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.
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Because the formation of the basement rocks and the deposition of these overlying sediments is not continuous deposition but broken 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 period 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 unconformity is called an angular unconformity. Disconformity , where is a break or stratigraphic absence between strata in an otherwise parallel sequence of strata.
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Block diagram to apply stratigraphic principles In the block diagram seen here , the 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 that 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 A narrow igneous intrusion that cuts through existing rock, not along bedding planes. 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 to 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 scientists of that time interpreted Earth history, until the end of the 19th Century, when radioactivity was discovered. This 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.
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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 number , but 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 H , when hydrogen has 1 proton and 1 neutron it is called deuterium 2 H , and when hydrogen has 1 proton and 2 neutrons it is called tritium 2 H.
Two basic types of rock dating
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.
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However, given a large number of radioactive atoms any measurable quantity of a substance contains trillions of atoms , the 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. The known half-life of an isotope can be used to calculate the age of a rock. The principles behind this dating method require two key assumptions. First, the mineral grains containing the isotope form at the same time as the rock, such as a mineral in an igneous rock that crystallized from magma. Second, the mineral crystals remain a closed system , that is they are not subsequently altered by elements moving in or out of them.
These requirements place some constraints on the kinds of rock that are suitable for dating, igneous rock being the best.
Metamorphic rocks are crystalline, but the processes of metamorphism may reset the clock and derived ages may represent a smear of different metamorphic events rather than the age of original crystallization. Detrital sedimentary rocks contain clasts from separate parent rocks from unknown locations and derived ages are thus meaningless. However, sedimentary rocks with precipitated minerals such as evaporites may contain elements suitable for radio-isotopic dating. Igneous pyroclastic layers and lava Liquid rock on the surface of the Earth. Cross-cutting igneous rocks and sill A type of dike that is parallel to bedding planes within the bedrock.
Knowing that the zircons in the metamorphosed sediments came from older rocks, their ages established the age of the source rocks , which are no longer available for study. Two protons and two neutrons leave the nucleus.
pop.mail.ruk-com.in.th/seven-stories-for-seven-sons-bedtime-stories-for.php When an atom decays by alpha decay , an alpha particle is emitted from its nucleus as an alpha ray. The alpha particle consists of two protons and two neutrons, a total of four particles. This happens also to be the nucleus of a helium atom; helium gas may thus be trapped in the crystal lattice of a mineral in which alpha decay has taken place. The loss of two protons from the nucleus of the atom lowers its atomic number by two forming an atom of an element two atomic numbers lower on the Periodic Table of the Elements.
The half-life of U is 4. This isotope of uranium U can be used to determine the age of the oldest materials found on Earth, even meteorites and materials from the earliest events in our solar system. When an atom decays by beta decay , a neutron in its nucleus splits into an electron and a proton.
The electron is emitted from the nucleus as a beta ray. The new proton increases the atomic number by one and a new element is formed, but the atomic mass does not change. The process of decay of radioactive elements like uranium leads to a series of parents and daughters , each one radioactive , until a stable non- radioactive daughter is formed. Such a series is called a decay chain.
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