![]() Let us start with textures associated with rocks formed by lava flows. ![]() Different cooling rate and gas content resulted in these different textures. Pyroclastic ( pyro = igneous, clastic = fragment) textures occur when explosive eruptions blast the lava into the air resulting in fragmental, typically glassy material which fall as volcanic ash, lapilli and volcanic bombs.įigure 1. 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. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. 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. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. Examples of phaneritic igneous rocks are gabbro, diorite and granite. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. As magma cools slowly the minerals have time to grow and form large crystals. Phaneritic ( phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth’s surface. 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). The result is a natural amorphous glass with few or no crystals. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. There are six main types of textures phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.Īphanitic ( a = not, phaner = visible) rocks in contrast to phaneritic rocks, typically form from lava which crystallize rapidly on or near Earth’ surface. Igneous textures are used by geologists in determining the mode of origin of igneous rocks and are used in rock classification. Igneous textures include the rock textures occurring in igneous rocks. 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. The way that magma turns into a solid rock gives it a distinctive igneous texture. For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle. Magma, and the igneous rock it becomes, has a range of chemical compositions. Igneous rocks are classified on the basis of their composition and their texture. Volcanic rocks break down into two more categories: (a) lava flows and (b) tephra (pyroclastic material). Classifying Igneous RocksĪs 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). There is little time for crystals to form, so extrusive igneous rocks have tiny crystals (figure 5). Why are there no visible crystals?Įxtrusive igneous rocks cool much more rapidly than intrusive rocks. Cooled lava forms basalt with no visible crystals. Figure 3 below shows a landscape in California’s Sierra Nevada made of granite that has been raised to create mountains.įigure 5. In some places, geological processes have brought igneous rocks to the surface. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. Igneous rocks make up most of the rocks on Earth. Granite is the most common intrusive igneous rock (see figure 2 for an example). Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. When magma cools within the Earth, the cooling proceeds slowly. A pluton is an igneous intrusive rock body that has cooled in the crust. Intrusive rocks form plutons and so are also called plutonic. ![]() Igneous rocks are called intrusive when they cool and solidify beneath the surface. Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), and biotite (black, platy).
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