billions of years ago. Water freezes at 0° C at 1 temper pressure level; this is always truthful.

Knowing that natural laws never change helps scientists sympathise Earth'due south past because information technology allows them to interpret clues virtually how things happened long ago. Geologists always use nowadays-solar day processes to interpret the by. If y'all find a fossil of a fish in a dry terrestrial environment did the fish flop effectually on land? Did the rock class in water and then move? Since fish practise not flop around on land today, the explanation that adheres to the philosophy that natural laws exercise non alter is that the rock moved.

Fossils were Living Organisms

In 1666, a immature physician named Nicholas Steno dissected the head of an enormous keen white shark that had been defenseless by fisherman near Florence, Italy. Steno was struck past the resemblance of the shark's teeth to fossils plant in inland mountains and hills (Figure below).

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

Most people at the time did not believe that fossils were in one case part of living creatures. Authors in that twenty-four hours idea that the fossils of marine animals plant in tall mountains, miles from any ocean could be explained in i of two means:

The shells were washed up during the Biblical flood. (This caption could not account for the fact that fossils were not just establish on mountains, but besides within mountains, in rocks that had been quarried from deep beneath Earth's surface.)
The fossils formed inside the rocks as a result of mysterious forces.

Just for Steno, the close resemblance betwixt fossils and modernistic organisms was impossible to ignore. Instead of invoking supernatural forces, Steno concluded that fossils were once parts of living creatures. He then sought to explicate how fossil seashells could be found in rocks and mountains far from whatsoever ocean. This led him to the ideas that are discussed below.

Superposition of Rock Layers

Steno proposed that if a rock independent the fossils of marine animals, the stone formed from sediments that were deposited on the seafloor. These rocks were and so uplifted to become mountains. Based on these assumptions, Steno fabricated a remarkable series 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 found tilted, the layer was tilted after it was formed. (b) Lateral continuity: Sediments are deposited in continuous sheets that span the torso of water that they are deposited in. When a valley cuts through sedimentary layers, it is causeless that the rocks on either side of the valley were originally continuous. (c) Superposition: Sedimentary rocks are deposited i on top of some other. The youngest layers are found at the top of the sequence, and the oldest layers are found at the lesser.

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 plant with certain other fossil types (eastward.g. human ancestors are never found with dinosaurs) meaning that fossils in a rock layer represent what lived during the menses the rock was deposited.
Older features are replaced by more modern features in fossil organisms equally species change through time; e.g. feathered dinosaurs precede birds in the fossil record.
Fossil species with features that change distinctly and speedily can be used to determine the historic period of rock layers quite precisely.

Scottish geologist, James Hutton (1726-1797) recognized the principle of cantankerous-cutting relationships. This helps geologists to make up one's mind the older and younger of two stone units (Figure beneath).

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

The K Canyon provides an excellent illustration of the principles above. The many horizontal layers of sedimentary rock illustrate the principle of original horizontality (Effigy below).

The youngest rock layers are at the meridian and the oldest are at the bottom, which is described by the law of superposition.
Distinctive rock layers, such as the Coconino Sandstone, are matched across the wide expanse of the canyon. These rock layers were one time continued, as stated by the rule of lateral continuity.
The Colorado River cuts through all the layers of rock to class the coulee. Based on the principle of cross-cutting relationships, the river must be younger than all of the rock layers that it cuts through.

At the Grand Canyon, the Coconino Sandstone appears across canyons. The Coconino is the distinctive white layer; information technology is a vast surface area of ancient sand dunes.

Determining the Relative Ages of Rocks

Steno'southward and Smith'southward principles are essential for determining the relative ages of rocks and stone layers. In the process of relative dating, scientists do not determine the exact historic period of a fossil or rock simply wait at a sequence of rocks to try to decipher the times that an effect occurred relative to the other events represented in that sequence. The relative age of a rock and so is its age in comparison with other rocks. If you know the relative ages of 2 stone layers, (1) Do you know which is older and which is younger? (2) Do y'all know how old the layers are in years?

An interactive website on relative ages and geologic time is found here: http://www.ucmp.berkeley.edu/didactics/explorations/tours/geotime/gtpage1.html

In some cases, information technology is very catchy to determine the sequence of events that leads to a certain formation. Can you lot figure out what happened in what order in (Figure below)? Write it downwards and so check the following paragraphs.

A geologic cross section: Sedimentary rocks (A-C), igneous intrusion (D), mistake (Due east).

The principle of cantankerous-cut relationships states that a fault or intrusion is younger than the rocks that information technology cuts through. The error cuts through all three sedimentary rock layers (A, B, and C) and besides the intrusion (D). So the mistake must exist the youngest feature. The intrusion (D) cuts through the 3 sedimentary rock layers, and so it must be younger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is notwithstanding younger.

The full sequence of events is:

i. Layer C formed.

2. Layer B formed.

3. Layer A formed.

4. After layers A-B-C were nowadays, intrusion D cutting across all 3.

five. Fault E formed, shifting rocks A through C and intrusion D.

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

Earth's Age

During Steno's time, most Europeans believed that the World was around 6,000 years old, a effigy that was based on the amount of fourth dimension estimated for the events described in the Bible. Ane of the beginning scientists to question this assumption and to empathise geologic time was James Hutton. Hutton traveled around Great United kingdom in the late 1700s, studying sedimentary rocks and their fossils (Figure below).

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

Often described as the founder of modern geology, Hutton formulated uniformitarianism: The nowadays is the key to the past. Co-ordinate to uniformitarianism, the same processes that operate on Earth today operated in the by every bit well. Why is an acceptance of this principle absolutely essential for u.s.a. to exist able to decipher Earth history?

