Day 1 - Due 10/24(A) 10/25(B) |
2.2 - Geologic History & Mapping |
AT HOME - Complete the Reading Assignment and watch the notes videos before coming to class
Geologic Time
The first principle you need to understand about geologic time is that the laws of nature never change. This means that the laws describing how things work are the same today as they were billions of years ago. For example, water freezes at 0oC. This law has always been true and always will be true. Knowing that natural laws never change helps you think about Earth’s past, because it gives you clues about how things happened very long ago. It means that we can use present-day processes to interpret the past. Imagine you find fossils of sea animals in a rock. The laws of nature say that sea animals must live in the sea. That law has never changed, so the rock must have formed near the sea. The rock may be millions of years old, but the fossils in it are a clue for us today about how it formed.
Now imagine that you find that same rock with fossils of a sea animal in a place that is very dry and nowhere near the sea. How could that be? Remember that the laws of nature never change. Therefore, the fossil means that the rock definitely formed by the sea. This tells you that even though the area is now dry, it must have once been underwater. Clues like this have helped scientists learn that Earth's surface features have changed many times. Spots that were once covered by warm seas may now be cool and dry. Places that now have tall mountains may have once been low, flat ground. These kinds of changes take place over many millions of years, but they are still slowly going on today. The place where you live right now may look very different in the far away future.
Relative and Absolute Age Dating of RocksThe clues in rocks help scientists put together a picture of how places on Earth have changed. Scientists noticed in the 1700s and 1800s that similar layers of sedimentary rocks all over the world contain similar fossils. They used relative dating to order the rock layers from oldest to youngest. In the process of relative dating, scientists do not determine the exact age of a fossil but do learn which ones are older or younger than others. They saw that the fossils in older rocks are different from the fossils in younger rocks. For example, older rock layers contain only reptile fossils, but younger rock layers may also contain mammal fossils.
Scientists divided Earth’s history into several chunks of time when the fossils showed similar things living on the Earth. They gave each chunk of time a name to help them keep track of how Earth has changed. For example, one chunk of time when many dinosaurs lived is called the Jurassic. We find fossils of Earth’s first green plants from the chunk of time named the Ordovician. Many of the scientists who first assigned names to times in Earth’s history were from Europe. As a result, many of the names they used came from towns or other local places where they studied in Europe.
Ordering rock layers from oldest to youngest was a first step in creating the geologic time scale. It showed the order in which life on Earth changed. It also showed us how certain areas changed over time in regard to climate or type of environment. However, the early geologic time scale only showed the order of events. It did not show the actual years that events happened. With the discovery of radioactivity in the late 1800’s, scientists were able to measure the exact age in years of different rocks. Measuring the amounts of radioactive elements in rocks let scientists use absolute dating to give ages to each chunk of time on the geologic time scale. For example, they are now able to state that the Jurassic began about 200 million years ago and that it lasted for about 55 million years.
Geologic Time Scale
Today, the geologic time scale is divided into major chunks of time called eons. Eons may be further divided into smaller chunks called eras, and each era is divided into periods. Figure below shows you what the geologic time scale looks like. We now live in the Phanerozoic eon, the Cenozoic era, and the Quarternary period. Sometimes, periods are further divided into epochs, but they are usually just named “early” or “late,” for example, “late Jurassic,” or “early Cretaceous.” Note that chunks of geologic time are not divided into equal numbers of years. Instead, they are divided into blocks of time when the fossil record shows that there were similar organisms on Earth.
The first principle you need to understand about geologic time is that the laws of nature never change. This means that the laws describing how things work are the same today as they were billions of years ago. For example, water freezes at 0oC. This law has always been true and always will be true. Knowing that natural laws never change helps you think about Earth’s past, because it gives you clues about how things happened very long ago. It means that we can use present-day processes to interpret the past. Imagine you find fossils of sea animals in a rock. The laws of nature say that sea animals must live in the sea. That law has never changed, so the rock must have formed near the sea. The rock may be millions of years old, but the fossils in it are a clue for us today about how it formed.
Now imagine that you find that same rock with fossils of a sea animal in a place that is very dry and nowhere near the sea. How could that be? Remember that the laws of nature never change. Therefore, the fossil means that the rock definitely formed by the sea. This tells you that even though the area is now dry, it must have once been underwater. Clues like this have helped scientists learn that Earth's surface features have changed many times. Spots that were once covered by warm seas may now be cool and dry. Places that now have tall mountains may have once been low, flat ground. These kinds of changes take place over many millions of years, but they are still slowly going on today. The place where you live right now may look very different in the far away future.
