Topic 13: Geologic History

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Topic 13: Geologic History

 Dating Geologic Events

Relative dating is the process where rocks are placed in their proper sequence or order -- only the chronological order of events is determined (Ex/ what’s older and younger)

Absolute dating is the process which determines the actual age of the rock or structure

Relative Dating

We assume that present geologic processes (weathering, erosion, etc.) have not changed throughout history. The principle of uniformitarianism means “the present is the key to the past.” [The opposite of this is catastrophism: the belief that changes occur because of sudden catastrophes.]

The Law of Superposition states that the bottom layer of a series of sedimentary layers is the oldest, unless the series has been overturned or has had older rock thrust over it.

General rules used in relative dating:

1. An igneous intrusion is younger than the rock it cuts through.

2. An extrusion of igneous rock is younger than the rock it rests on, but older than any rocks on top of it.

3. Rocks are older than faults, joints, folds, or veins appearing in them.

4. Fragments of unmelted material within a rock are older than the surrounding rock.

5. In sedimentary rocks, the sediments are older than the cementing material and the rock formation.

When rocks have been formed, then eroded, then more rocks form on top, there is a buried erosional surface between the old and new which is called an unconformity, and represents a gap in the sequence of events.

Correlation

Correlation is the process of showing that rocks or geologic events occurring at different locations are of the same age.

Correlation techniques:

 “Walking the outcrop”: physically following a particular layer or formation from one location to another

 Similarity of rock -- similarities in appearance, thickness, composition, and position

 Index fossils -- the remains of imprints of a particular plant or animal that:

1. existed for a relatively short period of time

2. lived in many different parts of the world

 Volcanic ash deposits -- deposited over a very large area during a brief period of time

When using these techniques, if the results are misleading or incorrect, it’s referred to as an anomaly (something that doesn’t fit the normal pattern)

An unconformity is a buried erosional surface. Where the surface has been eroded away, there’s a gap in the rock record. The presence of an unconformity indicates uplift, weathering, and erosion (leaving a gap), then subsidence, then more deposition.

In the above diagrams, rock outcrops were taken from two different locations. The index fossils and similarities of rock strata suggest that rock layers B and D are the same, and rock layers C and F are the same. The wavy line between layers B and C is an unconformity and suggests a long period of erosion, especially due to the fact that rock layer E is missing from the diagram on the left. The material was probably there at one time, but was eroded away from the rock record in this location.

Absolute Dating

Finding the absolute age of a rock or fossil is possible because of radioactivity.

Elements are composed of atoms, which are made up of 3 types of particles:

Protons: positive charge, found in the nucleus

Neutrons: no charge, found in the nucleus

Electrons: negative charge, orbit the nucleus

One element is different from another because of the number of protons in its nucleus. However, the same element can have many forms, called isotopes. Different isotopes of a given element have the same number of protons but different numbers of neutrons.

Radioactive Decay

- The nuclei of some isotopes are unstable and undergo radioactive decay in order to form more stable elements. These are called radioactiveisotopes or radioisotopes, and they emit subatomic particles and electromagnetic energy during the decay process.

- The number of atoms that decay within a given amount of time is predictable, depending on the element and the original amount of material.

- Radioactivity decreases at a steady and predictable rate, not affected by temperature, pressure, etc.

- The length of time necessary for half of a sample to disintegrate (change into a more stable form) is called the half-life of that element [see ESRT].

EX/ A rock has a certain amount of radioactive element X upon formation. After one half-life, half of the original element X will remain. In another half-life, half of what is remaining will be gone, then half of that, then half of that, and so on.

Radioactive Dating

When a radioactive isotope decays, the amount of the original (parent) isotope decreases, while the amount of the new (daughter) isotope increases. By using the ratio of the amounts of parent and daughter isotopes in a sample, and knowing the half-life of the radioisotope, scientists can determine the age of a sample. These radioisotopes occur naturally in many rock-forming minerals.

