Day 2 - Due 10/28(A) 10/29(B) |
2.2 - Geologic History & Mapping |
AT HOME - Complete the Reading Assignment and watch the notes videos before coming to class
The Fossil Record
Fossils are the preserved remains or traces of organisms that lived in the past. The soft parts of organisms almost always decompose quickly after death. On occasion, the hard parts—mainly bones, teeth, or shells—remain long enough to mineralize and form fossils. An example of a complete fossil skeleton is shown in Figure below . The fossil record is the record of life that unfolded over four billion years and pieced back together through the analysis of fossils.
Fossils are the preserved remains or traces of organisms that lived in the past. The soft parts of organisms almost always decompose quickly after death. On occasion, the hard parts—mainly bones, teeth, or shells—remain long enough to mineralize and form fossils. An example of a complete fossil skeleton is shown in Figure below . The fossil record is the record of life that unfolded over four billion years and pieced back together through the analysis of fossils.
To be preserved as fossils, remains must be covered quickly by sediments or preserved in some other way. For example, they may be frozen in glaciers or trapped in tree resin. Sometimes traces of organisms—such as footprints or burrows—are preserved The conditions required for fossils to form rarely occur. Therefore, the chance of an organism being preserved as a fossil is very low.
The process of a once-living organism becoming a fossil is called fossilization. Fossilization is very rare: Only a tiny percentage of the organisms that have ever lived become fossils.
Why do you think only a tiny percentage of living organisms become fossils after death? Think about an antelope that dies on the African plain (Figure below).
The process of a once-living organism becoming a fossil is called fossilization. Fossilization is very rare: Only a tiny percentage of the organisms that have ever lived become fossils.
Why do you think only a tiny percentage of living organisms become fossils after death? Think about an antelope that dies on the African plain (Figure below).
Most of its body is eaten by hyenas and other scavengers and the remaining flesh is devoured by insects and bacteria. Only bones are left behind. As the years go by, the bones are scattered and fragmented into small pieces, eventually turning into dust. The remaining nutrients return to the soil. This antelope will not be preserved as a fossil.
Is it more likely that a marine organism will become a fossil? When clams, oysters, and other shellfish die, the soft parts quickly decay, and the shells are scattered. In shallow water, wave action grinds them into sand-sized pieces. The shells are also attacked by worms, sponges, and other animals (Figure below).
Is it more likely that a marine organism will become a fossil? When clams, oysters, and other shellfish die, the soft parts quickly decay, and the shells are scattered. In shallow water, wave action grinds them into sand-sized pieces. The shells are also attacked by worms, sponges, and other animals (Figure below).
Despite these problems, there is a rich fossil record. How does an organism become fossilized?
Usually it’s only the hard parts that are fossilized. The fossil record consists almost entirely of the shells, bones, or other hard parts of animals. Mammal teeth are much more resistant than other bones, so a large portion of the mammal fossil record consists of teeth. The shells of marine creatures are common also.
Quick burial is essential because most decay and fragmentation occurs at the surface. Marine animals that die near a river delta may be rapidly buried by river sediments. A storm at sea may shift sediment on the ocean floor, covering a body and helping to preserve its skeletal remains (Figure below).
Usually it’s only the hard parts that are fossilized. The fossil record consists almost entirely of the shells, bones, or other hard parts of animals. Mammal teeth are much more resistant than other bones, so a large portion of the mammal fossil record consists of teeth. The shells of marine creatures are common also.
Quick burial is essential because most decay and fragmentation occurs at the surface. Marine animals that die near a river delta may be rapidly buried by river sediments. A storm at sea may shift sediment on the ocean floor, covering a body and helping to preserve its skeletal remains (Figure below).
Clues from Fossils
Fossils are our best form of evidence about Earth history, including the history of life. Along with other geological evidence from rocks and structures, fossils even give us clues about past climates, the motions of plates, and other major geological events.
History of Life on Earth
That life on Earth has changed over time is well illustrated by the fossil record. Fossils in relatively young rocks resemble animals and plants that are living today. In general, fossils in older rocks are less similar to modern organisms.
Environment of Deposition
By knowing something about the type of organism the fossil was, geologists can determine whether the region was terrestrial (on land) or marine (underwater) or even if the water was shallow or deep. The rock may give clues to whether the rate of sedimentation was slow or rapid. The amount of wear and fragmentation of a fossil allows scientists to learn about what happened to the region after the organism died; for example, whether it was exposed to wave action.
