Discordants:: Geology Rock Stone
Discordants:: Geology Rock Stone
Discordants:: Geology Rock Stone
sill - (geology) a flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock geology - a science that deals with the history of the earth as recorded in rocks rock, stone - a lump or mass of hard consolidated mineral matter; "he threw a rock at me"
discordants:
(geology) of a differing type of rock cutting across a formation dikes may be discordant to country rock if they intrude at a high angle to the bedding A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cutsdiscordantly across planar wall rock structures, such as bedding or foliation massive rock formations, like igneous/magmatic intrusions and salt diapirs.
Dikes are vertical wall-like structures that form as a result of magma being injected into the fractures of rocks. Dikes are discordant features. This means that they cut through existing layers of rocks. Being intrusive, dikes crystallize below the ground. When the surrounding rock is eroded, dikes are exposed and often appear as dark walls of rock. Pictured to the left is a rhyolite dike that cuts through a bed of shale
A phacolith is a pluton parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticlineor the trough of a syncline. In rare cases the body may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that in cross section it has an Sshape.
A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body
Laccoliths are a second category of intrusive structures that are visible in Big Bend National Park. Laccoliths form when magma is injected between layers of rock. The pressure of the magma being injected is high enough that the overlying strata are forced upward; this forms the laccolith's characteristic mushroom shape. A laccolith is a concordant feature
Volcanic necks form from feeder pipes that were once inside volcanoes, but once the volcano goes extinct and is in an erosional environment, the volcano weathers away but the neck remains. They are intrusive bodies, but near the surface the morphology of the necks can be cylindrical. At depth necks can become elliptical or even clover shaped. At great depths they can merge into several feeder dikes.
A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement ofigneous intrusive (also called plutonic) rock that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite (see also granite dome).
Phaneritic -- Large crystals that are clearly visible to the eye with or without a 10-power hand lens. The entire rock is made up of large crystals, which are generally mm to cm in size; no fine matrix material is present. This texture forms by slow cooling of magma deep underground in the plutonicenvironment. Aphanitic -- Small crystals that cannot be seen by the eye with or without a 10power hand lens. The entire rock is made up of small crystals, which are generally less than mm in size. This texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. Porphyritic -- Texture in which an aphanitic matrix makes up part of the rock, and large crystals are present within the matrix. The large crystals are called phenocrysts. This texture implies two-stage cooling--an early stage of
slow cooling in which the phenocrysts grow, followed by a later stage of rapid cooling that forms the matrix. Pegmatitic -- Pegmatites are composed of quite large crystals--cm to tens of cm in size. They typically occupy veins or layers within a larger plutonic body. The large crystals form by slow cooling of magma. Glassy -- Non-crystalline (glassy) structure of the rock, in which no minerals are present. Glass results from cooling that is so fast that minerals do not have a chance to crystallize. This may happen when magma or lava comes into quick contact with much cooler materials near the Earth's surface. Pure volcanic glass is known as obsidian. Vesicular -- This term refers to vesicles (holes, pores, or cavities) within the igneous rock. Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions. Pumice and scoria are common types of vesicular rocks. Breccia -- A rock composed of broken, angular fragments of mixed composition. Such texture forms in volcanoes, along fault zones, and in landslides.
70%. Potassium feldspar makes up more than one-third of total feldspars; plagioclase (Na & Ca) feldspars are less than two-thirds of total feldspars. Typical of continental crust. 2. Intermediate -- Between felsic and mafic. Silica content ranges from about 55% to 65%. Plagioclase feldspars make up more than two-thirds of total feldspars. Na-rich plagioclase predominates over Ca-rich plagioclase. Found in association with subduction zones. 3. Mafic -- Rich in magnesium and iron with less silica. Silica content is 45% to 50%. Ca-rich plagioclase is the dominant feldspar with little or no K- or Nafeldspars. Typical of oceanic crust. 4. Ultramafic -- Still more magnesium and iron and even less silica. Silica content is less than 45%, and little or no feldspar is present. Derived from the mantle.
Igneous Intrusion (also called a pluton): a large body of igneous rock formed when magma is injected into the surrounding (country rock) and crystallizes. Plutons are classified based on their size, shape and relation to the surrounding rock. I. Concordant plutons : intrusive bodies that do not cut across the layered country rock Most concordant intrusions are reletively small. Two common types are: a. Sill: a concordant pluton with parallel sides*
II. Discordant Plutons: Discordant plutons cut across the country rock. They may be either small or large. a. Dike: a small discordant pluton with parallel sides
b.Batholiths and Stocks: are large, irregularly-shaped, discordant plutons. Batholiths have an exposed area greater than 100 km2 and a stock has an exposed area less than 100 km2