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Fossils and the Rock Record
Transcript of Fossils and the Rock Record
Vista Grande High School
Earth Science Class
Fossils and the Rock Record
The Geologic Time Scale
A hike down the Kaibab Trail in the Grand Canyon reveals the multicolored layers of rock that make up the canyon walls. These layers, or strata, are made of different types of sedimentary rock. Some of the layers have fossils in them. At the bottom of the Grand Canyon, is the Colorado River, which has been cutting downward through the rocks for millions of years.
Also at the bottom are rocks that date back some 400 million years or more. These rocks record events and hold the key to unlocking Earth's history. They record the many advances and retreats of oceans and the development of plants and animals. By studying these rocks we can interpret the environments the rocks were deposited in, reconstruct Earth's history, and possibly predict events in the future.
To help with the analysis of Earth's rocks, geologists have divided the history of Earth into time based units based on the fossils contained in the rocks. These time units are part of the Geologic Time Scale, a record of Earth's history from its origin 4.6 billion years ago to the present.
The Rock Record
The oldest division of time is at the bottom of the geologic time scale. Moving upward on the scale, each division is younger, just as the rock layers in the rock record grow generally younger as you move upward. The time scale is divided into units called eons, eras, periods, and epochs.
An eon is the longest time unit measured in billions of years. The Archean, the Proterozoic, and the Phanerozoic are eons.
An era is the next longest span of time, it is measured in hundreds of millions to billions of years. Eras are defined by the differences in life-forms found in the rocks. The names are based on the relative ages of these life-forms found in the rocks.
Example of the names. In Greek "paleo" means old, "meso" means middle, and "ceno" means recent. "Zoic" means "of life" in Greek, and so Mesozoic means middle life.
Precambrian time, which makes up approximately 90% of geologic time, is divided into the Archean and Proterozoic Eons. The Proterozoic is the more recent of the two, and the end of it is marked by the appearance of the first organisms with hard parts. All life forms up until then had soft bodies and no shells or skeletons. Some of these resembled organisms that exist today, such as sponges, snails, and worms, while others cannot be accurately assigned to any known animal or plant group.
Since the naming of the first geologic time period, the Jurassic, in 1797. development of the time scale continued to the present. The names of the periods do not change, but the years marking the start and end of each period are continually being refined. The scale enables scientists to correlate events in Earth's history
Plants and Animals Evolve
During the Paleozoic Era, the oceans became full of a wide diversity of plants and animals. Trilobites dominated the oceans in the Cambrian period; land plants appeared and were followed by land animals; and swamps provided the plant material that became the coal deposits of the Pennsylvanian.
Plants and Animals Evolve
The end of the Paleozoic is marked by the largest extinction event in Earth's history. As many as 90% of all marine invertebrates species became extinct. The era following the Paleozoic Era, the Mesozoic Era, is known for the emergence of dinosaurs, but other important developments occurred as well. Reef-building corals and large predatory reptiles developed in the oceans. Amphibians began living on land as well as in the water. Dinosaur populations began to slowly decline in numbers throughout the Cretaceous period as mammals evolved during the Cretaceous period.
The end of the Mesozoic is also marked by a large extinction event. In addition to the remaining dinosaurs, many other groups of organisms, became extinct. Mammals increased both in number and diversity in the Cenozoic. Human ancestors developed at this time. Grass and flowering plants expanded on land while ocean life remained relatively unchanged throughout this era.
Periods of Geologic Time
Geologic time periods are defined by the life forms that were abundant and became extinct during the time in which specific rocks were deposited. Periods are usually measured in terms of tens of millions of years. Some were named for the geographic region in which the rocks of that age were first observed, studied, and described. For example, the Mississippian Period was named for the distinctive limestone bluffs along the Mississippi River as shown below.
The Jurassic Period was named for the rocks that were described in the Jura Mountains of Europe.
Historically, The Cenozoic Era was divided into two periods, the Tertiary and the Quaternary. Currently however, the Cenozoic is divided into three periods; the Paleogene, Neogene, and Quaternary. In contrast to the boundaries between the Paleozoic and the Mesozoic Eras, the boundaries between the Cenozoic are not marked by extinction events.
Epochs of Geologic Time
Epochs are even smaller divisions of geologic time and are usually measured in millions of years to tens of millions of years. The fossil record of the Cenozoic Era is relatively complete because there has been less time for weathering and erosion to remove the evidence of this part of Earth's history. Thus, the rocks and fossils from this era are easily accessed and studied. Accordingly, the Cenozoic periods have been further divided into epochs, such as the Paleocene and the Oligocene. Different groups of organisms have been used to distinguish the various epochs. For example, marine fossils were used to identify the Oligocene epoch, and terrestrial plant fossils were used to mark the boundaries of the Eocene Epoch.