Geologic History
The presence of marine organisms in a rock indicates that the region where the rock was deposited was once marine. Sometimes fossils of marine organisms are found on tall mountains indicating that rocks that formed on the seabed were uplifted (Figure below).
Fossils are our best form of evidence about Earth history, including the history of life. Along with other geological evidence from rocks and structures, fossils even give us clues about past climates, the motions of plates, and other major geological events.
History of Life on Earth
That life on Earth has changed over time is well illustrated by the fossil record. Fossils in relatively young rocks resemble animals and plants that are living today. In general, fossils in older rocks are less similar to modern organisms.
Environment of Deposition
By knowing something about the type of organism the fossil was, geologists can determine whether the region was terrestrial (on land) or marine (underwater) or even if the water was shallow or deep. The rock may give clues to whether the rate of sedimentation was slow or rapid. The amount of wear and fragmentation of a fossil allows scientists to learn about what happened to the region after the organism died; for example, whether it was exposed to wave action.
Geologic History
The presence of marine organisms in a rock indicates that the region where the rock was deposited was once marine. Sometimes fossils of marine organisms are found on tall mountains indicating that rocks that formed on the seabed were uplifted (Figure below).
Climate
By knowing something about the climate a type of organism lives in now, geologists can use fossils to decipher the climate at the time the fossil was deposited. For example, coal beds form in tropical environments but ancient coal beds are found in Antarctica. Geologists know that at that time the climate on the Antarctic continent was much warmer. Recall from the chapter about plate tectonics that Wegener used the presence of coal beds in Antarctica as one of the lines of evidence for continental drift.
Index Fossils
An index fossil can be used to identify a specific period of time. Organisms that make good index fossils are distinctive, widespread, and lived briefly. Their presence in a rock layer can be used to identify that period of time over a large area.
In order for fossils to “tell” us the story of life, they must be dated. Then they can help scientists reconstruct how life changed over time. Fossils can be dated in two different ways: relative dating and absolute dating. Both are described below.
By knowing something about the climate a type of organism lives in now, geologists can use fossils to decipher the climate at the time the fossil was deposited. For example, coal beds form in tropical environments but ancient coal beds are found in Antarctica. Geologists know that at that time the climate on the Antarctic continent was much warmer. Recall from the chapter about plate tectonics that Wegener used the presence of coal beds in Antarctica as one of the lines of evidence for continental drift.
Index Fossils
An index fossil can be used to identify a specific period of time. Organisms that make good index fossils are distinctive, widespread, and lived briefly. Their presence in a rock layer can be used to identify that period of time over a large area.
In order for fossils to “tell” us the story of life, they must be dated. Then they can help scientists reconstruct how life changed over time. Fossils can be dated in two different ways: relative dating and absolute dating. Both are described below.
- Relative dating determines which of two fossils is older or younger than the other, but not their age in years. Relative dating is based on the positions of fossils in rock layers. Lower layers were laid down earlier, so they are assumed to contain older fossils. This is illustrated in Figure below .
- Absolute dating determines about how long ago a fossil organism lived. This gives the fossil an approximate age in years. Absolute dating is often based on the amount of carbon-14 or other radioactive element that remains in a fossil.
Superposition of Rock Layers
Danish scientist, Nicholas Steno proposed that if a rock 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 made a remarkable series of conjectures that are now known as Steno’s Laws. These laws are illustrated in Figure below.
Danish scientist, Nicholas Steno proposed that if a rock 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 made a remarkable series of conjectures that are now known as Steno’s Laws. These laws are illustrated in Figure below.
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) meaning that fossils in a rock layer represent what lived during the period the rock was deposited.
- Older features are replaced by more modern features in fossil organisms as species change through time; e.g. feathered dinosaurs precede birds in the fossil record.
- Fossil species with features that change distinctly and quickly can be used to determine the age of rock layers quite precisely.
The Grand Canyon provides an excellent illustration of the principles above. 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 bottom, which is described by the law of superposition.
- Distinctive rock layers, such as the Coconino Sandstone, are matched across the broad expanse of the canyon. These rock layers were once connected, as stated by the rule of lateral continuity.
- The Colorado River cuts through all the layers of rock to form the canyon. Based on the principle of cross-cutting relationships, the river must be younger than all of the rock layers that it cuts through.
