Igneous rocks

Characterizing Color and Texture 

If you wander around a city admiring building façades, you'll find that many façades consist of igneous rock, for such rocks tend to be very durable. If you had to describe one of these rocks to a friend, what words might you use? You would  probably start by noting the rock’s colour. Overall, is the rock dark or light? More specifically, is it Gray, pink, white, or black? Describing colour may not be easy, because some igneous rocks contain many visible mineral grains, each with a different colour; but even so, you’ll probably be able to characterize the overall hue of the rock. Generally, the colour reflects the rock’s composition, but it isn't always so simple, because colour may also be influenced by grain size and by the presence of trace amounts of impurities. (For example, the presence of a small amount of iron oxide gives rock a reddish tint.) Next, you would probably characterize the rock’s texture. A description of igneous texture indicates whether the rock consists of glass, crystals, or fragments. If the rock consists of crystals or fragments, a description of texture also specifies the grain size. Here are the common terms for defining texture:

Textures and types of igneous rocks.

1. Crystalline texture: Rocks that consist of minerals that grow when a melt solidifies interlock like pieces of a jigsaw puzzle (a in figure above). Rocks with such a texture are called crystalline igneous rocks. The interlocking of crystals in these rocks occurs because once some grains have developed, they interfere with the growth of  later-formed grains. The last grains to form end up filling irregular spaces between already existing grains. Geologists distinguish subcategories of crystalline igneous rocks according to the size of the crystals. Coarse-grained (phaneritic) rocks have crystals large enough to be identified with the naked eye. Fine-grained (aphanitic) rocks have crystals too small to be identified with the naked eye. Porphyritic rocks have larger crystals surrounded by a mass of fine crystals. In a porphyritic rock, the larger crystals are called phenocrysts, while the mass of finer crystals is called ground mass. 
2. Fragmental texture: Rocks consisting of igneous chunks and/ or shards that are packed together, welded together, or cemented together after having solidified are fragmental igneous rocks (a in figure above). 
3. Glassy texture: Rocks made of a solid mass of glass, or of tiny crystals surrounded by glass, are glassy igneous rocks. Glassy rocks fracture conchoidally (b in figure above). 

What factors control the texture of igneous rocks? In the case of non-fragmental rocks, texture largely reflects cooling rate. The presence of glass indicates that cooling happened so quickly that the atoms within a lava didn't have time to arrange into crystal lattices. Crystalline rocks form when a melt cools more slowly. In crystalline rocks, grain size depends on cooling time. A melt that cools rapidly, but not rapidly enough to make glass, forms fine-grained rock, because many crystals form but none has time to grow large (c figure above). A melt that cools very slowly forms a coarse-grained rock, because a few crystals have time to grow large.
Because of the relationship between cooling rate and texture, lava flows, dikes, and sills tend to be composed of fine grained igneous rock. In contrast, plutons tend to be composed of coarse-grained rock. Plutons that intrude into hot wall rock at great depth cool very slowly and thus tend to have larger crystals than plutons that intrude into cool country rock at shallow depth, where they cool relatively rapidly. Porphyritic rocks form when a melt cools in two stages. First, the melt cools slowly at depth, so that phenocrysts form. Then, the melt erupts and the remainder cools quickly, so that groundmass crystallizes around the phenocrysts.
There is, however, an exception to the standard cooling rate and grain size relationship. A very coarse-grained igneous rock called pegmatite doesn't necessarily cool slowly. Pegmatite contains crystals up to tens of centimetres across and occurs in dikes. Because pegmatite occurs in dikes, which generally cool quickly, the coarseness of the rock may seem surprising. Researchers have shown that pegmatites are coarse because they form from water-rich melts in which atoms can move around so rapidly that large crystals can grow very quickly.

Classifying Igneous Rocks 

Because melts can have a variety of compositions and can freeze to form igneous rocks in many different environments above and below the surface of the Earth, we observe a wide spectrum of igneous rock types. We classify these according to their texture and composition. Studying a rock’s texture tells us about the rate at which it cooled, as we've seen, and therefore the environment in which it formed. Studying its composition tells us about the original source of the magma and the way in which the magma evolved before finally solidifying. Below, we introduce some of the more important igneous rock types. 

Crystalline igneous rocks

Igneous rocks are classified based on composition and texture.

The scheme for classifying the principal types of crystalline igneous rocks is quite simple. The different compositional classes are distinguished on the basis of silica content ultramafic, mafic, intermediate, or felsic whereas the different textural classes are distinguished according to whether the grains are coarse or fine.  The chart in figure above gives the texture and composition of the most commonly used crystalline igneous rock names. As a rough guide, the colour of an igneous rock reflects its composition: mafic rocks tend to be black or dark Gray, intermediate rocks tend to be lighter Gray or greenish Gray, and felsic rocks tend to be light tan to pink or maroon. Note that rhyolite and granite have the same chemical composition but differ in grain size. Which of these two rocks develops from a melt of felsic composition depends on the cooling rate. A felsic lava that solidifies quickly at the Earth’s surface or in a thin dike or sill turns into fine-grained rhyolite; but the same magma, if solidifying slowly at depth in a pluton, turns into coarse-grained granite. A similar situation holds for mafic lavas a mafic lava that cools quickly in a lava flow forms basalt, but a mafic magma that cools slowly forms gabbro. 

Glassy igneous rocks

Glassy texture develops more commonly in felsic igneous rocks because the high concentration of silica inhibits the easy growth of crystals. But basaltic and intermediate lavas can form glass if they cool rapidly enough. In some cases, a rapidly cooling lava freezes while it still contains a high concentration of gas bubbles these bubbles remain as open holes known as vesicles. Geologists distinguish among several different kinds of glassy rocks.

Pumice, a vesicle-filled volcanic rock, is so light that paper can hold it up. The vesicles it contains tend to be small.

• Obsidian is a mass of solid, felsic glass. It tends to be black or brown (b in first figure). Because it breaks conchoidally, sharp-edged pieces split off its surface when you hit a sample with a hammer. Pre- industrial people worldwide used such pieces for arrowheads, scrapers, and knife blades. 
• Pumice is a felsic volcanic rock that contains  abundant vesicles, giving it the appearance of a sponge. Pumice forms by the quick cooling of frothy lava that  resembles the head of foam in a glass of beer. In some cases, pumice contains so many air-filled pores that it can actually float on water, like styrofoam (figure above). 
• Scoria is a mafic volcanic rock that contains abundant vesicles (more than about 30%). Generally, the bubbles in scoria are bigger than those in pumice, and the rock, overall, looks darker.

Pyroclastic igneous rocks  

When volcanoes erupt explosively, they spew out fragments of lava. Geologists refer to all such fragments as pyroclasts. Accumulations of fragmental volcanic debris are called pyroclastic deposits, and when the material in these deposits consolidates into a solid mass, due either to welding together of still-hot clasts or to cementation by minerals precipitating from water passing through, it becomes a pyroclastic rock. Geologists distinguish among several types of pyroclastic rocks based on grain size. Let’s consider two examples. 

• Tuff is a fine-grained pyroclastic igneous rock composed of volcanic ash. It may contain fragments of pumice. 
• Volcanic breccia consists of larger fragments of volcanic debris that either fall through the air and accumulate, or form when a lava flow breaks into pieces.

Credits: Stephen Marshak (Essentials of Geology)

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