Sand Island) and the island of Hawaii.
a. What types of volcanoes are located on these islands, and how were they formed?
The Midway Islands were formed 27.2 million years ago, and Hawaii 0 to 0.4 million years ago. b. Use this information and your atlas to calculate the average velocity of the Pacific Plate (in
whole centimetres per year), and the direction of movement of the Pacific plate during the last 27.2 million years. Show your calculation. (You may assume the Earth to be flat for this calculation, as it involves only a small part of the Earth’s surface).
4.2 Formation of different types of igneous rocks
The previous section described how a shield volcano consists mainly of basalt and a stratovolcano mainly of andesite. In reality, many more types of igneous rocks can be distinguished (as you saw in chapter 1). We will now address these differences in more detail by looking at the composition of magma. This composition gives us important information about the relationship between volcanism and plate tectonics.
Igneous rocks are formed when liquid magma solidifies (see Chapter 1). These rocks are marked by differences in texture. For instance, basalt is much finer grained than granite. If it is not possible to identify any crystals at all, the rock consists of volcanic glass. The texture of the rock tells us how fast the magma cooled. Liquid magma needs time to form crystals. When the liquid starts cooling, small crystals of one or more minerals will form and will grow gradually as ions are attached to the surfaces. The longer the cooling takes, the more of these crystals will grow. If magma cools very quickly, no crystals (volcanic glass) or only very small crystals (e.g. in basalt) will form. Magma cools quickly when it reaches the Earth’s surface; the result is an extrusive rock (like basalt). Slower cooling rates result in larger crystals; this happens when the magma stays deep in the Earth’s crust where it remains warm over a much longer period of time. Such rocks are called ‘intrusive rocks’ (like granite).
You can obtain more information by looking at the chemical composition and mineral content of a rock. Igneous rocks can also be classified according to their chemical composition and the type of silicates they contain. Examples of such minerals are quartz, feldspar, muscovite, biotite, amphibole, pyroxene, and olivine. Two types of igneous rocks can be distinguished, based on their mineral content: mafic rocks and felsic rocks (see Figure 4.4). Basaltic magmas (section 4.1) form mafic rocks like basalt (extrusive rock) or gabbro (intrusive rock). Examples of felsic rocks are rhyolite (extrusive rock) and granite (intrusive rock).
Mafic rocks (ma = magnesium (Mg), f = iron (Fe)) have a relatively low silicon content but contain relatively large amounts of magnesium and iron. These are important building blocks for pyroxenes and olivines. These minerals have a dark colour, which is why mafic rocks are usually dark. Basalt is the best known and most abundant mafic rock. The entire ocean floor consists of basalt. On some continents, thick layers of basalt can be found. For example, the Deccan Traps in India and the Siberian Traps in Russia.
Felsic rocks (fel = feldspar, si = silica) have a high silicon content and contain little iron and magnesium. These rocks are much lighter in colour because they contain light-coloured minerals like feldspar and quartz. Granite is a felsic rock. It is one of the most common igneous rocks found on Earth.
Figure 4.4: Different types of igneous rocks. Source: Grotzinger et al,
Looking at texture, chemistry and mineral content, it is possible to make the following rough classification (compare to Figure 4.4):
Mafic Felsic
Fine-grained – extrusive Basalt Rhyolite
Coarse grained – intrusive Gabbro Granite
Andesite, the extrusive rock found in many stratovolcanoes, is not mentioned in the table. Its composition lies somewhere between basalt and rhyolite.
There are also rocks that we call ultramafic. These contain relatively little silicon but a lot of magnesium and iron. The most common ultramafic rock is peridotite. This coarse grained, dark green rock is the dominant rock in the mantle, and is the source of basaltic magmas (see chapter 6). It consists mainly of olivine and pyroxene.
4.2.1 Do felsic and mafic magmas have different properties?
You may wonder why a distinction between felsic and mafic rocks is useful. There is a relationship between the composition of a rock and its melting temperature. Mafic rocks melt at higher temperatures, felsic rocks at lower temperatures. Mafic magmas are usually hotter and start crystallising at higher temperatures than felsic magmas. The more silicon is added, or the lower the temperature gets, the higher the viscosity of the magma will become. Viscosity is a measure for how difficult it is for a liquid to flow. So, the more felsic or colder a magma is, the more inhibited is its capacity to flow, especially when it carries abundant crystals. In contrast, a hot mafic magma, which is low in silicon, has a low viscosity and will flow easily.
4.3 Magma formation
We know how magma is formed, but much remains still to be discovered. The processes take place so deep inside the Earth that direct observation is impossible. Instead, they must be simulated in lab experiments. These experiments show that the melting point of a rock not only depends on its composition, but also on the pressure applied to it.
Rocks consist of various minerals, each with its own chemical composition, and do not usually melt completely when heated. The minerals contribute to the melt in certain proportions that may change when melting proceeds; thus, magma that is created by partial melting of a rock will have a chemical composition other than the rock itself. This is the reason why basaltic magma (mafic) can be formed from a mantle rock that was originally a peridotite (ultramafic). Depending on the amount of melting, magmas will have different compositions, even if the source rock from which they are formed is the same. (Of course, magma compositions can also differ simply because their sources are different; for example, if not the mantle but the lower crust is melting).
Theoretically, there are three ways to melt a rock in the mantle:
1. By increasing the temperature. Surprisingly, this is not an important cause of magma formation and volcanism.
2. By lowering the pressure. This is the cause of volcanism at mid-oceanic ridges and at hotspots. In both cases hot mantle rock rises very slowly; in the case of mid-oceanic ridges because of the upward motion in mantle convection, and in the case of hotspots because of the rise of an isolated mantle plume. The rock remains hot (thermal conductivity in rocks is very low), while the pressure decreases, causing the rock to melt.
