The Geologic Time Scale
Earth formed 4.5 billion years ago. Geologists divide this time span into smaller periods. Many of the divisions mark major events in life history.
Dividing Geologic Time
Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic time scale. So, many of the names of the time periods are from places in Europe. The Jurassic Period is named for the Jura Mountains in France and Switzerland, for example.
Putting Events in Order
To create the geologic time scale, geologists correlated rock layers. Steno's laws were used to determine the relative ages of rocks. Older rocks are at the bottom, and younger rocks are at the top. The early geologic time scale could only show the order of events. The discovery of radioactivity in the late 1800s changed that. Scientists could determine the exact age of some rocks in years. They assigned dates to the time scale divisions. For example, the Jurassic began about 200 million years ago. It lasted for about 55 million years.
Divisions of the Geologic Time Scale
The largest blocks of time on the geologic time scale are called “eons.” Eons are split into “eras.” Each era is divided into “periods.” Periods may be further divided into “epochs.” Geologists may just use “early” or “late.” An example is “late Jurassic,” or “early Cretaceous.” Pictured below is the geologic time scale (Figure below).
Earth formed 4.5 billion years ago. Geologists divide this time span into smaller periods. Many of the divisions mark major events in life history.
Dividing Geologic Time
Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic time scale. So, many of the names of the time periods are from places in Europe. The Jurassic Period is named for the Jura Mountains in France and Switzerland, for example.
Putting Events in Order
To create the geologic time scale, geologists correlated rock layers. Steno's laws were used to determine the relative ages of rocks. Older rocks are at the bottom, and younger rocks are at the top. The early geologic time scale could only show the order of events. The discovery of radioactivity in the late 1800s changed that. Scientists could determine the exact age of some rocks in years. They assigned dates to the time scale divisions. For example, the Jurassic began about 200 million years ago. It lasted for about 55 million years.
Divisions of the Geologic Time Scale
The largest blocks of time on the geologic time scale are called “eons.” Eons are split into “eras.” Each era is divided into “periods.” Periods may be further divided into “epochs.” Geologists may just use “early” or “late.” An example is “late Jurassic,” or “early Cretaceous.” Pictured below is the geologic time scale (Figure below).
Relative Age Dating
Early geologists had no way to determine the absolute age of a geological material. If they didn't see it form, they couldn't know if a rock was one hundred years or 100 million years old. What they could do was determine the ages of materials relative to each other. Using sensible principles they could say whether one rock was older than another and when a process occurred relative to those rocks.
Early geologists had no way to determine the absolute age of a geological material. If they didn't see it form, they couldn't know if a rock was one hundred years or 100 million years old. What they could do was determine the ages of materials relative to each other. Using sensible principles they could say whether one rock was older than another and when a process occurred relative to those rocks.
- 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.
- Lateral continuity: Sediments are deposited in continuous sheets that span the body of water that they are deposited in. When a valley cuts through sedimentary layers, it is assumed that the rocks on either side of the valley were originally continuous.
- Superposition: Sedimentary rocks are deposited one on top of another. The youngest layers are found at the top of the sequence, and the oldest layers are found at the bottom.
|
More Principles of Relative Dating
Other scientists observed rock layers and formulated other principles. Geologist William Smith (1769-1839) identified the principle of faunal succession, which recognizes that:
|
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 Grand Canyon
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).
|
|
- What do you determine when you're doing relative dating? What are you not determining?
- What is the Law of Superposition? What is the exception?
- What is the Law of Original Horizontality? If rocks are not horizontal what does that mean?
- What is the Law of Cross-Cutting Relationships?
- What is the Law of Inclusions?
- What is an unconformity? What can cause an unconformity?
- What does an angular unconformity look like? What does this indicate?
- What happened during an unconformity and how do we know that?
- How do you know where there is a disconformity?
- What happened to create an nonconformity? What can you look for to identify a nonconformity?
This text was adapted from CK12.com. It is licensed under the Creative Commons (CC BY-NC 3.0)
What Is Radioactive Dating?
Radioactive isotopes, or radioisotopes, can be used to estimate the ages of not only of rocks, but also of fossils and artifacts made long ago by human beings. Even the age of Earth has been estimated on the basis of radioisotopes. The general method is called radioactive dating. To understand how radioactive dating works, you need to understand radioisotopes and radioactive decay.
Radioisotopes and Radioactive Decay
A radioisotope has atoms with unstable nuclei. Unstable nuclei naturally decay, or break down. They lose energy and particles and become more stable. As nuclei decay, they gain or lose protons, so the atoms become different elements. This is illustrated in the Figure below. The original, unstable nucleus is called the parent nucleus. After it loses a particle (in this case a type of particle called an alpha particle), it forms a daughter nucleus, with a different number of protons.
Radioactive isotopes, or radioisotopes, can be used to estimate the ages of not only of rocks, but also of fossils and artifacts made long ago by human beings. Even the age of Earth has been estimated on the basis of radioisotopes. The general method is called radioactive dating. To understand how radioactive dating works, you need to understand radioisotopes and radioactive decay.