Oligocene Marine Fossils
Plant Fossil of the Eocene
Regardless of how a geologic time period was defined, each unit contains specific characteristics that set it apart from the rest of geologic history.
Relative Dating of Rocks
As late as the turn of the 19th century, the, majority of the world believed that the Earth was only about 6,000 years old. This age had been determined by Archbishop James Ussher of Ireland, who used a chronology of human and Earth history to calculate Earth's age. As early as 1770, James Hutton, a Scottish physician and geologist, had begun to observe and attempt to explain Earth's landscapes. Hutton's observations in Great Britain helped him to develop the principle of uniformitarianism, which attempts to explain the forces that continually change the surface features of the Earth.
Such processes include mountain building, erosion, earthquakes, and sea-level changes. The principle of uniformitarianism states the the processes of today have been occurring since Earth formed.
Principles for Determining Relative Age
The concept of relative-age dating places the ages of the rocks and the events that formed them in order, but without exact dates. This is done by comparing one event or rock layer to another.
Many different horizontal or nearly horizontal layers of rocks make up the walls of the Grand Canyon. Most of the rocks are sedimentary and were originally deposited millions of years ago by water or wind.
The Principle of Original Horizontality states that sedimentary rocks are deposited in horizontal or nearly horizontal layers. While we may not know the actual ages of the rocks, we can assume that the oldest rocks are at the bottom and that each successive layer going upward is younger.
This is an application of the Principle of Superposition, which states that in an undisturbed rock sequence, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath. Rocks exposed at the bottom of the Grand Canyon are some of the oldest in North America. These are are mostly igneous and metamorphic rocks. Within the Vishnu Group at the bottom of the Grand Canyon sequence are dykes of granite. The principle of Cross-Cutting Relationships states that an intrusion or fault is younger than the rock it cuts across.
So, with that in mind, the granite is younger than the schist, because the granite cuts across the schist.
The dark rock pictured here is the Vishnu Schist and the lighter colored material is younger because it "cuts" across it.
This same principle applies to faults as well. A fault is a fracture in the Earth along which movement takes place.
A fault is younger than the strata and surrounding geologic features because it cuts across them.
Relative age can be determined where an overlying rock layer contains particles of rock material from the layer beneath it. The bottom layer was eroded, the loose material from the layer beneath became incorporated in the newly deposited top layer. These particles are called inclusions.
Other Means of Determining Relative Age
The fact that Earth is constantly changing as a result of processes such as weathering, erosion, earthquake, and volcanism makes it difficult to find an undisturbed sequence of rock layers.
For example, if rocks that record a volcanic eruption or the last occurrence of a particular fossil are eroded away, then the record of that particular event has been lost. Further changes may occur if the area is covered by a river during a flood or by the sea. Additionally, an erosional surface might become buried by the deposition of younger rocks.
This buried erosional surface results in a gap in the rock record and is called an unconformity.
Other Means of Determining Relative Age
When horizontal sedimentary rocks overlie horizontal sedimentary rocks, the unconformity is called a disconformity. A different type of unconformity exists when sedimentary rocks overlie non-sedimentary rocks
such as granite or marble.
Such an unconformity suggests a possible uplifting of the marble or granite and exposure at the surface by weathering and erosion. The contact point between the non-sedimentary and sedimentary rock is called a nonconformity.
When horizontal sedimentary rocks are uplifted and tilted, they are exposed to the processes of weathering and erosion. When deposition resumes, horizontal layers of sedimentary rocks are laid down on top of the erosional surface. The layers beneath the eroded surface of the folded layers remain in tact, but they are at an angle the the eroded surface. This type of unconformity is called an angular unconformity.
Correlation of Rock Strata
The Permian Kaibab Formation rims the top of the Grand Canyon, but is also found about 300 km away at the bottom of a 200 m gorge in Capital Reef National Park in Utah. How do geologists match rock layers such as these, which are far apart from each other? One method is by correlation. Correlation is the matching of outcrops of one geographic region to another. Geologists examine rocks for distinctive fossils and unique rock or mineral features to help correlate the rock layers. This information can be used to help in the exploration for oil or valuable minerals. For example, if a sandstone layer in one area contains oil, it is possible that the same layer in a different location may also contain oil. Correlation allows geologists to accurately locate the same sandstone layer in another location.