Relative and Absolute Age Dating of RocksThe clues in rocks help scientists put together a picture of how places on Earth have changed. Scientists noticed in the 1700s and 1800s that similar layers of sedimentary rocks all over the world contain similar fossils. They used relative dating to order the rock layers from oldest to youngest. In the process of relative dating, scientists do not determine the exact age of a fossil but do learn which ones are older or younger than others. They saw that the fossils in older rocks are different from the fossils in younger rocks. For example, older rock layers contain only reptile fossils, but younger rock layers may also contain mammal fossils.
Scientists divided Earth’s history into several chunks of time when the fossils showed similar things living on the Earth. They gave each chunk of time a name to help them keep track of how Earth has changed. For example, one chunk of time when many dinosaurs lived is called the Jurassic. We find fossils of Earth’s first green plants from the chunk of time named the Ordovician. Many of the scientists who first assigned names to times in Earth’s history were from Europe. As a result, many of the names they used came from towns or other local places where they studied in Europe.
Ordering rock layers from oldest to youngest was a first step in creating the geologic time scale. It showed the order in which life on Earth changed. It also showed us how certain areas changed over time in regard to climate or type of environment. However, the early geologic time scale only showed the order of events. It did not show the actual years that events happened. With the discovery of radioactivity in the late 1800’s, scientists were able to measure the exact age in years of different rocks. Measuring the amounts of radioactive elements in rocks let scientists use absolute dating to give ages to each chunk of time on the geologic time scale. For example, they are now able to state that the Jurassic began about 200 million years ago and that it lasted for about 55 million years.
Geologic Time Scale
Today, the geologic time scale is divided into major chunks of time called eons. Eons may be further divided into smaller chunks called eras, and each era is divided into periods. Figure below shows you what the geologic time scale looks like. We now live in the Phanerozoic eon, the Cenozoic era, and the Quarternary period. Sometimes, periods are further divided into epochs, but they are usually just named “early” or “late,” for example, “late Jurassic,” or “early Cretaceous.” Note that chunks of geologic time are not divided into equal numbers of years. Instead, they are divided into blocks of time when the fossil record shows that there were similar organisms on Earth.
Geologic Time Condensed to One YearIt's always fun to think about geologic time in a framework that we can more readily understand. Here are when some major events in Earth history would have occurred if all of earth history was condensed down to one calendar year.
January 1 12 am: Earth forms from the planetary nebula – 4600 million years ago
February 25, 12:30 pm: The origin of life; the first cells – 3900 million years ago
March 4, 3:39 pm: Oldest dated rocks – 3800 million years ago
March 20, 1:33 pm: First stromatolite fossils – 3600 million years ago
July 17, 9:54 pm: first fossil evidence of cells with nuclei – 2100 million years ago
November 18, 5:11 pm: Cambrian Explosion – 544 million years ago
December 1, 8:49 am: first insects – 385 million years ago
December 2, 3:54 am: first land animals, amphibians – 375 million years ago
December 5, 5:50 pm: first reptiles – 330 million years ago
December 12, 12:09 pm: Permo-Triassic Extinction – 245 million years ago
December 13, 8:37 pm: first dinosaurs – 228 million years ago
December 14, 9:59 am: first mammals -- 220 million years ago
December 22, 8:24 pm: first flowering plants – 115 million years ago
December 26, 7:52 pm: Cretaceous-Tertiary Extinction – 66 million years ago
December 26, 9:47 pm: first ancestors of dogs – 64 million years ago
December 27, 5:25 am: widespread grasses – 60 million years ago
December 27, 11:09 am: first ancestors of pigs and deer – 57 million years ago
December 28, 9:31 pm: first monkeys – 39 million years ago
December 31, 5:18 pm: oldest hominid – 4 million years ago
December 31, 11:02 pm: oldest direct human ancestor – 1 million years ago
December 31, 11:48 pm: first modern human – 200,000 years ago
December 31, 11:59 pm: Revolutionary War – 235 years ago
Earth's Geologic History
Precambrian
Plate TectonicsBy the end of the Archean, about 2.5 billion years ago, plate tectonics processes were completely recognizable. Small Proterozoic continents known as microcontinents collided to create supercontinents, which resulted in the uplift of massive mountain ranges.