Determining the Relative Ages of Rocks
Steno’s and Smith’s principles are essential for determining the relative ages of rocks and rock layers. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. The relative age of a rock then is its age in comparison with other rocks. If you know the relative ages of two rock layers, (1) Do you know which is older and which is younger? (2) Do you know how old the layers are in years?
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 order in (Figure below)? Write it down and then check the following paragraphs.
Steno’s and Smith’s principles are essential for determining the relative ages of rocks and rock layers. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. The relative age of a rock then is its age in comparison with other rocks. If you know the relative ages of two rock layers, (1) Do you know which is older and which is younger? (2) Do you know how old the layers are in years?
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 order in (Figure below)? Write it down and then check the following paragraphs.
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 be the youngest feature. The intrusion (D) cuts through the three sedimentary rock layers, 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 still younger.
The full sequence of events is:
1. Layer C formed.
2. Layer B formed.
3. Layer A formed.
4. After layers A-B-C were present, intrusion D cut across all three.
5. 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 Earth was around 6,000 years old, a figure that was based on the amount of time estimated for the events described in the Bible. One of the first scientists to question this assumption and to understand geologic time was James Hutton. Hutton traveled around Great Britain in the late 1700s, studying sedimentary rocks and their fossils (Figure below).
The full sequence of events is:
1. Layer C formed.
2. Layer B formed.
3. Layer A formed.
4. After layers A-B-C were present, intrusion D cut across all three.
5. 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 Earth was around 6,000 years old, a figure that was based on the amount of time estimated for the events described in the Bible. One of the first scientists to question this assumption and to understand geologic time was James Hutton. Hutton traveled around Great Britain in the late 1700s, studying sedimentary rocks and their fossils (Figure below).
Often described as 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 Earth today operated in the past as well. Why is an acceptance of this principle absolutely essential for us to be 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 lie on an eroded surface. At this gap in rock layers, or unconformity, some rocks were eroded away. For example, consider the famous unconformity at Siccar Point, on the coast of Scotland (Figure below).
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 lie on an eroded surface. At this gap in rock layers, or unconformity, some rocks were eroded away. For example, consider the famous unconformity at Siccar Point, on the coast of Scotland (Figure below).
1. A series of sedimentary beds was deposited on an ocean floor.
2. The sediments hardened into sedimentary rock.
3. The sedimentary rocks are uplifted and tilted, exposing them above sea 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 Earth today worked at the same rate in the past, he had to account for all of these events and the unknown amount of missing time represented by the unconformity, Hutton realized that this rock sequence alone represented a great deal of time. He concluded that Earth’s age should not be measured in thousands of years, but in millions of years.
Changes of fossils over time led to the development of the geologic time scale, which illustrates the relative order in which events on Earth have happened.
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 listing of rock layers from oldest to youngest. Then they divided Earth’s history into blocks of time with each block separated by important events, such as the disappearance of a species of fossil from the rock record. Since many of the scientists who first assigned names to times in Earth’s history were from Europe, they named the blocks of time from towns or other local places where the rock 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 top and the oldest on the bottom.
2. The sediments hardened into sedimentary rock.
3. The sedimentary rocks are uplifted and tilted, exposing them above sea 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 Earth today worked at the same rate in the past, he had to account for all of these events and the unknown amount of missing time represented by the unconformity, Hutton realized that this rock sequence alone represented a great deal of time. He concluded that Earth’s age should not be measured in thousands of years, but in millions of years.
Changes of fossils over time led to the development of the geologic time scale, which illustrates the relative order in which events on Earth have happened.
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 listing of rock layers from oldest to youngest. Then they divided Earth’s history into blocks of time with each block separated by important events, such as the disappearance of a species of fossil from the rock record. Since many of the scientists who first assigned names to times in Earth’s history were from Europe, they named the blocks of time from towns or other local places where the rock 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 top and the oldest on the bottom.
Questions
1. What conditions are necessary for a fossil to form?
2. Which are more abundant, land or marine fossils? Why?
3. What is the principle of original horizontality?
4. What is superposition?
5. What is the principle of cross-cutting?
6. What is uniformitarianism?
7. Which of Steno’s Laws is illustrated by each of the images in Figure below?
1. What conditions are necessary for a fossil to form?
2. Which are more abundant, land or marine fossils? Why?
3. What is the principle of original horizontality?
4. What is superposition?
5. What is the principle of cross-cutting?
6. What is uniformitarianism?
7. Which of Steno’s Laws is illustrated by each of the images in Figure below?
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