Igneous rocks are formed by the cooling and crystallization (solidification) of magma.

1. Magma.

Magma is a complex, highly variable mixture of molten rock, dissolved gas and solid crystals. We will briefly discuss each of these components.

a. Molten rock.

The earth's temperature increases with depth at a rate of about 2 degrees Centigrade every 100 meters. At great depths, high temperatures cause rock to melt. Liquid rock is less dense than solid rock, and so may rise towards the earth's surface, much like a hot air balloon will rise through the surrounding, cooler atmosphere. In most cases, rising magma will cool and solidify somewhere within the earth. In some cases, the magma will break through to the surface.

b. Dissolved gasses.

The molten rock beneath the earth is subjected to enormous pressure from the weight of overlying rock. This pressure forces various gases to dissolve within the liquid rock. A good analogy is a bottle of Coke. At the bottling plant CO2 gas under high pressure is forced into the liquid Coke. To keep the pressure high, and the gas dissolved in the liquid, a cap is put on the bottle. If you uncap the bottle this removes the pressure and the dissolved gas bubbles out of solution, giving Coke its fizz. The gas may escape directly into the atmosphere, or it may mix with some of the liquid to form a frothy, foamy head. If the gas escapes rapidly, like when the bottle is shaken before it is opened, the force of the escaping gas is so powerful that it blows some of the liquid right out of the bottle. Magma behaves similarly. When magma reaches the surface, it is "uncapped", that is, the pressure of the overlying rock is removed. As a result, the dissolved gas bubbles out of the liquid rock. If you could look down into an erupting volcano the magma would be fizzing and the bubbles would create a frothy "head" on the lava. If large amounts of gas escape all at once, some of the red-hot, liquid rock will be blown into the air. Most of the violence associated with volcanic eruptions is due to the force of escaping gas.

c. Solid mineral crystals.

Magma typically contains dozens of different elements, and so the potential to create hundred of different minerals. Each of these minerals crystallizes at a different temperature. As a magma slowly cools, high temperature minerals will form first, low temperature minerals last. For example, consider two minerals: "A" with a melting point of 1000 degrees, and "B" with a melting point of 750 degrees. If the magma is at 1250 degrees, both "A" and "B" will be liquid. If the magma is at 700, both "A" and "B" will be solid crystals. If the magma happens to be at 850 degrees, there will be solid crystals of "A" contained in liquid "B". Except under extreme conditions, magma will almost always contain some crystals of high temperature minerals.

2. Classification of igneous rocks.

Once formed from the solidification of magma, the resulting igneous rocks are classified based on texture and on composition.

A. Texture.

Texture refers to the size of the individual crystals contained in a rock. The size of these crystals depends primarily on the rate at which magma cools and solidifies. In liquid magma, individual atoms are free to move around. As a body of magma cools, these individual atoms lose energy and come together to form solid mineral crystals. Crystal growth only occurs when liquids are present. Once all liquids solidify, crystal growth stops. The size of crystals produced depends on the rate at which the magma cools. If cooling is very slow, minerals have lots of time to grow, and so large crystals are produced. If cooling is rapid, little time is available for growth, and so crystals will be small. If cooling is extremely rapid, individual atoms will be frozen in place and crystals may not form at all.

 

a. Igneous textures: phaneritic.

Rate of cooling depends on where magma cools. Most bodies of magma rising to the surface get emplaced (stuck) inside the earths crust. Igneous rocks that solidify within the crust are called "intrusive rocks" or "plutonic rocks" (named for Pluto, the Roman God of the Underworld). Emplaced bodies of intrusive rock are called "plutons". Plutons are well-insulated by miles of overlying rock and so heat escapes very slowly. As a result, intrusive magmas cool very slowly, perhaps requiring tens of thousands of years, and crystals have the time to grow relatively large. Intrusive igneous rocks typically have crystals about the size of a match head. In general, intrusive rocks possess individual crystals that are visible to the naked eye. Rocks with crystals this size have a "phaneritic" texture. Look at your granite specimen. This is a phaneritic rock. You can easily see individual crystals of quartz (grayish-glassy), a pinkish feldspar, and biotite (black). The granite pegmatite has much larger crystals and a slightly different mineral composition. The feldspar here is a more whitish variety.

