3-1 Igneous Rocks and Processes Classification of Igneous Rocks If a rock is subjected to a high enough temperature it will melt. Igneous rocks are classified on the basis of two characteristics: Molten rock beneath the surface is called magma. 1. Composition Magma which reaches the surface is called lava. 2. Texture As this material cools, mineral crystals will form. As this material completely solidifies the solid formed from the crystals will be an igneous rock. 1. Composition 2. Texture Magma s on Earth typically contain the most abundant elements on Earth: O, Si, Al, Fe... However, they do not all have the exact same composition and thus they form igneous rocks of different compositions. Texture refers to the size, shape, and orientation of the mineral grains within an igneous rock. These depend mostly on the rate of cooling. The two main categories are: Rocks which are relatively poor in silica (45% 55% silica content) are called mafic (derived from magnesium and ferrum for iron). Those which are very poor in silica (<40%) are called ultramafic. 1. Coarse Grained Rocks (phaneritic): The crystals can be clearly seen by the naked eye (> 1mm in size). What does this suggest about the rate of cooling and where they formed? Rocks rich in silica (>65%) are called felsic. Note that the terms mafic and felsic can also be used to refer to the composition of a magma.
3-2 2. Fine Grained Rocks (aphanitic): What does this suggest about the rate of cooling and where they formed? Igneous Rocks: Coarse Grained Fine Grained felsic Granite Rhyolite light colored medium intermediate Diorite Andesite grey or green dark mafic Gabbro Basalt grey to black dark ultramafic Peridotite Komatiite green to black While most igneous rocks are phaneritic or aphanitic there are other classifications: 3. Glassy 4. Porphyritic Some igneous rocks contain crystals of widely differing sizes. Some large, some fine grained. These are rocks which cooled so quickly they did not have time to form crystals. How might such a rock form? Obsidian Pumice What does this suggest about the formation environment for these rocks? 5. Pyroclastic Tephra: volcanic dust, ash, cinders, bombs Fragments of rocks which have been blasted apart in an explosive volcanic eruption. Either preexisting rock or lava which cools rapidly in various shapes and sizes.
Creation of Magma 2. Pressure 3-3 What processes cause rock to melt? 1. Heat Raise the temperature enough and rock will melt. Radioactive elements within the Earth release energy in the planet s interior: Most materials melt at a higher temperature as the pressure is increased. Conversely, if pressure is released, the melting point will be lowered and may cause melting. This factor is particularly important at the midoceanic ridge: ==> The temperature increases with depth into the Earth s surface. Some radioactive elements are enhanced in the crust (uranium, potassium) leading to a great deal of heating in the lower crust. This heat can lead to melting at depths of ~250km. 3. Water The introduction of water generally lowers the melting point of rocks. This is one of the processes which creates magma at subduction zones: Bowen s Reaction Series Magma (or lava) eventually cools down and solidifies. Exactly how does this occur? What minerals form? Would you expect all the material in a melt to solidify at the same time?
3-4 Problem: Most magmas are silica poor (mafic). How can silica rich rocks be generated from them? What would happen to the composition of the magma if this occurred? While under ideal conditions the entire sequence may occur, often it does not. As crystals form they may not remain with the magma. If they are denser than the magma they may settle out of the magma. Or they may stick to the walls of the magma chamber. This process is called fractional crystallization or differentiation. Or even be physically filtered out as the magma continues to flow towards the surface. What would happen if one were to reverse the process heating and melting a rock? Intrusive Igneous Structures While volcanoes may be more spectacular, most igneous rocks are formed underground. These are called intrusive rocks because they intrude into the preexisting rock. The preexisting rock into which the intrusive rocks are emplaced is called country rock. As noted before, rocks buried underground tend to cool slowly ==> coarse grained rocks. However, this is not always the case. What types of structures do they form?
