Working out Earth history depended on realizing some key principles of relative time. The figure in section 7. Using this time scale as a calendar, all events of Earth history can be placed in order without ever knowing the numerical age. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the Scientific Revolution of the 17th and 18th centuries.
James Hutton realized that geologic processes are slow and his ideas on uniformitarianism i. This section discusses the principles of relative time that are used in all of geology but especially useful in stratigraphy.
Lower strata are older than those lying on top of them. In an otherwise undisturbed sequence of sedimentary strata rock layersthe layers on the bottom are the oldest and the layers above are younger.
Principle The method of numerical age dating that involves the polarity of the earth Original Horizontality: This holds true except for the margins of basins, where the strata can slope slightly downward into the basin. Principle of Lateral Continuity: Of course, all strata eventually end, either by hitting a geographic barrier or by a depositional process being too far from its source, either a sediment source or a volcano.
Strata that are subsequently by cut by a canyon remain continuous on either side of the canyon. Dark dike cutting across older rocks, the lighter of which is younger than the grey rock. Principle of Cross-Cutting Relationships: Principle of
The method of numerical age dating that involves the polarity of the earth nclusions.
When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock. Principle of Fossil Succession: Assemblages of fossils contained in strata are unique to the time they lived and can be used to correlate rocks of the same age across wide geographic distribution.
Evolution has produced a succession of life whose fossils are unique to the units of the Geologic time Scale.
The Grand Canyon of Arizona illustrates the stratigraphic principles. The figure shows the South Rim separated from the North Rim by
The method of numerical age dating that involves the polarity of the earth 18 miles. The predominant white layer just below the canyon rim is the Coconino Sandstone. This layer is laterally continuous, even though the intervening canyon separates its outcrops on either side by about 18 miles.
These layers of rock are continuous over a wide region of the Colorado Plateau surrounding the Grand Canyon even though several canyons cut through the strata. This is an example of the principle of lateral continuity. Formation names are designated by geologists to identify rock units that have recognizable characteristics that can identify them in a region.
Thus, formations are used as units for mapping purposes and communication. In the lowest parts of the Grand Canyon are the oldest formations with igneous and metamorphic rocks at the bottom.
The Vishnu Schist is the oldest and the cross-cutting intrusions of Zoroaster Granite are younger. As seen in the figure, the other layers on the walls of the Grand Canyon are numbered in reverse order with 15 being the oldest and 1 the youngest. The Colorado Plateau, on which the Grand Canyon region lies, is characterized by strata that are horizontal or nearly so.
These rocks were originally deposited horizontally Principle of Original Horizontality and have not been disturbed very much since they were deposited except by a broad regional uplift there are local exceptions. In the Grand Canyon, there is a gentle tilt of the strata to the south, thus the strata of the North Rim are about a thousand feet higher than those of the South Rim about 18 miles away. Applying the stratigraphic principles, one can interpret that the slight tilting of the strata occurred after their deposition and that the Grand Canyon was cut by the Colorado River after the regional tilting.
This is an application of Cross Cutting Relationships to establish relative time and Lateral Continuity to correlate them across the canyon. The red, layered rocks of the Grand Canyon Supergroup on the dark-colored rocks of the Vishnu Complex. On top of these basement rocks, lie the strata of the Grand Canyon Supergroup there are several formations included in this supergroup unit.
These formations were originally deposited flat on top of the basement rocks Original Horizontality and have since been broken into tilted blocks by normal faulting see Chapter 9 which cut through both them and the underlying basement.
Because the formation of the basement rocks and the deposition of these overlying sediments is not continuous deposition but "The method of numerical age dating that involves the polarity of the earth" by events of metamorphism, intrusion, and erosion, the contact between the Grand Canyon Supergroup and the older basement is termed an unconformity.
An unconformity represents a period during which deposition did not occur or erosion removed rock that had been deposited, so there are no rocks that represent events of Earth history during that span of time at that place. Unconformities are shown on cross sections and stratigraphic columns as wavy lines between formations.
There are three types of unconformities which will be discussed below. The first occurs when sedimentary rock lies on top of crystalline rock, and is a type of unconformity called a nonconformity. A nonconformity occurs when sediments are deposited on top of non-layered crystalline igneous and metamorphic rocks as is the case with the contact between the Grand Canyon Supergroup and the Vishnu basement rocks. All three of these formations have an erosional unconformities at the two contacts between them.
The pinching Temple Butte is the easiest to see, but even between the Muav and Redwall, there is an unconformity. The Grand Canyon Supergroup is a sequence of strata representing alternating marine transgressions and terrestrial deposition in this case regressions where the sea retreated. During formation of this sequence, sea-level rose or the land sank leaving marine deposits on the surface and then fell or the land rose leaving the land exposed to erosion and to deposition of terrestrial sediments.
In other words, layers of rock that could have been present, are absent. The time that could have been represented by such layers is instead represented by the disconformity. Disconformities are unconformities that occur between parallel layers of strata indicating that there was no deformation during the
The method of numerical age dating that involves the polarity of the earth of nondeposition or erosion.
