Geologic Time

When one mentions "geologic time" to a geoscientist, that person immediately starts to think in terms of millions to billions of years. The Earth organized as a large accreted body between 4.5 and 4.6 billion years ago. The unit of time measurement applied to geologic history remains the year, which probably has had only limited lengthening (with a larger solar orbit) since the beginning. For practical purposes, we continue to work with the standard year of 365.25 days (although the days were once shorter as the Earth rotated faster).

The rocks at the Earth's surface or accessible (e.g., by drilling) in its shallow interior have a variety of ages as one moves from place to place. The question becomes: How does one know at least the approximate age, i.e., how much earlier in the past did this rock or rock unit (e.g., a layer) become initially emplaced (it may be moved by erosion or crustal deformation, etc.)? Two solutions to the question are: 1) determine its relative age and estimate its time of formation by approximation techniques; and 2) use some sort of absolute time dating method.

Relative age determination involves some important rules: 1) the Law of Uniformity (James Hutton):The Present is the Key to the Past; this means that processes we observe in today's world likely operated throughout the past, back even billions of years ago; 2) the Law of Superposition: Under most circumstances when sedimentary layers are deposited in a sequence, the lowest in that sequence was deposited first and is the oldest whereas the highest was last deposited and is the youngest (this may become spatially "untrue" when rocks are displayed by faulting or severe overfolding); 3) the Law of Cross-cutting: any geologic feature (igneous intrusion; fault, etc) that cuts into/across rock units must be younger than these units; 4) the Law of Faunal Succession: in a sequence of rocks of significant thicknesses, there may likely be remains of animals/plants (fossils) that show some systematic evolutionary progression; most commonly the change is towards complexity and diversity and follows morphological and taxonomic modifications.

Until the 18th and 19th Centuries, the age of the Earth was estimated more from biblical interpretations than from scientific determinations. Then, following Hutton, Lyell, Lamarck, Darwin and others two things were realized: 1) it takes thousands to millions of years to deposit layered sedimentary rock units of thicknesses in the 10s to 100s of meters realm; and 2) progressive evolutionary changes also take millions of years for significant advances in the development of genera and species to become recognizable. By the end of the 19th Century, Earth had "aged" from the 4004 BC estimate by Bishop Usher to millions of years. In the 20th Century the age passed two billion years and with more precise methods (radiometric; see below) zeroed in on the present value near 4.6 billion years.

The most indicative method for reaching the billion year age category was simply to measure the thicknesses of units in one place and estimate the time to deposit them (millions of years), then go elsewhere and find at least some of the same units that had different units below or above them, which had to be older or younger. This involves correlation of units based on something in common from place to place, as for example certain animal/plant fossils that are the same or similar, or distinctive rock types in the same sequences. By roaming over a continent and between continents, sequences were recognized, described, and matched with sequences elsewhere that had new member units, within, up, or below. This leads to a general "worldwide" Geologic Column that tries to account for all the deposits laid down during given time intervals (spans) in various locations that can be matched and then expanded by overlapping correlation (see below).

Around the turn of the 18th Century into the 19th, various geologists who were adept at stratigraphic analysis (recognition of layers that had characteristic time markers such as fossils) began to publish their descriptions of the sequences they studied. Others found different sequences and/or some of the same sequences in the descriptions. As the Geologic Column grew, estimates of their ages were made using mainly deposition rates. Individual sequences within the column were assigned times in the past which resulted from the column estimates. It became conventional to give a name to a sequence that seemed to represent a long span of time but with certain diagnostic properties (e.g., a collection of life forms that, while evolving, possessed similarities). These sequences became Periods in a temporal-stratigraphic nomenclature and all rocks contained within the sequence made up a System. Subdivision of Periods into smaller time spans yielded Epochs, with their Rocks being Series. Broader divisions of time made up a some number of Periods (each younger one overlying an older Period - Law of Superposition) were called Eras. Names given to Periods either had some geographic significance (the Cambrian Period was first described and measured in Cambria within the British Isles) or were at the Era Level defined by an appraisal of the dominant life forms and their stages of evolution (the Mesozoic means "Middle" "Life" [zo- is part of a Greek word pertaining to life).

From this approach, a system for expressing geologic time and naming subdivisions has emerged, as shown in this diagram:

This scale has many subdivisions over the last 600,000 million years, since these rocks are well preserved in parts of the world. All rocks older than the beginning of the Paleozoic ("ancient life"), whose oldest period is the Cambrian, are said to be Precambrian - a general term, that is now undergoing further subdivisions. The bulk of geologic time (about 87%) is Precambrian, as shown in this diagram

With the introduction of this nomenclature, we can now look at two figures that use deposition rates, superposition, and correlation to build up a regional Geologic Column covering rocks in Arizona and Utah. The first shows the Column as determined from rock exposed within the Grand Canyon.

