Two methods to assess geologic time: RELATIVE dating and ABSOLUTE dating.
What’s the difference?
GEOLOGIC TIME Geologic time involves events ranging in time from a few seconds (earthquakes, meteorite impacts) to millions and even billions of years (the movement of tectonic plates, the formation of the earth itself). Geologists use two different methods to determine geologic time and the ages of geologic features and events: 1. Relative age dating involves establishing the order of features and events, without necessarily determining their actual age in years. 2. Absolute age dating involves determining the actual age (in thousands, millions, or billions of years) of a rock or other geologic feature.
Layers of rock are said to be conformable when they are found to have been deposited layer after layer without interruption. Although particular sites may exhibit conformable beds representing significant spans of geologic time, there is no place on earth that contains a full set of conformable strata.
Throughout earth history, the deposition of sediment has been interrupted over and over again. All such breaks in the rock record are termed unconformities . An unconformity represents a long period of time during which deposition ceased and/or erosion removed previously formed rocks.
Nonconformities are a type of unconformity in which the break separates older metamorphic or intrusive igneous rocks from younger sedimentary strata.
Intrusive igneous masses and metamorphic rocks originate far below the surface. Therefore, for a nonconformity to develop, there must be a period of uplift and the erosion of overlying rocks. Once exposed at the surface, the igneous or metamorphic rocks are subjected to weathering/erosion prior to sedimentation.
The Fossil Record What about life represented in the rock record? William Smith’s Principle of Faunal Succession Examined fossils in rocks. Found that you could match them up all over the world based on the assemblages (group of fossils) in rocks. Rocks around the world can be correlated based on their fossil content (formed at the same TIME under the same CONDITIONS just in different locations). Principle of Faunal Succession suggests that throughout geologic time, there has been a regular succession of organisms. Fossils succeed one another in a definite and recognizable order, and therefore any time period can be recognized by its fossil content.
When fossils are arranged according to their age by using the law of superposition on the rocks in which they are found, they do not present a random or haphazard picture. To the contrary, fossils show progressive changes from simple to complex and reveal the advancement of life through time.
For example, in the fossil record there is represented, in succession, an age of trilobites. an age of fishes, an age of reptiles, and an age of mammals. These ages pertain to groups that were especially plentiful and characteristic during particular time periods. Within each of the ages, there are many subdivisions based on certain species. This same succession of dominant organisms, never out of order, is found on every major landmass.
Because of faunal succession, each fossil species can be given a geologic range the first appearance of the fossil in the rock record to the last appearance of the fossil in the rock record. It is the range that an organism existed and left behind fossil evidence.
Geologic ranges vary among groups of organisms.
Correlation and Faunal Succession Correlation by fossils. Certain index fossils are keys to matching and dating sedimentary strata in widely separated outcrops
Fossils that are good for correlating and dating rocks all over the world.
Easy to identify.
Short life span for the species (small geologic range).
Fossil assemblages are generally more useful for dating rocks than a single fossil because the sediment must have been deposited at a time when all the species represented existed.
As you can see in this image the fossil remains of living things are present in the rock layers at definite intervals, and exist within a discrete period of time. In this instance, using the Law of Superposition, would the age Rock Unit A be older or younger than the age of Rock Unit B?
The geologic time scale was based on relative dating. Absolute ages have now been assigned by way of radiometric dating.
Radiometric dating is a reliable means of calculating the ages of rocks and minerals that contain radioactive elements. How?
Absolute Age Dating Determining the actual age of a rock, in years, requires some sort of natural “clock”. The clock that geologists use is the radioactive decay of certain elements within rocks. Radioactive decay is when an unstable element changes (decays) into another element, or a new variation of itself . This change occurs at a precise rate that can be determined by experimentation.
Radioactive elements are not ‘stable’ like atoms they vary in the number of neutrons in the nucleus forming different isotopes.
Atoms of different isotopes are the same element but they have different atomic weights (due to varying number of neutrons).
Absolute Dating In order to become stable radioactive elements must lose particles (radioactive decay). By convention, the unstable element is called the “ parent ”, while the element resulting from the decay of the parent is called the “ daughter ”. The speed (rate) at which a particular parent element changes into a daughter element is expressed as the half-life : the time required for half of the parent atoms to change into daughter atoms.
Alpha decay is when an unstable parent atom emits an alpha particle (a small atom consisting of 2 protons and 2 neutrons). Beta decay is when an unstable parent emits a beta particle (an electron). Both types of decay produce a new, stable daughter atom. Two types of radioactive decay are common.
Bottom Line: To determine the age when a rock formed, you need to know two things:
- the percentage of parent and daughter atoms in the rock
- the half-life (rate of decay)
An example follows on the next 2 slides…..
Notice in this figure how the percentage of parent atoms decreases, and the percentage of daughter atoms increases, as time goes by. By measuring the rate at which this occurs (the half-life), and measuring the percentage of parent and daughter atoms in the rock, we can determine the age of the rock!
Different radioactive elements decay at different rates. You will choose your radioactive element wisely depending on what you are dating.
C-14 = 5730 yrs; U-238 = 4.5byrs.
A form of radioactive decay that is particularly useful for determining the ages of the remains of plants and animals is the decay of carbon-14 to carbon-12. C-14 is made continuously in the upper atmosphere, so all living things absorb a fraction of C-14 in their tissues as they grow. After death, C-14 decays gradually to N-14. By measuring the percentage of C-14 versus N-14 in plant and animal remains, we can determine their age. Carbon-14 radiometric dating can be used to determine ages of organic remains up about 50,000 years old.
FORMATION and AGE of the EARTH Our solar system consists of 8 planets (not including Pluto) orbiting the Sun in a single plane, all traveling in the same direction. Most scientists think this reflects the formation of the solar system from a nebula - a spinning cloud of gas and dust that slowly contracted under its own gravity, flattened into a disc, and eventually coalesced into the Sun and the planets, including Earth.
When did the Earth and the rest of the solar system form? Radiometric dating (based on the decay of uranium into lead) shows us that the oldest rocks yet found on earth are nearly 4.0 billion years old . Furthermore, certain isolated mineral grains (eroded out of rocks that are no longer present) have been dated as old as 4.3 billion years . However, scientists presently accept 4.6 billion years as the age of formation of the earth. This is the age (based again on the decay of uranium into lead) of the oldest moon rocks and meteorites . Since we assume that all objects in the solar system formed at about the same time (see previous slide), we assume that the earth is about as old as the moon and meteorites. Our earth today is a result of geologic processes that have acted over an immense span of geologic time!