Convergence was especially active between 1.5 and 1.0 billion years ago. These lands came together to create the continent of Laurentia.
About 1.1 billion years ago, Laurentia became part of the supercontinent Rodinia (Figure below ). Rodinia probably contained all of the landmass at the time, which was about 75% of the continental landmass present today.
January 1 12 am: Earth forms from the planetary nebula – 4600 million years ago
February 25, 12:30 pm: The origin of life; the first cells – 3900 million years ago
March 4, 3:39 pm: Oldest dated rocks – 3800 million years ago
March 20, 1:33 pm: First stromatolite fossils – 3600 million years ago
July 17, 9:54 pm: first fossil evidence of cells with nuclei – 2100 million years ago
November 18, 5:11 pm: Cambrian Explosion – 544 million years ago
December 1, 8:49 am: first insects – 385 million years ago
December 2, 3:54 am: first land animals, amphibians – 375 million years ago
December 5, 5:50 pm: first reptiles – 330 million years ago
December 12, 12:09 pm: Permo-Triassic Extinction – 245 million years ago
December 13, 8:37 pm: first dinosaurs – 228 million years ago
December 14, 9:59 am: first mammals -- 220 million years ago
December 22, 8:24 pm: first flowering plants – 115 million years ago
December 26, 7:52 pm: Cretaceous-Tertiary Extinction – 66 million years ago
December 26, 9:47 pm: first ancestors of dogs – 64 million years ago
December 27, 5:25 am: widespread grasses – 60 million years ago
December 27, 11:09 am: first ancestors of pigs and deer – 57 million years ago
December 28, 9:31 pm: first monkeys – 39 million years ago
December 31, 5:18 pm: oldest hominid – 4 million years ago
December 31, 11:02 pm: oldest direct human ancestor – 1 million years ago
December 31, 11:48 pm: first modern human – 200,000 years ago
December 31, 11:59 pm: Revolutionary War – 235 years ago
Earth's Geologic History
Precambrian
Plate TectonicsBy the end of the Archean, about 2.5 billion years ago, plate tectonics processes were completely recognizable. Small Proterozoic continents known as microcontinents collided to create supercontinents, which resulted in the uplift of massive mountain ranges.
Convergence was especially active between 1.5 and 1.0 billion years ago. These lands came together to create the continent of Laurentia.
About 1.1 billion years ago, Laurentia became part of the supercontinent Rodinia (Figure below ). Rodinia probably contained all of the landmass at the time, which was about 75% of the continental landmass present today.
Rodinia broke up about 750 million years ago. The geological evidence for this breakup includes large lava flows that are found where continental rifting took place. Seafloor spreading eventually started and created the oceans between the continents.
The breakup of Rodinia may have triggered Snowball Earth around 700 million years ago. Snowball Earth is the hypothesis that much of the planet was covered by ice at the end of the Precambrian. When the ice melted and the planet became habitable, life evolved rapidly. This explains the rapid evolution of life in the Ediacaran and Cambrian periods.
Compared with the long expanse of the Precambrian, the Phanerozoic is recent history. Much more geological evidence is available for scientists to study so the Phanerozoic is much better known.
Paleozoic
The Paleozoic is the furthest back era of the Phanerozoic and it lasted the longest. But the Paleozoic was relatively recent, beginning only 570 million years ago. The paleogeography of the Paleozoic begins and ends with a supercontinent.
Marine Transgressions and Regressions
Some of the most important events of the Paleozoic were the rising and falling of sea level over the continents. Sea level rises over the land during a marine transgression. During a marine regression, sea level retreats. During the Paleozoic there were four complete cycles of marine transgressions and regressions.
One of two things must happen for sea level to change in a marine transgression: either the land must sink or the water level must rise. What could cause sea level to rise? When little or no fresh water is tied up in glaciers and ice caps, sea level is high. Sea level also appears to rise if land is down dropped. Sea level rises if an increase in seafloor spreading rate buoys up the ocean crust, causing the ocean basin to become smaller.
What could cause sea level to fall in a marine regression?
Geologists think that the Paleozoic marine transgressions and regressions were the result of the decrease and increase in the size of glaciers covering the lands.