 

b. Igneous textures: aphanitic

Magma that reaches the earth's surface forms "extrusive rocks". These magmas are exposed to the atmosphere and so cool rapidly, usually solidifying within a few days. As a result, crystals have very little time to grow and so are relatively small. Extrusive rocks do contain crystals, but most are too small to be seen with the unaided eye. Rocks with crystals this size have an "aphanitic" texture. Aphanitic rocks often have a fine-grained, homogenous appearance, very similar to concrete. Look at the rhyolite. This rock is composed of the same minerals as granite. The only difference between the two is that the magma forming the rhyolite cooled rapidly on the surface, while the magma forming the granite cooled slowly, deep within the earth. Compare the gabbro specimen and the basalt. Both are formed from mafic magmas. Gabbro is a phaneritic intrusive rock, while basalt is an aphanitic extrusive rock.

c. Igneous textures: glassy.

Magma may cool almost instantaneously if it is extruded directly into water. When this occurs, the magma solidifies instantly and no crystals are formed at all. The resulting rock has little or no crystalline structure and has a glassy texture. Examine obsidian that is also called "volcanic glass". The magma producing was very similar in composition to that which formed the granite and the rhyolite, but was cooled so quickly that no crystals were formed. People make "glass" by imitating this natural process. Pure quartz sand is melted in a furnace to form a magma. Small quantities of this magma are removed from the furnace, shaped into pop bottles or whatever, and rapidly cooled by exposure to the air. Since the quartz has no time for quartz crystals to form, a clear "glass" is produced. Obsidian is discolored by small amounts of iron and other impurities.

d. Igneous textures: porphyritic.

Magmas contain all the different elements necessary to produce a huge variety of different minerals. Different minerals crystallize at different temperatures. As a magma begins cooling inside the earth, high-temperature minerals are the first to form crystals. These solid crystals float within the remaining molten rock. This partially crystallized / partially molten magma is then extruded, so that it cools very rapidly. The result is a rock texture called "porphyritic", where large "phenocrysts" (the big, high-temperature crystals) are encased within a matrix of low-temperature, aphanitic crystals, called the "groundmass". Examine the rhyolite porphyry. This rock exhibits two distinctive crystal-sizes. The large, light-colored crystals are potassium feldspar. These crystals were the first to form, at a relatively high-temperature. The feldspar phenocrysts are surrounded by a dark-gray groundmass of aphanitic crystals, formed when the magma was later cooled on the surface. The specimens of andesite have a few phenocrysts, so this might be considered andesite porphyry. Here, both the phenocrysts and the groundmass are composed of dark-colored, relatively mafic minerals.

 

e. Igneous textures: vesicular

Examine both the scoria and pumice that are rocks produced by gassy, violent, volcanic eruptions. The holes in these rocks, called "vesicles", were formed as gas bubbled out of the liquid magma. The crystallization of the magma preserved these holes. The basalt specimen in your collection has some vesicles, also, and was called a vesicular basalt.

f. Igneous textures: pyroclastic.

Extrusions of magma are often very violent events. Volcanic gas bubbling out of de-pressurized magma has enormous force and is capable of hurling both solid rock and molten lava great distances. Rocks formed from the material that has blown out of an erupting volcano possess a "pyroclastic" texture. Pyroclastic literally means "fiery chunks. Examine the welded tuff. Tuff is a rock formed from volcanic ash. Ash is a fine, dusty material produced when very violent eruptions pulverize rock and blow it into the atmosphere. This ash originally settled out in fluffy piles, but has since been welded (consolidated) into a rock.

B. Composition.

Magmas vary tremendously in composition, with the potential to form hundreds of different minerals. For our purposes, we will generalize all magmas into only three types.

a. Felsic magma. Felsic magmas are high in silicon and low in iron. These magmas produce rocks that tend to be light in color and low in density, often producing rocks containing a lot of quartz.

b. Mafic magma. Mafic magmas are high in iron, but are relatively low in silicon. These magmas produce rocks that are dark in color and high in density, containing little quartz.

c. Intermediate magma. These magmas are between mafic and felsic in composition. The resulting rocks fall between felsic and mafic rocks in terms of their physical characteristics.

Look at your rhyolite specimen. This felsic rock is called rhyolite, and it is composed primarily of silicon-rich minerals, particularly quartz. Rhyolite is relatively light in color and low in density. Contrast rhyolite to specimen basalt, a mafic rock. It contains less silicon and more iron, so it is dark in color and high in density. Compare rhyolite and basalt to andesite. Andesite is intermediate in composition, containing roughly equal amounts of mafic and felsic minerals, and so is grayish in color with a moderate density.