Radioisotopes and Radioactive Decay
A radioisotope has atoms with unstable nuclei. Unstable nuclei naturally decay, or break down. They lose energy and particles and become more stable. As nuclei decay, they gain or lose protons, so the atoms become different elements. This is illustrated in the Figure below. The original, unstable nucleus is called the parent nucleus. After it loses a particle (in this case a type of particle called an alpha particle), it forms a daughter nucleus, with a different number of protons.
|
The nucleus of a given radioisotope decays at a constant rate that is unaffected by temperature, pressure, or other conditions outside the nucleus. This rate of decay is called the half-life. The half-life is the length of time it takes for half of the original amount of the radioisotope to decay to another element.
Q: How can the half-life of a radioisotope be used to date a rock? A: After a rock forms, nuclei of a radioisotope inside the rock start to decay. As they decay, the amount of the original, or parent, isotope decreases, while the amount of its stable decay product, or daughter isotope, increases. By measuring the relative amounts of parent and daughter isotopes and knowing the rate of decay, scientists can determine how long the parent isotope has been decaying. This provides an estimate of the rock’s age. |
Different Isotopes, Different Half-Lives
Different radioisotopes decay at different rates. You can see some examples in the Table below. Radioisotopes with longer half-lives are used to date older rocks or other specimens, and those with shorter half-lives are used to date younger ones. For example, the oldest rocks at the bottom of the Grand Canyon were dated by measuring the amounts of potassium-40 in the rocks. Carbon-14 dating, in contrast, is used to date specimens that are much younger than the rocks in the Grand Canyon. You can read more carbon-14 dating below.
Different radioisotopes decay at different rates. You can see some examples in the Table below. Radioisotopes with longer half-lives are used to date older rocks or other specimens, and those with shorter half-lives are used to date younger ones. For example, the oldest rocks at the bottom of the Grand Canyon were dated by measuring the amounts of potassium-40 in the rocks. Carbon-14 dating, in contrast, is used to date specimens that are much younger than the rocks in the Grand Canyon. You can read more carbon-14 dating below.
Focus on Carbon-14 Dating
One of the most familiar types of radioactive dating is carbon-14 dating. Carbon-14 forms naturally in Earth’s atmosphere when cosmic rays strike atoms of nitrogen-14. Living things take in and use carbon-14, just as they do carbon-12. The carbon-14 in living things gradually decays to nitrogen-14. However, as it decays, it is constantly replaced because living things keep taking in carbon-14. As a result, there is a constant ratio of carbon-14 to carbon-12 in organisms as long as they are alive.
After organisms die, the carbon-14 they already contain continues to decay, but it is no longer replaced. Therefore, the carbon-14 in a dead organism constantly declines at a fixed rate equal to the half-life of carbon-14. Half of the remaining carbon-14 decays every 5,700 years. If you measure how much carbon-14 is left in a fossil, you can determine how many half-lives (and how many years) have passed since the organism died.
One of the most familiar types of radioactive dating is carbon-14 dating. Carbon-14 forms naturally in Earth’s atmosphere when cosmic rays strike atoms of nitrogen-14. Living things take in and use carbon-14, just as they do carbon-12. The carbon-14 in living things gradually decays to nitrogen-14. However, as it decays, it is constantly replaced because living things keep taking in carbon-14. As a result, there is a constant ratio of carbon-14 to carbon-12 in organisms as long as they are alive.
After organisms die, the carbon-14 they already contain continues to decay, but it is no longer replaced. Therefore, the carbon-14 in a dead organism constantly declines at a fixed rate equal to the half-life of carbon-14. Half of the remaining carbon-14 decays every 5,700 years. If you measure how much carbon-14 is left in a fossil, you can determine how many half-lives (and how many years) have passed since the organism died.
|
|
|
This text was adapted from CK12.com. It is licensed under the Creative Commons (CC BY-NC 3.0)
Formation of Earth
Earth formed at the same time as the other planets. The history of Earth is part of the history of the Solar System.
Planets Form
Earth came together (accreted) from the cloud of dust and gas known as the solar nebula nearly 4.6 billion years ago, the same time the Sun and the rest of the solar system formed. Gravity caused small bodies of rock and metal orbiting the proto-Sun to smash together to create larger bodies. Over time, the planetoids got larger and larger until they became planets.
Molten Earth
When Earth first came together it was really hot, hot enough to melt the metal elements that it contained. Earth was so hot for three reasons:
When Earth was entirely molten, gravity drew denser elements to the center and lighter elements rose to the surface. The separation of Earth into layers based on density is known as differentiation. The densest material moved to the center to create the planet’s dense metallic core. Materials that are intermediate in density became part of the mantle (Figure below).
Earth formed at the same time as the other planets. The history of Earth is part of the history of the Solar System.
Planets Form
Earth came together (accreted) from the cloud of dust and gas known as the solar nebula nearly 4.6 billion years ago, the same time the Sun and the rest of the solar system formed. Gravity caused small bodies of rock and metal orbiting the proto-Sun to smash together to create larger bodies. Over time, the planetoids got larger and larger until they became planets.
