7 Geologic Time – An Introduction to Geology
Explain the relation between time units and corresponding rock units (period and system, . This quiz is for you to check your comprehension of this section. . granite would give the crystallization age of the rock, while dating the gneiss might. A Geologist suffers from a black eye after yelling "Gneiss!!! Marble is a metamorphic rock that forms from a ______ parent. granite limestone sandstone What is the relationship between metamorphic foliation and sedimentary bedding?. Demystified · Quizzes · Galleries · Lists · On This Day · Biographies · Newsletters The rock cycle illustrated in Figure 1 reflects the basic relationships among On the other hand, the texture of gneiss is often described by its distinct banding. rocks, the terms are generally used as modifiers—e.g., medium-grained granite .
Diagenesis is, as previously explained, the process of forming sedimentary rock by compaction and natural cementation of grains, or crystallization from water or solutions, or recrystallization. The conversion of sediment to rock is termed lithification.
Sediments composed of weathered rock lithify to form sedimentary rock, which then becomes metamorphic rock under the pressure of Earth's crust.
When tectonic forces thrust sedimentary and metamorphic rocks into the hot mantle, they may melt and be ejected as magma, which cools to form igneous, or magmatic, rock.
Created and produced by QA International. Abundance of rock types An estimate of the distribution of rock types in large structural units of the terrestrial crust is given in the Table. The relative abundance of main rock types and minerals in the crust is shown in the Table. Texture The texture of a rock is the size, shape, and arrangement of the grains for sedimentary rocks or crystals for igneous and metamorphic rocks.
The latter is the extent to which the bulk structure and composition are the same in all directions in the rock. Rocks have many different textures. Layered sandstone produces a gritty texture, whereas coquina may be rough with cemented shells occasionally producing a sharp edge. Likewise, breccia, which contains pieces of other rocks that have been cemented together, and porphyry, which contains interlocking mineral crystals, tend to be rough. In contrast, obsidian tends to have a smooth glassy feel, whereas serpentine may feel platy or fibrous, and talc schist often feels greasy.
On the other hand, the texture of gneiss is often described by its distinct banding.
Classification by grain or crystal size The common textural terms used for rock types with respect to the size of the grains or crystals, are given in the Table. The particle-size categories are derived from the Udden-Wentworth scale developed for sediment. For igneous and metamorphic rocks, the terms are generally used as modifiers—e.
Aphanitic is a descriptive term for small crystals, and phaneritic for larger ones. Very coarse crystals those larger than 3 centimetres, or 1. For sedimentary rocks, the broad categories of sediment size are coarse greater than 2 millimetres, or 0. The latter includes silt and claywhich both have a size indistinguishable by the human eye and are also termed dust. Most shales the lithified version of clay contain some silt.
Pyroclastic rocks are those formed from clastic from the Greek word for broken material ejected from volcanoes. Blocks are fragments broken from solid rock, while bombs are molten when ejected. Porosity The term rock refers to the bulk volume of the material, including the grains or crystals as well as the contained void space.
The volumetric portion of bulk rock that is not occupied by grains, crystals, or natural cementing material is termed porosity. That is to say, porosity is the ratio of void volume to the bulk volume grains plus void space.
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This void space consists of pore space between grains or crystals, in addition to crack space. In sedimentary rocks, the amount of pore space depends on the degree of compaction of the sediment with compaction generally increasing with depth of burialon the packing arrangement and shape of grains, on the amount of cementation, and on the degree of sorting.
Typical cements are siliceous, calcareous or carbonate, or iron-bearing minerals. Sorting is the tendency of sedimentary rocks to have grains that are similarly sized—i. Poorly sorted sediment displays a wide range of grain sizes and hence has decreased porosity. Well-sorted indicates a grain size distribution that is fairly uniform. Depending on the type of close-packing of the grains, porosity can be substantial.
It should be noted that in engineering usage—e. A well-graded sediment is a geologically poorly sorted one, and a poorly graded sediment is a well-sorted one.
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Total porosity encompasses all the void space, including those pores that are interconnected to the surface of the sample as well as those that are sealed off by natural cement or other obstructions. It thus measures the pore volume that is effectively interconnected and accessible to the surface of the sample, which is important when considering the storage and movement of subsurface fluids such as petroleumgroundwater, or contaminated fluids.
Physical properties Physical properties of rocks are of interest and utility in many fields of work, including geology, petrophysics, geophysics, materials sciencegeochemistry, and geotechnical engineering.
The scale of investigation ranges from the molecular and crystalline up to terrestrial studies of the Earth and other planetary bodies. Since rocks are aggregates of mineral grains or crystals, their properties are determined in large part by the properties of their various constituent minerals.
In a rock these general properties are determined by averaging the relative properties and sometimes orientations of the various grains or crystals.
As a result, some properties that are anisotropic i. What is a fossil? Describe ways by which fossils are preserved. Outline how natural selection takes place as a mechanism of evolution. Explain what stratigraphic correlation is and how rocks are correlated regionally and over wide geographic distances. Know the eras and periods of the geologic time scale and explain the purpose behind its divisions.
Explain the relation between time units and corresponding rock units period and system, epoch and series, age and stage. 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 of Original Horizontality: This holds true except for the margins of basins, where the strata can slope slightly downward into the basin.
Lateral continuity 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: When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock. Fossil succession showing correlation among strata. 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 figure shows the South Rim separated from the North Rim by approximately 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.