3-5 Shallow Igneous Structures: If it follows the layering found in the country rock it is said to be concordant and is called a sill. Tabular Structures In some cases we see igneous rocks which have squeezed between other rock layers (or perhaps forced their way between them). If the intrusive rock does not follow the preexisting layering it cuts across them it is said to be discordant. These features are called dikes. These often form features which are long and wide but not very thick. Suppose we had a lava flow over preexisting rock. Tabular igneous structures are given different names depending on their orientation with respect to the country rock. Lava flow was then buried under further layers. What would it look like? How can we distinguish between a sill and a volcanic flow which was later buried? 3. Look for evidence of heating of the surrounding rock if seen in rock atop an igneous formation this would suggest what type of feature? There are several characteristics which can help distinguish these cases: 1. Vesicular structure lavas which flow on the surface tend to have a frothy appearance on its top layer caused by escaping gases. 4. Signs of weathering Lava flow would be exposed at the surface for some time and thus the top edge might show signs of weathering. 2. Xenoliths xenoliths are foreign rock incorporated in the pluton. Sometimes xenoliths of the overlying rock are present within the igneous layer. If so would this suggest an intrusive or volcanic feature?
3-6 Tabular features are thought to be formed relatively near the Earth s surface. Deep Intrusive Structures Dikes are often the result of magma solidifying within the conduits of a volcano. Sills tend to form by intruding in spaces between layers of rocks. Sometimes igneous material can force its way into the country rock causing it to bulge upwards. Creates a dome shaped feature called a laccolith. If the intruding magma is heavier than the surrounding rock its weight may push down on the underlying rock. This will yield an upside down dome shaped structure called a lopolith. Sills, dikes, laccoliths, and lopoliths are all features seen near the Earth s surface (within a few kilometers). At too large a depth any cracks in the layers are closed by the high pressure of the overlying material. However, other structures are seen. These are called plutons. These underground structures can be enormous (1000s of km). Vast plutonic structures are called batholiths. Deep intrusive structures are almost always felsic in composition. Smaller features exposed at the surface and covering areas less than 100 square kilometers are called stocks. Most are granite. Though some are diorite.
Volcanoes Basaltic Volcanoes 3-7 Volcanic eruptions can be quite spectacular: Mt. St. Helens The island of Krakatau literally blew itself up. However, some volcanoes erupt in a quite peaceful manner, quietly flowing lava out onto the surface. The main factor which determines a volcano s eruptive style is the composition of the lava it erupts. Mafic (low silica content) magmas are much less viscous than felsic (high silica content) magmas. Thus it is relatively easy for gases in such magmas to escape. Thus are large pressures likely to develop? Clearly there are a variety of types of volcanoes. Why do they differ in form? Are they likely to be explosive? Why are some explosive and others not? Basalt volcanoes are the most common. Volcanism at Hot Spots Seen in two main geologic settings: 1. Mid-oceanic ridge In addition to the mid-ocean ridges, basaltic volcanoes are seen at intra-plate volcanoes overlying hot spots. ==> Ocean floor almost entirely made of basalt and gabbro. 2. Hot Spots The Hawaiian Islands are the classic example. Because the lava generated is of low viscosity, would you expect the lava to be able to flow over relatively large or small distances? Would you expect such volcanoes to be gently or steeply sloped? These are called shield volcanoes.
Types of Lava Underwater we often see pillow basalt. 3-8 Much of our knowledge of basaltic lava flows come from the Hawaiian islands. The water quickly cools the lava as it emerges. Thus, our names for types of lava come from Polynesian roots. Outside layers solidify instantly. pahoehoe ropy lava. Low viscosity and thus flows quickly yielding a ropy appearance. Typically seen near the eruption site. Lava then continues to ooze out within the solidified skin. Looks much as if someone were squeezing toothpaste out of its container. aa Looks much like pillows stacked on top of each other. Higher viscosity (cooler) lava. Moves more slowly and has a more blocky jagged look to it. Andesitic Volcanoes How do these volcanoes differ from those erupting more mafic material? Volcanoes which occur at subduction zone boundaries tend to exhibit a more felsic composition. The higher silica content makes the lava more viscous. Why? Will lava flow as far? As with more mafic volcanism, it is thought that the magma is created from melted mantle. What makes it more felsic? What about the slope of the volcano? Finally how would you expect the eruptive style to differ from more mafic volcanoes?
3-9 Some volcanoes are on the border between being explosive or effusing lava quietly. Rhyolite Volcanoes They may do both at various times. Very felsic volcanoes are rare. These types of volcanoes create composite cones or stratovolcanoes. However, when they do occur the lava is very viscous and thus they tend to be quite explosive. Both basaltic and andesitic volcanoes may also form pyroclastic cones or cinder cones. Often form volcanic domes. Why are they rare? These types of volcanoes tend to be small. The unconsolidated pyroclastic material also tends to be eroded quickly.