Molten Earth
When Earth first came together it was really hot, hot enough to melt the metal elements that it contained. Earth was so hot for three reasons:
- Gravitational contraction: As small bodies of rock and metal accreted, the planet grew larger and more massive. Gravity within such an enormous body squeezes the material in its interior so hard that the pressure swells. As Earth’s internal pressure grew, its temperature also rose.
- Radioactive decay: Radioactive decay releases heat, and early in the planet’s history there were many radioactive elements with short half lives. These elements long ago decayed into stable materials, but they were responsible for the release of enormous amounts of heat in the beginning.
- Bombardment: Ancient impact craters found on the Moon and inner planets indicate that asteroid impacts were common in the early solar system. Earth was struck so much in its first 500 million years that the heat was intense. Very few large objects have struck the planet in the past many hundreds of millions of year.
When Earth was entirely molten, gravity drew denser elements to the center and lighter elements rose to the surface. The separation of Earth into layers based on density is known as differentiation. The densest material moved to the center to create the planet’s dense metallic core. Materials that are intermediate in density became part of the mantle (Figure below).
First Crust
Lighter materials accumulated at the surface of the mantle to become the earliest crust. The first crust was probably basaltic, like the oceanic crust is today. Intense heat from the early core drove rapid and vigorous mantle convection so that crust quickly recycled into the mantle. The recycling of basaltic crust was so effective that no remnants of it are found today.
Early Solar System Materials
There is not much material to let us know about the earliest days of our planet Earth. What there is comes from three sources: (1) zircon crystals, the oldest materials found on Earth, which show that the age of the earliest crust formed at least 4.4 billion years ago; (2) meteorites that date from the beginning of the solar system, to nearly 4.6 billion years ago; and (3) lunar rocks, which represent the early days of the Earth-Moon system as far back as 4.5 billion years ago.
Lighter materials accumulated at the surface of the mantle to become the earliest crust. The first crust was probably basaltic, like the oceanic crust is today. Intense heat from the early core drove rapid and vigorous mantle convection so that crust quickly recycled into the mantle. The recycling of basaltic crust was so effective that no remnants of it are found today.
Early Solar System Materials
There is not much material to let us know about the earliest days of our planet Earth. What there is comes from three sources: (1) zircon crystals, the oldest materials found on Earth, which show that the age of the earliest crust formed at least 4.4 billion years ago; (2) meteorites that date from the beginning of the solar system, to nearly 4.6 billion years ago; and (3) lunar rocks, which represent the early days of the Earth-Moon system as far back as 4.5 billion years ago.
- What did the shock wave do to the material that was gathered in our nebula?
- Why do scientists think that a supernova was necessary for the solar system to form?
- Why did fusion start in the center?
- How do seeds of planets form?
- How did the moon and Earth form?
This text was adapted from CK12.com. It is licensed under the Creative Commons (CC BY-NC 3.0)
Plate Tectonics
The lithosphere is divided into a dozen major and several minor plates. A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both.
The movement of the plates over Earth's surface is termed plate tectonics. Plates move at a rate of a few centimeters a year, about the same rate fingernails grow.
How Plates Move
If seafloor spreading drives the plates, what drives seafloor spreading?
This goes back to Arthur Holmes’ idea of mantle convection. Picture two convection cells side by side in the mantle, similar to the illustration in Figure below.
The lithosphere is divided into a dozen major and several minor plates. A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both.
The movement of the plates over Earth's surface is termed plate tectonics. Plates move at a rate of a few centimeters a year, about the same rate fingernails grow.
How Plates Move
If seafloor spreading drives the plates, what drives seafloor spreading?
This goes back to Arthur Holmes’ idea of mantle convection. Picture two convection cells side by side in the mantle, similar to the illustration in Figure below.
- Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust.
- The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor.
- The outer limbs of the convection cells plunge down into the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches.
- The material sinks to the core and moves horizontally.
- The material heats up and reaches the zone where it rises again.
|
|
Plate Boundaries
Plate boundaries are the edges where two plates meet. How can two plates move relative to each other? Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. The features found at these plate boundaries are the mid-ocean ridges, trenches, and large transform faults (Figure below).
Plate boundaries are the edges where two plates meet. How can two plates move relative to each other? Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. The features found at these plate boundaries are the mid-ocean ridges, trenches, and large transform faults (Figure below).
- Divergent plate boundaries: the two plates move away from each other.
- Convergent plate boundaries: the two plates move towards each other.
- Transform plate boundaries: the two plates slip past each other.
|
|
Review
- How does the topography of the seafloor give evidence for seafloor spreading?
- How does seafloor spreading fit into the idea that continents move about on Earth’s surface?
- How do convection cells drive the plates around Earth’s surface?
- What are the three types of plate boundaries?
This text was adapted from CK12.com. It is licensed under the Creative Commons (CC BY-NC 3.0)