EX/ The half-life of element “Davenportium” is 100 years. Davenport decays into the more stable element, Oneontium. You start with a sample of volcanic rock that is 100% Davenportium and 0% Oneontium upon formation. In 100 years, half of the rock will be changed into Oneontium, the other half will still be Davenportium. In another 100 years (another half-life), half of the remaining Davenportium will decay, leaving 25% of the rock as Davenportium and 75% Oneontium. In another 100 years, there will be 12.5% Davenportium, 87.5% Oneontium, and so forth. So, for instance, if you find a rock that contains ¼ Davenportium and ¾ Oneontium, you know it has gone through two half-lives. You can look up the half-life of the element and find out that the rock is 200 years old.

For the remains of plants and animals, radiocarbon dating is used. All living organisms contain a constant proportion of Carbon-14, which decays to Nitrogen-14 when the organism dies. The half-life, however, is very short (5,770 years) so radiocarbon dating is used only on samples less than 50,000 years old.

How old is the Earth?

The oldest crustal rocks found have been dated at 4.1 billion years

Oldest moon rocks = 4.5 billion years

All meteorites = 4.5 billion years

Conclusion: The solar system, including Earth, is approximately 4.5 billion years old.

The Record in the Rocks

- Earth scientists used several “clues” to determine relative ages of rock formations (superposition, igneous intrusions, etc.) but they were related to physical events only, so correlation was limited to local regions. The big breakthrough came when it was discovered that fossils could be used to correlate rock formations.

- Geologists now recognize that a definite pattern exists in the fossil record.

- In general, the plants and animals found in younger rocks are more complex and more specialized than those found in older rocks.

- Based on this evolutionary trend and the presence of certain fossils, geologists have been able to piece together a chronological record extending from the time of the earliest fossils to the present. This rock record is sometimes referred to as the Geologic Column.

 Geologic Time Scale

- Geologic history can be divided into time units based on fossil evidence.

- For most of the 4 ½ billion years of the Earth’s history, there is little or no evidence of life. This timespan is known as the Precambrian Era.

- About 570 million years ago a dramatic change took place -- a sudden “flowering” of life forms. From these fossils to the present, we have a continuous record of life on Earth.

- The geologic time scale is divided into 4 major eras:

Precambrian (before life)

Paleozoic (ancient life)

Mesozoic (middle life)

Cenozoic (recent life)

- The boundaries between these eras are clearly marked by major crustal events, such as mountain building, subsidence, etc.

- Eras are divided into units called periods, which are divided into epochs.

- The geologic time scale was originally a scale of relative time, but was converted to absolute ages once radioactivity was discovered.

- Dates are expressed in millions of years before present (mybp or mya)

- Precambrian time begins with the origin of the Earth about 4.5 billion years ago, and ends about 570 million years ago

- Evidence of man appears in the rock record about 2 million years ago, representing only 1/2,250 of the entire record!

See your ESRT for more information on the Geologic Time Scale as it pertains to New York State.

The Fossil Record

- The fossil record starts about 570 million years ago. This doesn’t mean that life didn’t exist in Precambrian time, but that evidence of life is scarce…….Precambrian life was mostly algae, fungi, and wormlike animals that don’t preserve well.

- To become a fossil:

 the organism must possess hard body parts (shells or skeletons)

 the remains must be trapped in some medium (mud, tar, ash, etc.)

 the remains must be sealed off from scavengers and bacteria

Evolution

- The classification of living organisms is called taxonomy

- Living things are divided into groups (in order): kingdom, phylum, class, order, family, genus, species.

- A species is a group of organisms that live in similar habitats and interbreed in nature. Although members within a species are very similar, there is a great deal of variation between individual members, which can be important to survival.

- When the environment changes, some members of a species may be better adapted to survive under the new conditions, due to some minor variation or characteristic. With time, most members of the species will show this characteristic.

- Gradual change in a species over long periods of time is known as evolution. As time goes on, there may be so many changes that a new species is formed. If members of the old species all died out, they are extinct, and the new species takes its place.

- Sometimes the environment undergoes a sudden drastic change, causing some species to become extinct.

- The vast majority of life forms that have occurred on Earth have become extinct, and it is believed that we know of only about 2% of the total number of species that have ever existed.