Plate Tectonics
A mountain-building event is called an orogeny. Orogenies take place over tens or hundreds of millions of years. At the beginning of the Paleozoic, the supercontinent Rodinia began to split up. At the end, Pangaea came together. As continents smash into microcontinents and island arcs collided, mountains rise.
Geologists find evidence for these collisions in many locations. For example, Laurentia collided with the Taconic Island Arc during the Taconic Orogeny (Figure below). The remnants of this mountain range make up the Taconic Mountains in New York.
The breakup of Rodinia may have triggered Snowball Earth around 700 million years ago. Snowball Earth is the hypothesis that much of the planet was covered by ice at the end of the Precambrian. When the ice melted and the planet became habitable, life evolved rapidly. This explains the rapid evolution of life in the Ediacaran and Cambrian periods.
Compared with the long expanse of the Precambrian, the Phanerozoic is recent history. Much more geological evidence is available for scientists to study so the Phanerozoic is much better known.
Paleozoic
The Paleozoic is the furthest back era of the Phanerozoic and it lasted the longest. But the Paleozoic was relatively recent, beginning only 570 million years ago. The paleogeography of the Paleozoic begins and ends with a supercontinent.
Marine Transgressions and Regressions
Some of the most important events of the Paleozoic were the rising and falling of sea level over the continents. Sea level rises over the land during a marine transgression. During a marine regression, sea level retreats. During the Paleozoic there were four complete cycles of marine transgressions and regressions.
One of two things must happen for sea level to change in a marine transgression: either the land must sink or the water level must rise. What could cause sea level to rise? When little or no fresh water is tied up in glaciers and ice caps, sea level is high. Sea level also appears to rise if land is down dropped. Sea level rises if an increase in seafloor spreading rate buoys up the ocean crust, causing the ocean basin to become smaller.
What could cause sea level to fall in a marine regression?
Geologists think that the Paleozoic marine transgressions and regressions were the result of the decrease and increase in the size of glaciers covering the lands.
Plate Tectonics
A mountain-building event is called an orogeny. Orogenies take place over tens or hundreds of millions of years. At the beginning of the Paleozoic, the supercontinent Rodinia began to split up. At the end, Pangaea came together. As continents smash into microcontinents and island arcs collided, mountains rise.
Geologists find evidence for these collisions in many locations. For example, Laurentia collided with the Taconic Island Arc during the Taconic Orogeny (Figure below). The remnants of this mountain range make up the Taconic Mountains in New York.
Laurentia experienced other orogenies as it merged with the northern continents. The southern continents came together to form Gondwana. When Laurentia and Gondwana collided to create Pangaea, the Appalachians rose. Geologists think they may once have been higher than the Himalayas are now.
Pangaea was the last supercontinent on Earth. Evidence for the existence of Pangaea was what Alfred Wegener used to create his continental drift hypothesis.
As the continents move and the land masses change shape, the shape of the oceans changes too. During the time of Pangaea, about 250 million years ago, most of Earth’s water was collected in a huge ocean called Panthalassa (Figurebelow).
Pangaea was the last supercontinent on Earth. Evidence for the existence of Pangaea was what Alfred Wegener used to create his continental drift hypothesis.
As the continents move and the land masses change shape, the shape of the oceans changes too. During the time of Pangaea, about 250 million years ago, most of Earth’s water was collected in a huge ocean called Panthalassa (Figurebelow).
Mesozoic
The Mesozoic is known as the age of the dinosaurs, but things were happening geologically as well. The Mesozoic was dominantly warm and tropical.
The Breakup of Pangaea
At the end of the Paleozoic there was one continent and one ocean. When Pangaea began to break apart about 180 million years ago, the Panthalassa Ocean separated into the individual but interconnected oceans that we see today on Earth.
Why would a supercontinent break up after being together for tens of millions of years? A continent is a giant insulating blanket that does not allow mantle heat to escape very effectively. As heat builds up beneath a supercontinent, continental rifting begins. Basaltic lavas fill in the rift and eventually lead to seafloor spreading and the formation of a new ocean basin.
The Atlantic Ocean basin formed as Pangaea split apart. The seafloor spreading that pushed Africa and South America apart is continuing to enlarge the Atlantic Ocean.