3. By lowering the melting temperature. This happens when water comes into contact with the mantle rocks (just like when salt is sprinkled on roads during winter to lower the melting point of water so that it will not freeze). It is the main explanation for the origin of subduction-zone volcanism (discussed below).
4.3.1 The composition of first-formed magma and final igneous rock
Magmas of different composition are formed, depending on source rock and melting conditions. The next question is: will the same magma always yield the same igneous rock? The answer is no. Lab research shows us that different factors play a role:
- Firstly, the crystallisation history, which will determine the texture of the rock, in particular the sizes of the visible grains. Rising magma can accumulate in a magma chamber, for example somewhere within the crust. Because this environment is usually colder than where the magma came from, it will start cooling and crystals will form. Ultimately, it could crystallise completely inside the chamber and form a coarse-grained intrusive rock such as granite or gabbro. But (part of) the magma could also find its way further up during this process; it could rise to the surface and solidify there after a volcanic eruption. It would then form a fine grained or even glassy extrusive rock (for example basalt or rhyolite) that will often carry larger grains of minerals that crystallised previously at depth.
- Secondly, magma could change chemically in the course of its ascent to the surface. This is known as magmatic differentiation. While it resides in a magma chamber, different minerals will crystallise if the temperature drops. If these crystals do not stay floating in the liquid but sink to the bottom, the remaining magma will change in composition. It is “differentiated” because these removed minerals are different in composition than the original melt in which they crystallised. Therefore, a rock resulting from such a differentiated magma has not the same chemical composition as the first magma that formed during melting of the deep source.
In a basaltic magma, olivine will start to crystallise first, followed by pyroxene and other minerals. The composition of the remaining magma changes, depending upon the type of minerals formed (provided that these do indeed sink to the bottom). Bowen’s reaction series (Figure 4.5: The varying composition of igneous rocks is explained by fractional crystallisation. Source: Grotzinger et al, Understanding Earth (2007)) describes the order in which minerals crystallise. It usually holds quite well but it is no more than a general guideline. Several factors (such as the depth of crystallisation) can affect the order of crystallisation.
Basalt and granite are the two most common igneous rocks. Is there a relationship between the formation of these two rock types? We have seen that their compositions differ greatly: basalt is mafic and granite is felsic. Long ago it was thought that all granite originates from basalt by magmatic differentiation. As a basaltic magma crystallises, olivine and pyroxene (dark minerals, rich in magnesium and iron) are removed from it. What remains is a magma that is rich in silicon and poor in magnesium and iron, yielding a light coloured rock (like granite or rhyolite). The
Figure 4.5: The varying composition of igneous rocks is explained by fractional crystallisation. Source: Grotzinger et al, Understanding Earth (2007)
problem is that you would need an enormous amount of basaltic magma to account for all the granitic rocks on Earth.
The melting of entirely different source rocks could explain the difference between basalt and granite: when upper-mantle rocks melts partially, a basaltic magma will form. Granite can be formed through differentiation of basaltic magma, but also directly when the lower part of the continental crust melts. This part of the crust can consist of all kinds of sedimentary, igneous and metamorphic rocks, and has thus a composition that is very different from an ultramafic mantle rock.
4.3.2 Magma formation and plate tectonics
So how does magma formation fit into the framework of plate tectonics? In section 4.1 we saw that magma is formed at two types of plate boundaries: MORs and subduction zones
At MORs (Mid-Oceanic Ridges), the source material is the upper mantle, which is mostly peridotite. The pressure is reduced as the hot rock rises slowly, causing it to melt partially. The difference in density between the newly formed basaltic melt and the surrounding rocks will cause the melt to escape and rise even faster independently, until it accumulates in a magma chamber in the crust. There, it will partially crystallise as a gabbro, but some of it will reach the ocean floor and form so- called pillow lavas. These pillow-shaped blobs of basalt with a glassy crust are created as outpouring lava cools quickly in the seawater. Gabbros and pillow lavas are present in MORs below all oceans. If a basaltic magma is extruded above sea level it may form a shield volcano.
At subduction zones, oceanic (basaltic) crust is subducted beneath a continental or another oceanic plate. Not only the basaltic part of oceanic crust is subducted, but a thin layer of deep-sea sediments on top of it may go down as well. All the subducting material is gradually brought to depths where the temperature and pressure become higher and higher. The rocks will be metamorphosed because minerals of which they consist are transformed into other minerals that are better “adapted” to the conditions at greater depths. During these reactions, minerals that carry fluids in their crystal structure break down so that the fluids (mostly water) are released. The water is driven out of the subducting slab, moves upward and enters the mantle of the overlying plate. The addition of water lowers the melting point of the mantle material so much that it will melt, which usually occurs at a depth of about 100-150 km. Again, the source rock is upper-mantle peridotite so that the initial magma has a basaltic composition also here. This usually changes when it rises and concentrates in magma chambers, as we discussed before. The concentration of silicon in the magma increases, so that andesitic or even felsic magmas are formed. Compositions can also become felsic if portions of the continental crust melt because of the heat of basaltic magma that came up from the mantle. Granite forms if such a felsic magma crystallises at depth. If it reaches the surface, violent eruptions may occur. The increased silica content makes the magma very viscous and gases remain trapped inside. Enormous pressures can build up, which are released in violent eruptions.
Figure 4.6 shows a schematic overview of the relationship between plate tectonics and the different types of volcanism. Note that hot-spot volcanism is not related to any plate boundary.
Figure 4.6: Relation between plate tectonics and volcanism. Source: Grotzinger, et al, Understanding Earth, 2007.