Volcanism and the Composition of Rocks

There are three types of rocks on Earth. Sedimentary rocks are solidified layered deposits of sand and mud (produced by weathering) or by carbonate shells (from dead animals) laid down in lakes, river mouths or in the sea. Igneous rocks are formed by the solidification of melted magma that originates from deep in the Earth's crust or mantle. Metamorphic rocks are transformations of originally sedimentary or igneous rocks under the influence of high pressure and temperature. Rocks are composed of minerals that are naturally occurring inorganic solid substances with a distinct crystal structure and specific chemical composition.

The composition of minerals is controlled by the abundance of the chemical elements found in the Earth's crust and mantle. As shown in the previous chapter (Table 2.1), the most abundant elements on Earth are O, Si, Al, Fe, Ca, Na, K, Mg of which two, O with 49% and Si with 26%, make up three quarters of all elements by weight. It is therefore not surprising that the most important constituent of minerals that make up igneous rocks from the Earth's crust and mantle is the chemical compound silica, SiO2.

Table 3.1 shows the chemical composition, density and structure of minerals that are the constituents of igneous rocks. It is arranged by increasing density that also corresponds roughly to the depth where these minerals occur on Earth. It thus is not surprising that the lightest of these minerals, quartz, a special form of silica, is the most common mineral of the Earth's crust. Basic constituents of the minerals are tetrahedra of SiO^ 4) that are

Table 3.1. Chemical composition, density and silicate structure of common minerals in igneous rocks of the Earth's crust and mantle arranged by density (modified after Press et al. 2004)

Mineral

Composition

Density

Silicate structure

(g/cmä)

Quartz

SiOa

2. er,

spatial structures

Orthoclase feldspar

KAlSi3Og

2.D-2.Ü

spatial structures

Plagioclase feldspar

NaAlSijOs- CaALSi.O»

2.&-2.S

spatial structures

Muscovite :r:i< a

KAl3Si3Oio(OH)a

2.8-3.1

sheets

liiotitc :i:i' i

{K,Mii,Fc,Al}SiaOio(OH)2

3.11-3.1

sheets

Aniphibole

{Mg, Fe, Ca, NaJSi8()22(t)lI)2

3.1-3.3

double rliaijis

Pyroxene group

{Mg. h'e. Ca. Al}Si->0<;

3.1-3.6

single chains

Olivine group

(Mg, FVhSi04

3.3-3.5

isolated tetrahedra

Isolated Tetrahedra Crystals
Fig. 3.11. Silicate minerals. Clockwise from upper left: feldspar, mica, pyroxene, quartz, olivine

arranged in different ways in spatial structures, sheets, chains and isolated tetrahedra, and where the four excess bonds indicated by the superscript couple with strategically placed ions of Al, Fe, Ca, Na, K and Mg in the crystal lattices. Note that Si has four, O, Ca and Mg have two, Na and K one, Fe two or three and Al three bonds. Figure 3.11 shows samples of principal silicate minerals.

In addition to the mineral content (where felsic compositions are rich in feldspar and quartz (silica), and mafic compositions have lots of magnesium and ferrum (iron)), igneous rocks are classified by their texture (see Fig. 3.12). While for instance granite has an easily visible grainy crystal structure, one needs a microscope to see the small crystals in rhyolite that has exactly the same composition as granite. Similarly, diorite and andesite as well as gabbro and basalt have similar mineral content but different textures (Fig. 3.12). The texture is a result of the depth (temperature) in which the rocks formed from the solidifying magma and the available time for the growth of the crystals.

Because molten rock, called magma, is less dense than the surrounding solid rock, it rises and upon reaching the surface erupts as lava in volcanoes. Since this rise is rapid, one has three major types of fine-grained magma, basaltic magma contains about 50% SiO2, andesitic magma roughly 60% and rhyolitic magma about 70% (Fig. 3.12). Roughly 80% of the terrestrial volcanoes have basaltic magmas coming from deep to very deep in the mantle. As these magmas are very hot (1000-1200 °C), the lavas are fluid and move rapidly. This can lead to the formation of shield volcanoes that are flat and non-explosive since the lava has no problem running off a gentle slope. Examples of shield volcanoes with basaltic magmas are the Hawaiian volcanoes (see Fig. 3.13b).