Absolute-Age Dating of Rocks
As we have seen, relative-age dating of rocks is a method of comparing past geologic events based on the observed order of strata in the rock record. In contrast, absolute-age dating of rocks enables scientists to determine the exact age of a rock, fossil, or other object. Scientists have devised a way for dating very old objects using the decay rate of radioactive isotopes.
Absolute-Dating of Rocks
Radioactive isotopes are found in both igneous and metamorphic rocks, some fossils, and organic remains also contain radioactive isotopes. Radioactive substances emit nuclear particles at a constant rate. As the number of protons and neutrons change with each nuclear emission, the element is converted to a different element. The original element is referred to as the "parent", and the new element that is produced is referred to as the "daughter" element.
For example Uranium 238, U-238, will decay into an isotope of Lead 206, Pb-206, over a specific period of time.
The emission of radioactive particles and the resulting change into other elements over time is called radioactive decay. Once the emission of these atomic particles begins, the rate remains constant regardless of environment, pressure, temperature, or any other physical changes. Because of this, the atomic particles become a very accurate indicator of the absolute age of the object.
Use of Radioactive Isotopes
In the process called radiometric dating, scientists attempt to determine the ratio of parent nuclei to daughter nuclei within a given sample of rock or fossil. This ratio is then used to determine the absolute age of the rock or fossil. As the number of parent atoms decreases, the number of daughter atoms increases by the same amount and indicates the increasing age of an object. Because it often takes a long time for the entire amount of an isotope to decay, geologists use the length of time it takes for one-half of the original amount to decay. This period of time is called the "half-life."
Other ways to Determine Age
Determining the relative age of an object or event is not limited to the use of rocks or chemical elements. Naturally occurring materials, such as trees, lake-bottom sediment, and volcanic ash can also be used to help geologists determine the age of an object or event, such as a forest fire, a drought, a flood, or a volcanic eruption.
With the use of a technique from science of forestry, the age of a tree can be determined by counting the number of annual tree rings in a cross section of the tree. Trees experience their great growth in the spring and their least amount of growth in the winter. One spring and winter cycle equals one annual cycle, and can be counted as one years growth. Dendrochronology is the science of comparing annual growth rings in trees to date events and changes in past environments.
In Mesa Verde National Park in Colorado, the age of the wood rafters used to build the pueblos of the Anasazi
Seasonal Climatic Changes
About 11,000 years ago, continental glaciers covered the northern part of the United States. During the summer months, the ice would partially melt. Large volumes of water containing fine glacial sediment were carried downstream and deposited in large lakes. Summer deposits are generally light-colored and relatively thick compared to the thinner, organically enriched, and dark colored sediments of winter. These bands of alternating light and dark colored sediments are called varves. Varves are similar to tree rings in that they show evidence of cyclic events - in this case, a cycle of summer to winter. Varves from different lakes can be compared to determine the ages of glacial sediments from about 15,000 to 12,000 years ago.
Remains of Organisms in the Rock Record
Fossils are the evidence or remains of once living plants or animals. The fossil record also provides evidence that populations have undergone change through time in response to changes in their environments. This change in populations as a result of environmental changes is called evolution.
Types of Fossils
Fossils with original preservation are the soft and hard parts of plant and animal remains have not undergone any kind of change since the organisms' deaths. Such fossils are uncommon because their preservation requires extraordinary circumstances such as freezing, drying out, or oxygen free environments.
Altered Hard Parts
When all the organic material has been removed and the hard parts of a plant or animal have been changed either by mineral replacement or by recrystallization, their fossils are referred to as altered hard parts. The process by which pore spaces are filled in with minerals is called permineralization. The most common type of fossil with mineral replacement is petrified wood, or a fossilized tree.
Some fossils are more useful than others in relative age dating. Index fossils are the remains of plant or animals that can be used by geologists to correlate rock layers over large geographic regions or to date a particular rock layer. An index fossil is easily recognizable, abundant, and widely distributed geographically.
Molds and Casts
Some fossils do not contain any shell or bone material. They may be molds and casts of shelled organisms such as clams. A mold is formed when the original shell parts of an organism within a sedimentary rock are weathered and eroded. A hollowed-out impression, or mold, of the shells left in their place. This cavity might later become filled with minerals or sediment to create a cast of the organism.
Indirect evidence of plant and animal life are called trace fossils. Examples of this type of fossil would be a worm trail, burrows, and footprints.