With a background on how melts form and freeze, we can now introduce key features of the two settings intrusive and extrusive in which igneous rocks form.
Different volcanoes extrude molten rock in different ways. Some volcanoes erupt streams of low-viscosity lava that flood down the flanks of the volcano and then cover broad swaths of the countryside. When this lava freezes, it forms a relatively thin lava flow. Such flows may cool in days to months. In contrast, some volcanoes erupt viscous masses of lava that pile into rubbly domes. And still others erupt explosively, sending clouds of volcanic ash and debris skyward, and/or avalanches of ash tumbling down the sides of the volcano.
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| Examples of eruptions and extrusive materials. |
Which type of eruption occurs depends largely on a magma’s composition and volatile content. Volatile-rich felsic lavas tend to erupt explosively and form thick ash and debris deposits (figure above a, b). Mafic lavas tend to have low viscosity and spread in broad, thin flows (figure above c, d).
Intrusive Igneous Settings
Magma rises and intrudes into pre-existing rock by slowly percolating upward between grains and/or by forcing open cracks. The magma that doesn't make it to the surface freezes solid underground in contact with pre-existing rock and becomes intrusive igneous rock. As we noted, geologists commonly refer to the pre-existing rock into which magma intrudes as wall rock. The boundary between wall rock and an intrusive igneous rock is called an intrusive contact.
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| Igneous sills and dikes, examples of tabular intrusions. |
Geologists distinguish among different types of intrusions on the basis of their shape. Tabular intrusions, or sheet intrusions, are planar and are of roughly uniform thickness. Most are in the range of centimetres to tens of meters thick, and tens of meters to tens of kilometres long. A dike is a tabular intrusion that cuts across pre-existing layering (bedding or foliation), whereas a sill is a tabular intrusion that injects parallel to layering (figure above a–d). In places where tabular intrusions cut across rock that does not have layering, a nearly vertical, wall-like tabular intrusion is called a dike, and a nearly horizontal, tabletop-shaped tabular intrusion is called a sill. Some intrusions start to inject between layers but then dome upward, creating a blister-shaped intrusion known as a laccolith.
Plutons are blob-shaped intrusions that range in size from tens of meters across to tens of kilometres across (figure below a–e). The intrusion of numerous plutons in a region creates a vast composite body that may be several hundred kilometres long and over 100 km wide; such immense masses of igneous rock are called batholiths. The rock making up the Sierra Nevada of California is a batholith formed from plutons that intruded between 145 and 80 million years ago.
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| Igneous plutons, "blob shaped" intrusions. |
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| Making room for an igneous intrusion. |
Where does the space for intrusions come from? Dikes form in regions where the crust is being stretched horizontally, such as in a rift. Thus, as the magma that makes a dike forces its way up into a crack, the crust opens up sideways (figure above a). Intrusion of sills occurs near the surface of the Earth, so the pressure of the magma effectively pushes up the rock above the sill, leading to uplift of the Earth’s surface (figure above b).
How does the space for a pluton develop? Some geologists propose that a pluton is a frozen “diapir,” meaning a light-bulb-shaped blob of magma that pierced overlying rock and pushed it aside as it rose (figure above c). Another explanation involves stoping, a process during which magma assimilates wall rock, and blocks of wall rock break off and sink into the magma (Fig. 4.11d). If a stoped block does not melt entirely, but rather becomes surrounded by new igneous rock, it is a xenolith, after the Greek word xeno, meaning foreign. More recently, geologists have proposed that plutons form by injection of numerous superimposed dikes or sills, which coalesce and recrystallize to become a single, massive body.
Credits: Stephen Marshak (Essentials of Geology)