Several comments are in order: 1) the horizontal rocks all fall within the Paleozoic Era; since they are horizontal, they rest in the same positions as when deposited - they have not been folded; 2) some rocks within individual Periods are distinct from one another; they constitute Formations (mappable units that are named from where [geographically] they were first described); 2) representatives from some Periods or Epochs are missing - either they were never deposited in the ancient seas that produce the Formations or if deposited have since been eroded away - a fact termed an Unconformity; 4) rocks below the Paleozoic are Precambrian; the upper group are sedimentary but tilted (folded) indicating they were deformed in some type of mountain building, partially eroded and then covered with the lowest Paleozoic units, making an Angular Unconformity (Orogeny; 5) these in turn rest on metamorphic rocks, much older with some upper units having been removed by erosion (reaching into lower levels of the crust) to form a Nonconformity; 6) the metamorphic rocks were intruded by granite, which by the Law of Cross-cutting must be younger; and 7) even now, rocks at the surface (top of the column) are being eroded and probably were undergoing erosion since the Paleozoic; this absebce implies that Mesozoic and Cenozoic rocks are missing, either through erosion or absence of depositing seas; if at some future time seas roll into the region a new unconformity will develop.

The next diagram shows how the Geologic Column has been compiled for this region using correlation:

The link between the Grand Canyon and Bryce columns is the Kaibab Limestone. Bryce contains Mesozoic units, of which the Jurassic Navajo Sandstone is most distinctive. That sandstone unit is near the base of the section at Zion Canyon which preserves Upper (higher) Mesozic Units and an early Tertiary Formation - the Wasatch - in the Cenozoic. So the composite of the three columns or sections has representative deposits in Periods from the Precambrian through the Paleozoic and Mesozoic into the young Cenozoic.

The skills of stratigraphers over the last 200+ years has produced a general geologic column and a time scale that proves to be quite accurate. But this accuracy had to be confirmed by some independent method that measures absolute time. That had to await the discovery of radioactivity in the late 19th Century followed by the realization that radioactive elements (as isotopes) decay (their nucleus is changed to another radionuclide of the same element or more commonly to new elements) at very precise fixed rates. The decay is said to be exponential (for example, a series proceeding as 1-->2-->4-->8-->16-->....). For example, some given amount of the radioactive form of Potassium, K40 will have half its atoms come apart to form radioactive Argon A40 over a long finite time = 1.251 billion years. Consider this diagram:

Half the potassium-40 is gone after 1.251 billion years; half remains. Now if another 1.251 billion years elapses while the mineral containing the potassium (several isotopes), then in 2.502 billion years only 1/4th of the original K-40 will remain and a larger amount of A-40 has developed. The clock on dating begins when the original potassium is incorporated in the rock; argon, a gas, should not be present. Assuming none of the A-40 escapes over time, then a geochronologist need only measure (using a mass spectrometer) the amounts of K-40 and A-40 now present in the rock to set up a ratio and to use the decay rate (given in half-life, the time required for half of any of the radioisotope to decay) to determine the age. If that ratio is K-40/A-40 = 1/8th, the age would be 3753 million years - the time between incorporation of the potassium in the rock (likely, a granite) and the present.

Very accurate ages of incorporation are possible using radiometric dating, provided nothing escape or no element contamination occurs. There are a number of radioactive element that have their own decay schemes. Radioisotopes of Uranium (U) decay at various rates to isotopes of other elements (e.g., Radon) and eventually to isotopes of Lead (Pb). The element Rubidium (found in micas) decays to Strontium. Some elements decay over short half lifes and are confined to dating younger rocks; others are especially suited to determining Precambrian times. If a shale is intruded by a granitic dike (narrow tabular cross-cutting body), the two rock types can be dated by different radioisotope methods, so that the time when each event took place can be fixed fairly accurately.

Thus, since the early 20th Century geoscientists have had a powerful tool to reconstruct when different specific events took place in a complex assemblage of rocks, so that a precise geologic history can unfold. The table below summarizes a generalized history of, mainly, the primitive Earth.

The oldest dated mineral, a zircon from Australia, is age-fixed at 4.1 billion years, but most early ages for rocks fall around 3.6 b.y. Thus most of the Earth's original, and some subsequent, crust has been destroyed (remelted; subducted; broken down by weathering). When oxygen was nearly absent from the atmosphere, the most characteristic rock type was BIF (Banded Iron Formation); its production consuming any oxygen released. As photosynthesis in plants emerged as a working process, oxygen increased, producing iron oxides in the form of Red Beds; then also carbonate rocks became commonplace in sedimentary sequences.

The most important events, from the human perspective, have been the origin and time of appearance of the first living forms, and the subsequent development of the major phylla and orders of life on land and sea. The first indications of life extend now to earlier than 3.5 billion years ago. Early life was single-celled - procaryotic. Multi-celled life - eucaryotic - appeared in the Precambrian. The greatest diversity ("explosion") of life occurred at then close of the Precambrian into the Cambrian Period. Since then at least 6 mass extinctions (significant fraction of all life types at the time) have occurred. The time sequence of life on Earth is depicted in this "cartoon", in which principles of evolution govern the progression and emergence of new phyla:

The above taken from "Life in the Universe", Steven Weinberg, Scientific American, Oct. 1994

Time to leave Geologic Time, and move forward to Landforms, or Earthquakes and Natural Disasters that affect People, or back to Earth Science Introduction.

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