Igneous Rock Identification Nature’s Fiery Cauldron
When a volcano erupts, which involves the ash falls and the magna flowing from the crater, it is a breathtaking view to look at despite it fiery damages. The solidified product of this eruption known as volcanic rocks are part of a larger group of rocks commonly called as igneous rocks. When viewed under the ore microscope, igneous rocks are crystalline or glassy rocks. They are formed by the cooling and solidification of molten magma, which is a hot molten or partially molten rock material. The Earth is composed mainly of a large bulk of igneous rock covered thinly by sedimentary rock. Sedimentary rocks are by products of processes operating mainly at the Earth’s surface such as weathering and erosion. Igneous and metamorphic rocks on the other hand are formed by internal processes that cannot be directly observed.
Magma is believed to be produced within the asthenosphere approximately at a depth below about 60-100 kilometers. Magma is less dense than the surrounding solid rocks, thus it easily goes up toward the surface. The rocks formed due to the cooling and solidification of magma deep within the crust are distinct from those erupted at the surface basically because of the differences in conditions in the two environments. In the Earth’s crust the temperatures and pressures are much higher which results to the slow cooling of the hot magma and complete crystallization. The slow cooling encourages the growth of minerals big enough to be identified visually without the aid of a microscope. These are referred to as phaneritic, from the Greek phaneros, meaning visible. Magma erupted at the surface is chilled so fast. Under this condition, the individual minerals have slim or no chance to grow.
As a result, the rock is made up of minerals that can be seen only with the use of an ore microscope. These are known as aphanitic, from the Greek aphanes, meaning invisible or contains no minerals at all. There are two groups of igneous rocks. First are the plutonic or intrusive igneous rocks found solidified deep within the earth. The other is volcanic, or extrusive, igneous rocks formed at the surface of the earth.
Igneous rocks can be classified based on mineralogy and texture. As to texture the igneous rocks can be classified into the plutonic rocks which are mineral grain sizes visible to the unaided eye, and the volcanic types, which are too fine-grained or glassy for their mineral structure to be examined without the use of a microscope.
Mineralogically a plutonic rock may be classified according to the actual proportion of the various minerals of which it is composed. Some classification uses a simple association that occurs among the different silicate minerals. Volcanic rocks are hard to classify since many of the mineral grains are not visible. Mineralogical classification becomes problematic. Most field classifications of volcanic rocks depend on the few phenocrysts or the rock’s color. The identification of the volcanic rock samples can be possible based on texture, color as well as phenocryts. Article link
