Geologic Time

Geologic Perspective

        Geologists attempt to unravel events and materials that may have occurred or formed millions to billions of years ago. The immensity of geologic time is very difficult to appreciate from our human perspective, but appreciation is necessary to understand the history of the Earth. There are two basic ways we try to make sense of geologic time:

• Relative Dating - Placing geologic events in sequential order as determined by their position in the geologic record. Produced the Geologic Time Scale.
• Absolute Dating - Gives specific dates for events and materials expressed in years before the present. Radiometric dating (radioactivity was discovered in the late 1800s) is the most common method used to obtain absolute dates. Allowed dates to be placed on the Geologic Time Scale.

        Through genealogies and history recorded in the Bible, James Ussher determined the date of creation to be 4004 BC, which required the Earth and all its features to be no more than about 6,000 years old. These ideas dominated Western thinking about Earth history before the 18th century.

        During the 18th and 19th centuries, different attempts were made to more scientifically determine the Earth's age.

• Georges Louis de Buffon (mid-1700s) - Assumed the Earth was originally molten and calculated age of Earth to be at least 75,000 years from the Earth's present temperature and assuming a rapid rate of cooling.
• Rate of Sedimentation - Several scientists attempted to calculate the age of the Earth from the deposition rate of various sediments and the thickness of sediments in the Earth's crust. Problem with this approach is that deposition rate is not uniform for a particular sediment; sediment is removed by erosion, modified by compaction, or not deposited, so a complete record of sedimentation does not exist. Gave ages from 3 million to 1.5 billion years.
• Ocean Salinity - John Joly (1800s) calculated the age of the Earth from the current salinity of the ocean, assuming that it was originally pure water and that the salt in it was derived from erosion of the continents. Problems are that erosion rate is not constant, loss and recycling of salt was not considered, and salt is not only obtained from the continents. Gave an age of 90 million years.

        James Hutton (mid-1700s) recognized that the present-dav processes have operated throughout geologic time. With enough time, small changes could have tremendous effects. Hutton's ideas were popularized by Charles Lyell's book, Principles of Geology (1830).

        Lord Kelvin (1866), the most influential physicist of his time, made calculations which indicated the Earth could not be more than 100 million years old or younger than 20 million years. Calculations assumed an originally molten Earth and conventional heat sources for the Earth and Sun. The discovery of radioactivity in late 1800s proved his calculations to be invalid.
 
 

Geologic time was originally subdivided based on the relative positions of sedimentary rocks.

    The chronological sequence of rock units can be determined by six fundamental principles:

1. Principle (Law) of Superposition - In an undeformed sequence of sedimentary rocks, the youngest beds are at the top and the oldest beds are at the bottom (also applies to volcanic rocks). Unfortunately, there is no place on earth where the entire history of sedimentation is preserved.
2. Principle of Original Horizontality - The observation that sediment particles deposited from water under the influence of gravity form essentially horizontal layers. Non-horizontal rocks have been disturbed after deposition and lithification.
3. Principle of Lateral Continuity - Sediment extends laterally in all directions until it thins, pinches out, or terminates against the edge of the depositional basin.
4. Law of Cross-Cutting Relationships - An intrusion or fault that cuts through another rock is younger than the rock it cuts. [Sills vs buried lava flow? Must look for heat effects.]
5. Principle of Inclusion - Inclusions are older than the rock that contains them.
6. Principle of Faunal Succession - Fossil organisms succeed one another in a definite and determinable order, so any time period can be recognized by its fossil content. General evolution pattern is from simple to complex organisms.

        Unconformities are surfaces of erosion or non-deposition of sediment that separate younger rocks from older rocks. The time gap in the rock record is known as a hiatus. Result in incomplete rock records. Three types of unconformities:

• Disconformity - Break in the rock record caused by erosion or non-deposition of sediment. Rocks on either side of the break are essentially parallel and may look like a bedding plane. Fossils must be used to determine the length of the break in deposition.
• Angular Unconformity - Tilted or folded sedimentary rocks are overlain by more flat-lying strata.
• Nonconformity - An unconformity between two very different rock types. Example: Boundary between an eroded igneous intrusion and overlying sedimentary rocks. May look like an intrusive contact, but there are no heat effects. Inclusions may be useful in distinguishing a nonconformity.