In the lower part of the picture, note the dipping toward the right rocks. These intersect the non-dipping rocks above at an angle, making an angular unconformity. On top of the Grand Canyon Supergroup lie the horizontal layers of the canyon walls showing unconformable contacts with the tilted layers of the Grand Canyon Supergroup below i. The lower strata were tilted by tectonic processes that disturbed their original horizontality which of course also affected the underlying basement rocks.
Thus there were cross-cutting processes that affected those rocks before the younger strata were deposited horizontally on top of them. After the deposition of the Grand Canyon Supergroup and the tectonic events that tilted and faulted them, there was an erosion-produced landscape with hills and valleys over which the sea transgressed again and deposited layers of three horizontal formations of sedimentary rock called the Tonto Group.
The upturned and eroded edges of the tilted older rocks of the Grand Canyon Supergroup lay at angles with the overlying Tonto Group. This third type of is called an angular unconformity. Disconformitywhere is a break or stratigraphic absence between strata in an otherwise parallel sequence of strata.
Block diagram to apply stratigraphic principles In the block diagram seen herethe sequence of events from oldest to youngest that took place can be interpreted using the stratigraphic principles and interpretations from the chapters on rocks chapters Here is the sequence of these events in order. T he oldest rock is a body of deformed rock composed of brown and gray layers. Its deformation includes pretty severe deformation shown as folding. From the symbols used in the drawing, this rock looks like it was probably metamorphosed.
The oldest event, therefore, is the formation of the brown and grey rock, followed by its deformation and metamorphism which we might call basement rock here. The brown and gray basement rock was cut by the fault A which cuts across and displaces it.
Both the basement rock and fault A are crosscut by rock mass B. Its irregular outline suggests The method of numerical age dating that involves the polarity of the earth it is an igneous intrusion emplaced as magma into the region. Since it cuts across both the basement rocks and the fault, it is younger than both. Next, both the basement rock and rock B were eroded forming an unconformity. This was actually an ancient landscape surface on which sedimentary rock C was subsequently deposited perhaps by a marine transgression.
Because C is sedimentary rock that was deposited on top of crystalline igneous rock B and crystalline metamorphic rock, this unconformity is called a nonconformity. Deposition of sedimentary rock E suggests that there was a period of erosion or non-deposition producing a disconformity between C and E, the nonconformity between dike D and rock E establishes that erosion in fact did take place forming a quite flat landscape.
The final events affecting this area are the current erosion processes working on the land surface, rounding off the edge of the fault scarp, and producing the modern landscape on the top of the diagram.
Relative time allows science tell the story of the Earth, but does not provide specific numeric ages of events, and thus, the rate at which geologic processes operate.
Because science advances as the technology of its tools advances, the discovery of radioactivity in the late s provided a new scientific tool by which actual ages in years can be assigned to mineral grains within a rock. This was how
The method of numerical age dating that involves the polarity of the earth of that time interpreted Earth history, until the end of the 19th Century, when radioactivity was discovered.
The method of numerical age dating that involves the polarity of the earth discovery introduced a new dating technology that allows scientists to determine specific numeric ages of some rocks, called absolute dating. The next sections discuss this absolute dating system called radio-isotopic dating. All elements on the Periodic Table of Elements see Chapter 3 contain isotopes.
An isotope is an atom of an element with a different number of neutrons. For example, hydrogen H always has 1 proton in its nucleus the atomic numberbut the number of neutrons can vary among the isotopes 0, 1, 2. Recall that the number of neutrons added to the atomic number gives the atomic mass. When hydrogen has 1 proton and 0 neutrons it is sometimes called protium 1 Hwhen hydrogen has 1 proton and 1 neutron it is called deuterium 2 Hand when hydrogen has 1 proton and 2 neutrons it is called tritium 2 H.
Note that the atomic mass of elements on the Periodic Table is usually expressed with decimal digits. This indicates that the atomic mass of that element in nature is made of all its natural isotopes so the average atomic mass including all these isotopes is a decimal value.
Many elements like hydrogen have both stable and unstable isotopes. Unstable isotopes called radioactive isotopes spontaneously decay over time releasing radiation.
When this occurs, that isotope becomes an isotope of another element. This process of radioactivity is called radioactive decay. On the left, 4 simulations with only a few atoms. On the right, 4 simulations with many atoms.
The radioactive decay of any individual atom is a completely unpredictable and random event. However, given a large number of radioactive atoms any measurable quantity of a substance contains trillions of atomsthe decay of half of the atoms in the specimen takes a specific amount of time.
This amount of time for half the atoms to decay is called the half-life. In other words, the half-life of an isotope is the amount of time it take for half of an initial quantity of unstable isotopes to decay into another isotope.
The half-life is constant for a given radioactive isotope and can be measured. Names of relative ages (such as Silurian) and accurate numerical dates these salts from weathering of rock only rarely are transported all the way to the ocean has been uniformly strong, with constant polarity, throughout Earth's history.
Dendrochonology involves dating of historic and geologic events through the. geochronology The umbrella term used for methods that provide ages for Absolute dating methods are mostly applied to the rocks in which the global events such as reversals in the direction of the Earth's magnetic field.
But researchers involved in dating have largely abandoned the absolute/relative dichotomy. the studying the Earth is a way of breaking down large, complex problems into smaller . refers to the dating of rocks according to their numerical ages.