What type of cooling results in large crystals

The rate of cooling is very important. If the magma cools slowly then the reactions proceed for a longer period of time and the resulting mineral crystals grains become large. If the magma cools rapidly then the reactions are much quicker and the mineral crystals grains are smaller.

Igneous rocks will have a distinctive appearance this is referred to as texture based on the rate of cooling fast or slow. Phaneritic coarse grained : rocks are composed of mineral grains large enough to be seen with the unaided eye. Aphanitic fine grained : rocks are composed of mineral grains too small to be seen by the unaided eye. Indicitive of faster cooling, usually at or near the Earth's surface.

Glassy : magma cooled so quickly that crystal formation could not occur. Indicative of almost instant cooling at the Earth's surface. Porphorytic : large mineral cyrstals surrounded by a fine grained matrix background. This texture forms when slow cooling begins to form large mineral crystals. Before the magma is completely cooled the magma and its large mineral grains moves closer to the surface and finishes cooling at a more rapid pace. Sialic granitic or felsic : rocks have highest silica, contain feldspar and ferromagnesium minerals.

Rocks are light in color. At that temperature, the plagioclase is calcium-rich anorthite see Figure 2. As the temperature drops, and providing that there is sodium left in the magma, the plagioclase that forms is a more sodium-rich variety. In , he joined the Carnegie Institution in Washington, D.

Working mostly with basaltic magmas, he determined the order of crystallization of minerals as the temperature drops. The method, in brief, was to melt the rock to a magma in a specially made kiln, allow it to cool slowly to a specific temperature allowing some minerals to form , and then quench it cool it quickly so that no new minerals form only glass.

The results were studied under the microscope and by chemical analysis. This was done over and over, each time allowing the magma to cool to a lower temperature before quenching. The Bowen reaction series is one of the results of his work, and even a century later, it is an important basis for our understanding of igneous rocks.

The word reaction is critical. As the temperature continues to drop, olivine becomes unstable while pyroxene becomes stable. The early-forming olivine crystals react with silica in the remaining liquid magma and are converted into pyroxene, something like this:. This continues down the chain, as long as there is still silica left in the liquid.

In cases where cooling happens relatively quickly, individual plagioclase crystals can be zoned from calcium-rich in the centre to more sodium-rich around the outside. This occurs when calcium-rich early-forming plagioclase crystals become coated with progressively more sodium-rich plagioclase as the magma cools. Figure 3. The composition of the original magma is critical to magma crystallization because it determines how far the reaction process can continue before all of the silica is used up.

The compositions of typical mafic , intermediate, and felsic magmas are shown in Figure 3. Note that, unlike Figure 3. There are two reasons for this: one is that in the early analytical procedures, the results were always expressed that way, and the other is that all of these elements combine readily with oxygen to form oxides. Obsidian is usually black. Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks.

A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in.

An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff. A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture.

However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye.

These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic. Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture. Sometimes an intrusion of magma that is crystallizing slowly underground releases large amounts of hot water.

The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it. A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than 1 cm in diameter and sometimes is much larger. The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite. The most common igneous compositions can be summarized in three words: mafic basaltic , intermediate andesitic , and felsic granitic.

Felsic composition is higher in silica SiO 2 and low in iron Fe and magnesium Mg. Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions. Composition influences the color of igneous rocks. Felsic rocks tend to be light in color white, pink, tan, light brown, light gray. Mafic rocks tend to be dark in color black, very dark brown, very dark gray, dark green mixed with black.

The color distinction comes from the differences in iron and magnesium content. Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors green, gray, brown.

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. If you have an aphanitic igneous rock, which has no crystals big enough to see without a microscope, you can estimate its composition based on its color: pink or nearly white, felsic; medium gray, intermediate; very dark or black, mafic. This color rule works most of the time but there are two problems that you need to keep in mind.

First, the rule does not work for glassy igneous rocks. Obsidian, which is volcanic glass, is usually black, even though it has a felsic composition. That is because a tiny amount of iron, too little to color minerals very darkly, can color glass darkly. The second problem is that when igneous rocks have been exposed to air and water for a long time, they start to weather, which changes their color. If you can see and identify the minerals in an igneous rock, you can gain further information about the igneous composition.