    Correlation involves matching up rock layers of similar age in different regions. Methods utilized include:

• Correlation by Physical Features - Matching rock units on a small scale by tracing the unit across an outcrop, noting the place of the unit in a sequence of rock strata, or by identifying the same bed in separate areas because of its distinctive composition (key beds).
• Correlation by Fossils - Matching rock units of similar age on a large scale by using index (or guide) fossils (fossils that were widespread geographically and lived only a short time). Allows widely separated rocks of different composition to be correlated. Overlapping time ranges of several sets of index fossils are typically used.

        The Geologic Column is the chronologic arrangement of rock units from oldest at the bottom to youngest at the top. Boundaries between different time periods are marked by a dramatic change in the fossil record.
 

• Precambrian - The oldest time period representing over 80% of geological time. Rocks contain only rare fossils. Divided into two eras:

1. Archean - Earliest evidence of life (cyanobacteria).
2. Proterozoic - First organisms with shells.

• Phanerozoic = Visible Life - Geologic time after the Precambrian. Because fossils are abundant, more time subdivisions can be made. Largest time subdivisions are eras:

1. Paleozoic = Ancient Life - Beginning marked by first abundant fossil evidence (570 mya).
2. Mesozoic = Middle Life - Beginning marked by first abundance of reptiles (245 mya).
3. Cenozoic = Recent Life - Beginning marked by the extinction of dinosaurs and the rise of mammals (66 mya).

        Radiometric dating has allowed dates to be placed on geologic events and ages to be placed on the formation of geologic materials. Oldest evidence for life is about 3.6 billion years. The oldest rocks found on Earth (Australia) are 3.96 billion years old. The Earth is about 4.6 billion years old (meteorites date from 4.5 to 4.8 billion years old).

        The process by which radioactive elements (unstable isotopes) break down by nuclear decay to a different element. Products of radioactivity include:

• Radiation - Emitted by the nucleus during decay.
1. Alpha Particle - Composed of 2 protons and 2 neutrons. Loss reduces mass number by 4 and atomic number by 2.
2. Beta Particles - An electron produced by the break-up of a neutron (neutron = proton + electron). Loss does not change mass number, but increases atomic number by 1. A nucleus can also gain a beta particle (electron-capture), which decreases the atomic number by 1.
3. Gamma Ray - High energy X-ray. Electromagnetic radiation. Nucleus looses energy without a change in mass or atomic number. Most dangerous form of radiation.
• Heat - Energy is emitted from the nucleus partly as heat.
• Daughter Isotopes - Different elements produced by changes in atomic number.

        Each unstable parent element decays to a more stable daughter element at a characteristic and fixed rate. The time it takes for one half of the atoms in the sample to decay is called the half-life. Decay proceeds at a geometric rate. By measuring the amount of parent and daughter isotopes in a sample and knowing the half life of the parent, an age can be determined for the sample.

        Sedimentary rock radiometric dates are generally meaningless because the minerals making up the rock are parts of other, preexisting rocks and therefore do not give the age of the sedimentary rock. The only exceptions to this rule are some sandstones and shales that contain a potassium-bearing mineral, glauconite, that forms at the time of sediment deposition. More often, sedimentary rock ages are bracketed by dating igneous and metamorphic rocks. The most accurate dates are obtained from igneous rocks because metamorphism can affect the parent/daughter ratio. For good dates, there must be no gain or loss of parent or daughter isotopes (closed system). Typically more than one isotope is used for cross-checking. There are six radioactive isotopes that are useful in geologic dating: Carbon 14, Potassium 40, Rubidium 87, Thorium 232, and Uranium 235 and 238.
 

1. Long-Lived Radioactive Isotope Pairs - All of the isotopes listed above except Carbon 14 have half-lives > 1 billion years. Used to date very old materials (meteorites, igneous intrusives, lunar samples, oldest rocks on Earth).
2. Fission-Track Dating - Uranium 238 spontaneously decays by fission. Particles from the nucleus make tracks in rock minerals which can be counted and tied to a number of years. This dating method has the largest useful age range of any radiometric method (40,000 to 1 million years).
3. Radiocarbon Dating - Nitrogen 14 is converted to Carbon 14 in the atmosphere through neutron bombardment (involves the loss of a proton). Carbon 14 is radioactive and spontaneously decays back to Nitrogen 14 through beta decay. Ratio of Carbon 14 to Carbon 12 remains constant until the organism dies. Useful for dating materials less than 70,000 years old. Correction factors must be applied to account for variations in the amount of Carbon 12 and 14 in the atmosphere over time. Good match with tree ring dating methods (up to 14,000 years old).

        The chronologic sequence of units of relative geologic time. Radiometric dates were added later. Same time subdivisions apply as for the Geologic Column. All dating methods prior to radiometric dating underestimated the age of the Earth.