Enigmatic Relationship Between Silicic Volcanic and Plutonic Rocks Archives - Elements
Among igneous rocks, i.e. those formed from magma or molten rock, the most important difference may be between plutonic and volcanic rocks. Plutonic rocks . The textural difference is that plutonic rocks are more coarse grained than volcanic rocks. Plutonic rocks are coarse grained while volcanic are fine grained (in. Igneous rocks are the most basic type of rocks. They are formed when magma ( molten rock, typically derived from the earth's mantle) solidifies. This can happen .
Volcanic rocks are igneous rocks which form from lava, molten rock which has been thrust out of a volcano onto the surface of a rocky body such as an asteroid, planet, or dwarf planet. Volcanic rocks are fine-grained and are found on most terrestrial planetary bodies in our solar system. Formation of volcanic rocks Volcanic rocks form on the surface of a planet from molten rock after it has been ejected or extruded out of a volcano, a point in the crust which has been ruptured due to the pressure of upwelling molten rock.
As the lava cools and hardens, mineral crystals will begin to form. The Crystals will continue to grow as long as there is still lava, so that the longer a lava takes to solidify, the larger the crystals will be in the resulting rock. Since volcanic rocks tend to form quickly and have little time to cool, the crystals within them tend to be very small, making volcanic rocks characteristically fine-grained. Types of volcanic rocks Mafic vs felsic Igneous rocks can be classified in several ways including based on their chemical and mineralogical composition.
Rocks that contain minerals with higher amounts of heavy elements such as iron and magnesium are considered mafic while rocks which have high silica content and minerals rich in lighter elements such as alkali feldspar are considered felsic.
There are also rocks which are intermediate between felsic and mafic. Basalt is a ubiquitous mafic volcanic rock. It forms in all active tectonic settings, but it often forms from lava originating from volcanoes that occur in continental rift basins, mid-oceanic ridges, and oceanic island arcs. Basalt also composes most of the surface rock of terrestrial planets and large asteroids.
This makes it one of the most common, if not the most common, rock in the solar system. An example of felsic volcanic rock is rhyolite which commonly forms at continental arcs. Rhyolite-forming lava tends to be more viscous and will occur alongside more explosive volcanic eruptions than those associated with basalt-forming mafic lavas.
Andesite is the intermediate between basalt and rhyolite in terms of its chemical composition. One place where andesites are found is at an oceanic island arc. What is Plutonic rocks? Plutonic rocks are igneous rocks which form from subterranean chambers of molten rock or magma. Plutonic rocks make up the base of continental crust as well as oceanic crust. Their slow rate of formation causes them to be coarse grained since there is plenty of time for large crystals to form before the magma solidifies into rock.
Continents can be eroded, broken up, or mashed over millions of years, but they cannot sink. The Sierra Nevada Batholith, which formed deep in the earth in the Mesozoic, has subsequently floated up, helping to raise the Sierra Nevada mountains in California. The weight and color of basalt is due to the inclusion of magnesium and iron in its constituent minerals.
Granite is light in color and density because it has little of those metals and substantially consists of aluminum and silica, i.
Plutonic and Volcanic Rocks
Granite is oversaturated with quartz and thus typically has pure quartz crystals in it. Rocks like basalt get called mafic from the magnesium Mg and iron Fe in them. Rocks like granite get called silicic or sialic from the silica and aluminum Al in them. But basalt has silica in it also, just in a very different form than in granite. Indeed, oxygen and silicon are the two most abundant elements in the earth's crust. The story of the rocks of the crust is thus paralleled by the story of silica.
It is now thought that the mantle of the Earth, the whole area between the crust and the core, consists of a rock that, where we can find it at or near the surface, is called peridotite. These are ultramafic rocks perhaps dunite is ultra-ultramafic. Olivine thus looks like one of the most basic, or most primitive, minerals that underlies the Earth's crust. It is not just that olivine contains magnesium and iron; silica occurs in olivine in a unique and simple form.
Thus, we get a molecule that becomes the negative ion SiO4 -4, the silicate anion. The oxygen atoms group around the silicon atom in a tetrahedron. See the discussion elsewhere for the coordination numbers produced by the relative sizes of such atoms. The olivines thus form a series from forsterite, Mg2SiO4, to fayalaite, Fe2SiO4, with a range of percentages of each in between. The olivine series thus can be written, Mg,Fe 2SiO4.
Olivine is characterized by its independent ionic tetrahedron. Tetrahedra, however, can begin to link together as oxygen atoms begin to share silicon atoms and thus form corners of two different tetrahedra.
If we get a single chain of tetrahedra, each one linked to the next, we get minerals in the pyroxene group, where the ratio of silicon to oxygen is Si2O6. A single chain can form a ring, as in beryl, Be3Al2 Si6O A double chain, were single chains link together, gives minerals in the amphibole group, with a ratio of Si4O And the silicate tetrahedra can link together into a continuous sheet, which characterized minerals that are clays and micas, Si4O As oxygens are eliminated, we are approaching a limit.
In olivine, the ratio of oxygen to silicon is 4: This is approaching the 2: Since this is electrically neutral, it can exist in pure form as the mineral quartz. Peridotite is olivines mixed with pyroxenes or amphiboles.
Dunite can be nothing but olivine and pyroxene. This already suggests a certain hierarchy. As the olivine-rich materials rise into the crust as magma, they begin to evolve. Perhaps not at first. Hawaiian volcanoes erupt a lot of olivine -- if the pure mineral erodes out, it can even form a green sand beach.
Plutonic and Volcanic Rocks
If such materials are buried, however, or subducted into the depths, under either an oceanic or continental plate, the material is reheated under pressure.
We can easily imagine the silicate tetrahedra begin to link together in greater numbers. The olivines evolve into pyroxenes, amphiboles, sheets, and framework silicates. As each of these approaches closer to electric neutrality, it needs fewer positive ions and thus become more silica rich, which means lighter and less dense. Now there is a clue about the relationships between granites and continents, basalts and oceans.
Oceanic crust is young, produced at mid-ocean ridges. It is straight out of the mantle. Hawaiian lavas are the newborn rocks of the earth. Continents, however, have a history. Granite is the result of the reworking, perhaps the multiple reworking, of previous crust.
When an oceanic plate is subducted, as under Japan or the west coast of South America, basalt and water is carried down to the depths. It melts and rises towards the surface. The silicate materials, however, are lighter and rise faster. The magnesium and iron are heavier and may rise more slowly or even settle. With pressure and the loss of positive ions, the silicate tetrahedra can link together.
Thus, when a volcano erupts in the Andes, it is no longer with the same mafic materials that were drawn down under the continent.
It may erupt andesite, which is quite close to basalt in composition, but lighter and with more silica. A magma that finally has the same composition as granite can erupt as a rhyolite. This may also happen were continental crust meets continental crust.