Obsidian. Cooled lava forms basalt with no visible crystals. Let us start with textures associated with rocks formed by lava flows. Diorite was used extensively by ancient civilizations for vases and other decorative artwork and is still used for art today (Figure 1). Examples includeobsidian and pumice. Most of the precious gemstones can be identified by looks from glossy, sparkly, see through, solid colors. Felsic rocks tend to be light in color (white, pink, tan, light brown, light gray). Igneous rocks are known for having gas bubbles or crystals. Mafic rocks tend to be dark in color (black, very dark brown, very dark gray, dark green mixed with black). 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. If you want to determine whether the rock is extrusive or intrusive, look for different colors or textures on the surface. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. Start studying Lesson 2 How Rocks are Classified. Such a rock is said to have a glassy texture. Mafic rocks are made of magnesium and iron. 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. One important use is as stone for buildings and statues. Mafic composition is higher in iron and magnesium and lower in silica. The intrusion of large bodies of granite—batholiths—is usually part of the origin of a mountain range. Felsic rocks are made up of feldspar and silica quartz. Some lava flows, however, are not purely fine-grained. This article was co-authored by our trained team of editors and researchers who validated it for accuracy and comprehensiveness. These rocks are usually formed into the Earth's crust. Intrusive igneous rocks are created from magma that has solidified underground. Large minerals form because it takes a long time for the magma to cool underground. Please consider making a contribution to wikiHow today. Your support helps wikiHow to create more in-depth illustrated articles and videos and to share our trusted brand of instructional content with millions of people all over the world. The bands in the rocks are formed by the temperature of the environment where the rock was formed. Learn vocabulary, terms, and more with flashcards, games, and other study tools. In this science printable, students will determine whether each rock is a clastic or a nonclastic, and will identify how the rock was formed. For more tips, like how to categorize metamorphic rocks, read on! 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. The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it. 1. For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle. In some places, geological processes have brought igneous rocks to the surface. Rock is defined as one or more minerals aggregate. The association between color and composition is useful because before you can name and interpret an igneous rock you need to determine both its texture AND its composition. Clastic sedimentary rocks are rocks composed predominantly of broken pieces or clasts of older weathered and eroded rocks. This earth science activity does not classify rocks in this way. Pegmatites are commonly found in or near the margins of bodies of granite. Collecting rocks is a fun hobby and being able to classify the rocks into different types makes it even better! The silica in the free state is called sand and in combined state is silicate. 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. If the cooling occurs underground, the rock is an intrusive, or plutonic, igneous rock. Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), and biotite (black, platy). Igneous rocks with olivine in them are usually mafic. Rocks are divided into three basic types depending on how they were formed: Igneous, Sedimentary, and Metamorphic. Igneous rocks are classified on the basis of their composition and their texture. ; The earth is made of igneous rock – at least at the surface where our planet is exposed to the coldness of space. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. These “frozen bubble holes” are called vesicles, and the texture of a rock containing them is said to be vesicular. Each type can be further divided into categories, such as based on what they are made out of. Magmas that erupt as lava onto the earth’s surface cool and solidify rapidly. Rocks made of volcanic ash are called tuff. Now let us briefly consider textures of tephra or pyroclastic rocks. This texture is called porphyritic. This is where you get your obsidian glass and basalts. Foliated metamorphic rocks appeared banded or layered. 1. Igneous: Igneous rocks form from the cooling of melted rock (either lava or magma) into solid form. Igneous. 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