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

Hutton's Unconformity on the Coast of Scotland. Can you discover the unconformity? What are the geological events that you can notice in this image? (Hint: There are 9.)

1. A series of sedimentary beds was deposited on an ocean flooring.

two. The sediments hardened into sedimentary rock.

3. The sedimentary rocks are uplifted and tilted, exposing them above body of water level.

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

5. A bounding main 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 body of water surface.

Since he thought that the same processes at piece of work on Earth today worked at the same rate in the by, he had to account for all of these events and the unknown corporeality of missing time represented by the unconformity, Hutton realized that this rock sequence lonely represented a bully deal of time. He ended that Earth's age should non be measured in thousands of years, merely in millions of years.

Matching Up Rock Layers

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

ane. Distinctive rock formations may be recognizable across large regions (Figure beneath).

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

ii. 2 separated rock units with the aforementioned index fossil are of very similar historic period. What traits practise yous remember an index fossil should have? To become an index fossil the organism must take (1) been widespread and then that it is useful for identifying stone layers over large areas and (two) existed for a relatively brief period of time and so that the approximate historic period of the rock layer is immediately known.

Many fossils may authorize as index fossils (Figure beneath). Ammonites, trilobites, and graptolites are often used as index fossils.

Several examples of index fossils are shown here. Mucrospirifer mucronatus is an alphabetize fossil that indicates that a stone was laid down from 416 to 359 million years agone.

Microfossils, which are fossils of microscopic organisms, are also useful alphabetize fossils. Fossils of animals that drifted in the upper layers of the bounding main are peculiarly useful as alphabetize fossils, since they may exist distributed over very big areas.

A biostratigraphic unit of measurement, or biozone, is a geological stone layer that is divers by a single index fossil or a fossil assemblage. A biozone can as well exist used to place stone layers across distances.

3. A fundamental bed tin be used similar an index fossil since a fundamental bed is a distinctive layer of stone that can be recognized across a large area. A volcanic ash unit could be a good key bed. I famous primal bed is the clay layer at the boundary between the Cretaceous Period and the Tertiary Period, the time that the dinosaurs went extinct (Figure below). This thin clay contains a high concentration of iridium, an element that is rare on Earth but common in asteroids. In 1980, the male parent-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 clay is a primal bed that marks the Cretaceous-Tertiary Boundary.

The Geologic Time Scale

To be able to discuss Earth history, scientists needed some way to refer to the time periods in which events happened and organisms lived. With the information they collected from fossil evidence and using Steno's principles, they created a list of stone layers from oldest to youngest. Then they divided Earth's history into blocks of time with each block separated past important events, such as the disappearance of a species of fossil from the rock tape. Since many of the scientists who first assigned names to times in Globe'south history were from Europe, they named the blocks of time from towns or other local places where the rock layers that represented that fourth dimension were establish.

From these blocks of time the scientists created the geologic time scale (Figure below). In the geologic fourth dimension scale the youngest ages are on the superlative and the oldest on the lesser. Why do y'all think that the more than recent time periods are divided more finely? Practice you call up the divisions in the Figure below are proportional to the amount of time each time catamenia represented in Globe history?

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

In what eon, era, menstruation and epoch do we at present live? We alive in the Holocene (sometimes called Contempo) epoch, Quaternary period, Cenozoic era, and Phanerozoic eon.

Lesson Summary

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

Review Questions

A 15th century farmer finds a rock that looks exactly like a clamshell. What did he likely conclude about how the fossil got in that location?
Which of Steno's Laws is illustrated by each of the images in Effigy below?
What is the sequence of stone units in Figure below, from oldest to youngest?
What kind of geological germination is shown in the outcrop in Figure below, and what sequence of events does information technology represent?
The iii outcrops in Figure below are very far apart. Based on what you run across, which fossil is an index fossil, and why?
Why didn't the early on geologic fourth dimension scale include the number of years ago that events happened?
Dinosaurs went extinct about 66 million years ago. Which period of geologic time was the last in which dinosaurs lived?
Suppose that while you're hiking in the mountains of Utah, you find a fossil of an animal that lived on the ocean floor. You learn that the fossil is from the Mississippian menstruation. What was the environment like during the Mississippian in Utah?
Why are sedimentary rocks more than useful than metamorphic or igneous rocks in establishing the relative ages of rock?
Which is likely to be more often found in rocks: fossils of very erstwhile sea creatures or very old country creatures?

Farther Reading / Supplemental Links

A The states Geological Survey paper on Rocks, Fossils and Time: http://pubs.usgs.gov/gip/fossils/contents.html
Try to guess the mystery fossils in these pictures and see if you're right. At that place are more in the archives: http://world wide web.ucmp.berkeley.edu/exhibits/mysteryfossil/mysteryfossil.php
An interactive "Virtual Museum of Fossils" http://fossils.valdosta.edu/.
The fossil tape in North America http://world wide web.paleoportal.org/.
An excerpt of the book "The Seashell on the Mountaintop" is found hither http://alan-cutler.com/excerpt.html
Determining the ages of rocks and fossils: http://www.ucmp.berkeley.edu/fosrec/McKinney.html

Points to Consider

How did preconceived ideas in Steno's fourth dimension make people bullheaded to the reality of what fossils stand for?
How did Steno explain the presence of marine fossils in high mountains?
Why was Hutton'due south recognition of unconformities so pregnant?
Tin can the relative ages of ii rock layers that are very far apart be determined?
Tin can the same principles used to study Globe'south history too be used to report the history of other planets?

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