Cenozoic
The Cenozoic began around 65.5 million years ago and continues today. Although it accounts for only about 1.5% of the Earth’s total history, as the most recent era it is the one scientists know the most about.
Plate Tectonics
The paleogeography of the era was very much like it is today. Early in the Cenozoic, blocks of crust uplifted to form the Rocky Mountains, which were later eroded away and then uplifted again. Subduction off of the Pacific Northwest formed the Cascades volcanic arc. The Basin and Range province that centers on Nevada is where crust is being pulled apart.
The San Andreas Fault has grown where the Pacific and North American plates meet.
Although most plate tectonic activity involves continents moving apart, smaller regions are coming together. Africa collided with Eurasia to create the Alps. India crashed into Asia to form the Himalayas.
Earth's Life History
The Origin of Life
No one knows how or when life first began on the turbulent early Earth. There is little hard evidence from so long ago. Scientists think that it is extremely likely that life began and was wiped out more than once; for example, by the impact that created the Moon.
Photosynthesis and the Changing Atmosphere
Sometime around 3 billion years ago (about 1.5 billion years after Earth formed!), photosynthesis began. Photosynthesis allowed organisms to use sunlight and inorganic molecules, such as carbon dioxide and water, to create chemical energy that they could use for food. To photosynthesize, a cell needs chloroplasts (Figure below).
The Mesozoic is known as the age of the dinosaurs, but things were happening geologically as well. The Mesozoic was dominantly warm and tropical.
The Breakup of Pangaea
At the end of the Paleozoic there was one continent and one ocean. When Pangaea began to break apart about 180 million years ago, the Panthalassa Ocean separated into the individual but interconnected oceans that we see today on Earth.
Why would a supercontinent break up after being together for tens of millions of years? A continent is a giant insulating blanket that does not allow mantle heat to escape very effectively. As heat builds up beneath a supercontinent, continental rifting begins. Basaltic lavas fill in the rift and eventually lead to seafloor spreading and the formation of a new ocean basin.
The Atlantic Ocean basin formed as Pangaea split apart. The seafloor spreading that pushed Africa and South America apart is continuing to enlarge the Atlantic Ocean.
Cenozoic
The Cenozoic began around 65.5 million years ago and continues today. Although it accounts for only about 1.5% of the Earth’s total history, as the most recent era it is the one scientists know the most about.
Plate Tectonics
The paleogeography of the era was very much like it is today. Early in the Cenozoic, blocks of crust uplifted to form the Rocky Mountains, which were later eroded away and then uplifted again. Subduction off of the Pacific Northwest formed the Cascades volcanic arc. The Basin and Range province that centers on Nevada is where crust is being pulled apart.
The San Andreas Fault has grown where the Pacific and North American plates meet.
Although most plate tectonic activity involves continents moving apart, smaller regions are coming together. Africa collided with Eurasia to create the Alps. India crashed into Asia to form the Himalayas.
Earth's Life History
The Origin of Life
No one knows how or when life first began on the turbulent early Earth. There is little hard evidence from so long ago. Scientists think that it is extremely likely that life began and was wiped out more than once; for example, by the impact that created the Moon.
Photosynthesis and the Changing Atmosphere
Sometime around 3 billion years ago (about 1.5 billion years after Earth formed!), photosynthesis began. Photosynthesis allowed organisms to use sunlight and inorganic molecules, such as carbon dioxide and water, to create chemical energy that they could use for food. To photosynthesize, a cell needs chloroplasts (Figure below).
In what two ways did photosynthesis make the planet much more favorable for life?
1. Photosynthesis allowed organisms to create food energy so that they did not need to rely on nutrients floating around in the environment. Photosynthesizing organisms could also become food for other organisms.
2. A byproduct of photosynthesis is oxygen. When photosynthesis evolved, all of a sudden oxygen was present in large amounts in the atmosphere. For organisms used to an anaerobic environment, the gas was toxic, and many organisms died out.
With more oxygen in the atmosphere, ultraviolet radiation could create ozone. With the formation of an ozone layer to protect the surface of the Earth from UV radiation, more complex life forms could evolve.
Early Organisms
What were these organisms that completely changed the progression of life on Earth by changing the atmosphere from anaerobic to aerobic? The oldest known fossils that are from organisms known to photosynthesize are cyanobacteria (Figure below). Cyanobacteria were present by 2.8 billion years ago, and some may have been around as far back as 3.5 billion years.
