How do aphanitic rocks form

Laccoliths are blister-like, concordant intrusions of magma that form between sedimentary layers. The Henry Mountains of Utah are a famous topographic landform formed by this process. Laccoliths bulge upwards; a similar downward-bulging intrusion is called a lopolith.

Peacock, M. Classification of Igneous Rock Series. The Journal of Geology 39 , 54—67 Arndt, N. Chapter 1 Archean Komatiites. Condie 11 , 11—44 Elsevier, Glazner, A. Are plutons assembled over.

The different colors are unique minerals. The black colors are likely two or three different minerals. This sample is mostly fine groundmass with a few small green phenocrysts that are the mineral olivine. Note conchoidal fracture. The percentage of minerals is shown on the vertical axis. The percentage of silica is shown on the horizontal axis. Rock names at the top include a continuous spectrum of compositions grading from one into another. Fel sic refers to a predominance of the light-colored felsic minerals fel dspar and si lica in the form of quartz.

These light-colored minerals have more silica as a proportion of their overall chemical formula. Minor amounts of dark-colored mafic minerals like amphibole and biotite mica may be present as well. Intermediate is a composition between felsic and mafic. It usually contains roughly-equal amounts of light and dark minerals, including light grains of plagioclase feldspar and dark minerals like amphibole.

Maf ic refers to an abundance of ferromagnesian minerals with magnesium and iron, chemical symbols M g and F e plus plagioclase feldspar.

It is mostly made of dark minerals like pyroxene and olivine, which are rich in iron and magnesium and relatively poor in silica. Ultramafic refers to the extremely mafic rocks composed of mostly olivine and some pyroxene which have even more magnesium and iron and even less silica. These rocks are rare on the surface, but make up peridotite, the rock of the upper mantle. Rhyolite source: Michael C. Rygel via Wikimedia Commons Granite is a course-crystalline felsic intrusive rock.

The presence of quartz is a good indicator of granite. Granite commonly has large amounts of salmon pink potassium feldspar and white plagioclase crystals that have visible cleavage planes. Granite is a good approximation for the continental crust, both in density and composition. Rhyolite is a fine-crystalline felsic extrusive rock.

Rhyolite is commonly pink and will often have glassy quartz phenocrysts. Because felsic lavas are less mobile, it is less common than granite. Examples of rhyolite include several lava flows in Yellowstone National Park and the altered rhyolite that makes up the Grand Canyon of the Yellowstone.

Intermediate Composition Diorite Andesite Diorite is a coarse-crystalline intermediate intrusive igneous rock. Andesite is a fine crystalline intermediate extrusive rock. It is commonly grey and porphyritic. It can be found in the Andes Mountains and in some island arcs see Chapter 2. It is the fine grained compositional equivalent of diorite. Mafic Composition Gabbro Vesicular Basalt Gabbro is a coarse-grained mafic igneous rock, made with mainly mafic minerals like pyroxene and only minor plagioclase.

Basalt is a fine-grained mafic igneous rock. It is commonly vesicular and aphanitic. When porphyritic, it often has either olivine or plagioclase phenocrysts.

Igneous Rock Bodies Igneous rocks are common in the geologic record, but surprisingly, it is the intrusive rocks that are more common. Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals figure 5. As we just learned, there are two main types of igneous rocks: intrusive rocks also known as plutonic rocks and extrusive rocks also known as volcanic rocks.

Volcanic rocks break down into two more categories: a lava flows and b tephra pyroclastic material. Igneous rocks are classified on the basis of their composition and their texture. Magma, and the igneous rock it becomes, has a range of chemical compositions. For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle. The way that magma turns into a solid rock gives it a distinctive igneous texture.

For example, magma that becomes a pluton by slowly crystallizing growing minerals within the crust will develop a very different texture from magma that becomes an ash flow tuff as a result of semi-molten volcanic ash spewing across a landscape and then settling down and welding itself together into solid rock. Igneous textures include the rock textures occurring in igneous rocks.

Igneous textures are used by geologists in determining the mode of origin of igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals.

The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur.

The result is a natural amorphous glass with few or no crystals. Examples include obsidian and pumice. Pegmatitic texture occurs during magma cooling when some minerals may grow so large that they become massive the size ranges from a few centimetres to several metres. This is typical of pegmatites.

As magma cools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Examples of phaneritic igneous rocks are gabbro, diorite and granite. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly.

The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained aphanitic matrix. The result is an aphanitic rock with some larger crystals phenocrysts imbedded within its matrix.

Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.

Figure 1. Different cooling rate and gas content resulted in these different textures. Let us start with textures associated with rocks formed by lava flows. Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye.

This is sometimes referred to as a fine-grained igneous texture. Some lava flows, however, are not purely fine-grained. If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix.

This texture is called porphyritic. If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy. Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals glass by definition is not a mineral, because it does not have a crystal lattice. Emplacement in high-level dykes or eruption on to the surface can result in the development of aphanitic fabrics.

Crystalline rocks with mineral grains that cannot be distinguished from one another without magnification have an aphanitic igneous texture. Igneous rocks form by crystallization of minerals from liquid magma rising into the upper portion of Earth's crust from the lower crust and underlying mantle. Igneous rock texture indicates the rate of magmatic cooling. Crystallization takes place either slowly in deeply buried intrusions called plutons, or rapidly at the earth's surface where magma has been extruded as lava by volcanic activity.

Igneous rocks are therefore classified as either intrusive plutonic or extrusive volcanic. Slow, undisturbed cooling in a well-insulated pluton is conducive to orderly arrangement of atoms and molecules into large, well-formed crystals.