dating geological events
Geologic time covers the whole sweep of earth's history, from how and when the earth first formed, to everything that has happened on, in, and to the planet since then, right up to now. Geologists analyze geologic time in two different ways: The combination of these two types of geologic ages makes a complete record of earth's geologic history in terms of the order of events and in terms of how many years ago each event occurred.
Relative geologic age refers to the order in which geologic events occurred. Relative geologic age is established, based on such evidence as the order in which layers of sediment are stacked, with the younger layer originally on top. By using the principles of relative geologic age, the sequence of geologic events -- what happened first, what happened next, what happened last -- can be established.
Absolute geologic age refers to how long ago a geologic event occurred or a rock formed, in numeric terms, such as Some rocks and minerals can have their absolute age directly measured by analyzing the ratios of certain radioactive and non-radioactive isotopes they contain. The units commonly used for geologic age are dating geological events in years Ma for millions of years, giga-annum Ga for billions of years, and kiloannum ka ka for thousands of years.
Because these units are used according to the rules of the metric ih, the M in Ma and the G in Ga must be capitalized, and the k in ka must gfological be capitalized. Much of the most detailed and precise information that geologists have gleaned of earth's history comes from a branch of geology known as stratigraphy.
Stratigraphy studies stratified rocks, - layered rocks, in other words, which are either sedimentary or volcanic - establishes their age sequence based on principles of relative geologic age, and reconstructs, from the evidence in the rocks and from their field relations as depicted on maps and cross-sections, the geologic history that they represent. Dating geological events in years may have already completed introductory laboratory studies of igneous, sedimentary, and metamorphic rocks.
If so, you have already practiced interpreting details of earth's history from the evidence contained in rocks. By dating geological events in years the information that can be gathered from a single rock sample into the broader context of where the body of rock that it comes from fits into the sequence of the earth's rocks, and what its age is in absolute numeric terms, the geologic history of that part of the earth can be reconstructed.
Stratigraphy started to become a formal science due to daating work of a man who published under the name Nicolas Steno in the 17th century. Steno made careful geologic observations and illustrations. He published the results of his work and established a basic set of principles for interpreting sedimentary strata. Geologists still use Steno's principles, with some refinements and additions.
They are summarized as the datung of relative geologic age determination, sometimes referred to as the principles of relative dating. In the two centuries after Steno developed the first set of principles for determining relative geologic age, other geologists contributed major ideas that have allowed scientists to grapple with, measure, and come to terms with geologic time.
James Hutton of Scotland, dating geological events in years same man who developed the modern concept of the rock cycle as a geolobical of the endlessly dynamic and constantly changing dating geological events in years, advanced the theory of unconformities. An unconformity is a buried erosional surface or non-depositional surface, a contact between the rocks below and the layer of stratified rock above that is missing a significantly large interval of geologic time.
For example, deep in the Grand Canyon in Arizona, there dating geological events in years places where fvents layer of rock of Devonian age is dating geological events in years on top of a layer of rock of Cambrian age. That means that tens of millions of years of geologic time dating geological events in years between those two rock layers forming, and there is no sedimentary rock, no rock record, to record the details of what happened during that geologic time.
The contact between the Cambrain rock over million years old and the Devonian rock less than million years old is a type of unconformity, which you will read more about below. In the gsological s, soon after James Hutton died, William Smith in England made the scientific case for what came to be called the principle datinng faunal succession.
The key to this principle is that during a specific geologic time, only certain types dating geological events in years organisms existed, so if fossils of those organisms are found in a layer of rock, the rock is of that geologic age, the age when those organisms were species that lived on earth. This principle was based on applying other methods of determining geolgoical rocks are older and which rocks are younger, which verifies that there is indeed a faunal or fossil, if you prefer succession that occurs in the same order in the rock layers everywhere on yearx.
Charles Lyell developed a key idea known as uniformitarianism, which also underlies the geological study of earth's history. Lyell was a significant scientific presence through much of the time of Victorian England in the s. He had a large influence on the development and spread of the practice of geology as a science, partly through his textbook, The Principles of Geology.
This was not only the first complete geology textbook published in English, it was by far the most widely used textbook for decades, through several revised editions. A copy of The Principles of Geology that Charles Darwin read had a major influence in his thinking as he traveled around the world collecting samples, and Darwin consulted with Lyell at key stages during the time he developed and published his theory of evolution of species by natural selection.
The idea of uniformitarianism is that the laws and principles that nature follows in today's world, such as gravity, also dating geological events in years in the geologic past; in other words, "the present is the key to the past. The idea of uniformitarianisms is commonly misinterpreted in two different ways.