1. Photosynthesis allowed organisms to create food energy so that they did not need to rely on nutrients floating around in the environment. Photosynthesizing organisms could also become food for other organisms.
2. A byproduct of photosynthesis is oxygen. When photosynthesis evolved, all of a sudden oxygen was present in large amounts in the atmosphere. For organisms used to an anaerobic environment, the gas was toxic, and many organisms died out.
With more oxygen in the atmosphere, ultraviolet radiation could create ozone. With the formation of an ozone layer to protect the surface of the Earth from UV radiation, more complex life forms could evolve.
Early Organisms
What were these organisms that completely changed the progression of life on Earth by changing the atmosphere from anaerobic to aerobic? The oldest known fossils that are from organisms known to photosynthesize are cyanobacteria (Figure below). Cyanobacteria were present by 2.8 billion years ago, and some may have been around as far back as 3.5 billion years.
Phanerozoic EonThe Phanerozoic Eon is divided into three eras—the Paleozoic, the Mesozoic, and the Cenozoic--spanning from about 540 million years ago to the present (Table below). Life has undergone fantastic changes during the long span of the Phanerozoic Eon.
Notice that different types of organisms developed at different times. |
Extinction and Radiation
The eras of the Phanerozoic Eon are separated by mass extinctions. A mass extinction occurs when large numbers of species become extinct in a short amount of time. The causes of different mass extinctions are different: collisions with comets or asteroids, massive volcanic eruptions, or rapidly changing climate are all possible causes of some of these disasters (Figure below).
The eras of the Phanerozoic Eon are separated by mass extinctions. A mass extinction occurs when large numbers of species become extinct in a short amount of time. The causes of different mass extinctions are different: collisions with comets or asteroids, massive volcanic eruptions, or rapidly changing climate are all possible causes of some of these disasters (Figure below).
After a mass extinction, many habitats are no longer inhabited by organisms because they have gone extinct. A change in the environment from one in which organisms live in all the available habitats to one in which many habitats are available gives an advantage to organisms that can adapt to new environments. Evolutionary processes act rapidly during these times and many new species evolve to fill those available habitats. The process in which many new species evolve in a short period of time to fill available niches is called adaptive radiation.
Questions
1. Dinosaurs went extinct about 66 million years ago. Which period of geologic time was the last in which dinosaurs lived?
2. What are the Paleozoic, Mesozoic and Cenozoic Eras part of?
3. What are the periods within the Cenozoic Era?
4. What was Laurentia and what lands was it composed of? What happened to it?
5. What are the possible causes of a marine transgression? Of a marine regression?
6. How and when did the Appalachians form?
6. What was the configuration of oceans during the time of the supercontinent Pangaea?
7. Why were cyanobacteria important in the early Earth?
8. List the order in which the major types of animals appeared on Earth.
Questions
1. Dinosaurs went extinct about 66 million years ago. Which period of geologic time was the last in which dinosaurs lived?
2. What are the Paleozoic, Mesozoic and Cenozoic Eras part of?
3. What are the periods within the Cenozoic Era?
4. What was Laurentia and what lands was it composed of? What happened to it?
5. What are the possible causes of a marine transgression? Of a marine regression?
6. How and when did the Appalachians form?
6. What was the configuration of oceans during the time of the supercontinent Pangaea?
7. Why were cyanobacteria important in the early Earth?
8. List the order in which the major types of animals appeared on Earth.
IN CLASS:
- Find the missing dates on the chart below.
- Calculate the corresponding distance on a football field that each event represents. For this activity, 1 yard = 50 million years.
- Mark the events on the Geologic Time Football Field. One end zone is labeled "TODAY" and the other is "BEFORE EARTH." As you place your events on the football field, pay close attention to which end is which. Place events at the nearest yard.
- Lightly shade the Precambrian Era red, the Paleozoic Era blue, the Mesozoic Era yellow, and the Cenozoic Era green.
Answer the following questions on the back of your timescale.
- What event is exactly in the middle of the football field?
- Find where your birth date would be on the football field. Why is it almost impossible to show a human's life on this scale?
- In what eras did fish, mammals, reptiles, amphibians, and plants appear on Earth?
- How many years does the first 10 yards represent? What events are within the first 10 yards of the football field from the TODAY end zone?