Geologic time can be expressed in two ways
1) Relative dating--places geologic events into a sequence and refers to them in their order of occurrence. This is typically determined from their position in the rock record or from comparison of fossils. Studying the fossil record of life is called PALEONTOLOGY.
2) Absolute dating–results in an absolute age. Specific dating techniques such as radioactive dating or dendrochronoloy (counting tree rings) are expressed in years before the present.
Relative Dating and the Sequence of Events
James Hutton and Uniformitarianism
The great age of the earth and the discovery of a complex sequence events in the rocks of Scotland led the famous Scottish geologist James Hutton to the concept of UNIFORMITARIANISM. Hutton believed that the present was the key to the past and that uniformitarianism, the idea that physical processes had worked the same in the past as in the present, was the key to interpreting earth history. His observations of sequences of weathering, erosion, deposition, distortion of sedimentary strata, intrusion by igneous rocks, and so on, made him state, "No vestige of a beginning, no prospect of an end." which meant that the earth was very old for all of these things to have happened. His observations also lead to a concept of the rock cycle.
Five Key Concepts of interpreting Earth History
To interpret the past, several key concepts must be remembered. These ideas include those of Nichalous Steno, 15th century, Danish geologist. Steno proposed 3 rules:
I. Superposition. The oldest rock in a sequence of layered rocks is on the bottom of the pile
II. Original horizontality. The layered rocks such as sediment, pyroclastics, and lava were originally deposited in level, flat-lying layers, as in a cake.
III. Lateral continuity. Layers of sediment no matter how much was removed by later erosion at first extended to the sides of the basin of deposition.
To these concepts Hutton added:
I V. Inclusion. Anything that is included in one rock--such as a granite pebble in conglomerate, or a piece of the rock surrounding an igneous intrusion, some of which has fallen into the still molten magma but has not remelted–is an inclusion and is evidence that the inclusion came from an older preexisting rock.
V. Cross Cutting. Any geologic material that cuts across a rock is younger than the rock it cuts. For example, any igneous intrusive rock that cuts another rock is younger than the rock it cuts. Any fault that disrupts a geologic unit is younger in its faulting that the unit it disrupts. Any erosion surface that planes off the surface of the land or wears down the land is an new event.
Unconformities a special type of cross cutting done by weathering and erosion. Some erosion surfaces are preserved in the geologic record and are called unconformities. Significant amounts of rock and thus information on geologic time can be removed at unconformities by erosion.
Thus 5 rules are available to tell how rocks relate to each other and if we know other geologic information such as the succession of fossil life and combine this information with evidence of past depositional environment we may ultimately interpret the geologic history of an area.
The 5 rules can be used to draw a reconstruction of the sequence of events.
Often to practice this deductive reasoning a model or diagram is used that shows a sequence of events available for reconstruction (See page 201)
Completeness of the geologic sequence. Unlike written history, geologic history is almost always incomplete with much of geologic time missing. These gaps in the geologic record are missing time and are due to A) periods where no sediments were deposited, or B) postdepositional removal. Such removal events are recorded in the sequence of events as UNCONFORMITIES.
There are 3 types of unconformities–I. Disconformities, II. Nonconformities, and III. Angular unconformities.
I. Disconformity– a break between rock layers due to erosion. The erosion surface marks missing time. Layers are horizontal above and below.
II. Nonconformity–a break between layered rocks above and an eroded crystalline (igneous or metamorphic) rock below. Typically this represents a longer period of time than the disconformity. There can sometimes be recognizable fragment of the crystalline rock incorporated as clasts in the overlying sedimentary unit.
III. Angular unconformity. Tilted (at an angle to horizontal or folded) rocks are intersected by a layer of flat-lying rocks that was deposited on top of the tilted rocks after erosion. This unconformity typically represents a very long missing time period in which mountains were thrown up by plate tectonics, tilted, and then eroded away, perhaps to a flat surface and then sediments were deposited above the eroded tilted rocks that represented a tectonic mountain building event.
Because a relative dating method cannot be numeric, such questions as to how old the earth and the Universe are can not be answered by the methods mentioned above. Absolute age dating is done using either radioactive elements or by counting events that repeat at a regular interval such as tree rings or winter and summer sediment layers in a lake.
When unstable radioactive minerals are trapped in a mineral, they begin to decay. The mineral is like a tiny hour glass collecting products of decay and can be age-dated. The rate of decay varies with the radioactive element but is a constant. As an element such as uranium decays, it may lose protons and thus its atomic number and chemical identity changes. The new element created by the radioactive decay is called a daughter element. For example: uranium, the parent, element spontaneously decays to lead, the daughter element.
The amount of time it takes for half of parent to decay to daughter element is called the half-life. Since the rate of decay is constant, the half-life of an element is also constant. Thus a ratio of say 50 to 50 parent to daughter indicates that one half-life of time has gone by since the mineral crystallized.
Because different radioactive elements decay at different rates, different radioactive elements are valuable for dating at different amounts of time into the past. Radioactive carbon-14 decays quite rapidly and can be used only for relatively young samples back to about 70,000 year before present (bp). Uranium 238 decays to lead 206 with a half life of 4.5 billion years. It can to date things that are older than 10 million years or so. Potassium-Argon is often used for lavas of relatively young.
Carbon-14 is interesting as it is used to date living things. Since carbon is the most important element for life, all animals incorporate carbon-14. Once they die there is no longer any more carbon-14 being added to bones, wood, etc., and the carbon-14 clock begins its decay. Because it decays rapidly, too little carbon-14 is left after about 70,000 years to date.
Another method of absolute dating is by using tree rings; this is called dendrochronology. Living wood can have different rings thickness depending on how hard a winter or how dry a summer the particular year had. Living rings can be matched to fossil wood, or wood in someone's house and the record can be extended back to approximately 14,000 years before the present.
Almost anything that occurs at a regular rate has been used to try to date rocks. For example, the size of lichens growing on a rock. One idea tried by Lord Kelvin in 1866 was to measure the rate of cooling of the earth, much as a coroner uses decrease in body temperature to tell how long a person has been dead. His results did not take into account internal heat generated by radioactive elements and his result was too young an age for the earth. Kelvin calculated an age of 100 million years as the maximum age of the earth.
Radioactive dating now suggests that the earth is around 4.5 billion years old. Dating by the red shift of stars as far away as the Hubble telescope can see gives an age for the universe of about 14 billion years! And you thought twenty-one was old!
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