Composition Coarse grained Fine grained

FELSIC

INTERMEDIATE

MAFIC

ULTRAMAFIC

Granite Rhyoiite

Granodiorite Dlorite □acite Andesite

Gabbro Basalt

Peridotlte

Andesitic Volcanism

<—-

Sodium, pot asslum content

Iron, magnesium, calcium content

Color of rocks becomes Increasingly dark

W

700°C

Melting temperature

1200°C ^

2.6 g/cm 3

Density

3.5 g/cm

Fig. 3.12. Mineral content of various types of igneous rocks. The rocks are classified by their composition and texture, the crystal size of the contributing minerals. Rocks formed deep in the Earth's crust or mantle where the cooling is slow develop large crystals and are coarse grained. Rapid cooling of magma at shallow depths leads to fine-grained rocks (modified after Press et al. 2004)

Fig. 3.12. Mineral content of various types of igneous rocks. The rocks are classified by their composition and texture, the crystal size of the contributing minerals. Rocks formed deep in the Earth's crust or mantle where the cooling is slow develop large crystals and are coarse grained. Rapid cooling of magma at shallow depths leads to fine-grained rocks (modified after Press et al. 2004)

Major Types Volcanoes
Fig. 3.13. Main types of volcanoes: a. stratovolcanoe Mount Fujiyama, Japan. b. shield volcano Mauna Kea, Hawaii (courtesy of Smithsonian Institution)

About 10% of the volcanoes erupt andesitic magmas. The magmas come from the upper mantle and the volcanoes are usually associated with island arcs or with continental margins where oceanic crust subducts and together with mantle material gets remelted. This is called wet melting because whenever water is added to hot rocks, the melting temperature is lowered and they melt more easily. Since the temperature of this magma is lower (800-1000 °C) it is more viscous and sometimes explodes as a shower of solid pyroclastic debris, called tephra. This material interlaced with layers of lava flows creates steep slopes producing stratovolcanoes of which Mt. Fujiyama and Mt. Vesuvius are examples (see Fig. 3.13a). Explosive release of dissolved gases in the magma can cause violent eruptions in stratovolcanoes.

Another 10% of the volcanoes have rhyolitic magma that is even less hot (650-800 °C) and has even higher viscosity. These magmas are the result of wet melting of continental crust. Rhyolitic volcanoes tend to erupt mostly tephra that accumulates in a distinctive cinder cone. They are primarily found on continents where one has rifting. Examples are the Yellowstone Caldera in Wyoming and many volcanoes on the North Island of New Zealand.

Next to the shield volcanoes and stratovolcanoes, a third type of volcan-ism occurs along elongated fractures usually at submarine spreading centers such as the Mid-Atlantic Ridge but also on land as on Iceland. Such fissure eruptions produce large amounts of basaltic magma that under the sea appears in the form of pillow lava (see Fig. 3.14a) that is produced by rapid cooling of erupting lava packets due to sea water. These layers of pillow basalt looking like an endless bed of sandbags form most of the oceanic crust. Pillow basalts represent the most common igneous rock on Earth.

Volcanic Vents The Deep Sea
Fig. 3.14. Volcanic-type eruptions in the deep sea. a. Pillow basalt. b. Hydrothermal vent with a so-called black smoker (courtesy of NOAA)

Finally, there is volcanism in the form of hot springs and geysers on land as well as at numerous hydrothermal (hot water) vents in the deep sea. Here, water percolating down through cracks in the underlying rock layers comes in contact with hot magma where it produces super-heated water, rises again and gets laden with minerals by leaching the surrounding rocks. Figure 3.14b shows such a mineral saturated flow from a hydrothermal vent called a black smoker. Due to high pressures in the deep sea these hydrothermal vent flows are still liquid despite temperatures of up to 400 °C.

The distribution of currently active volcanoes on Earth is seen in Fig. 3.15. Comparison with Fig. 3.10 shows that there is a close correlation of most volcanoes with nearby locations of earthquake centers and plate boundaries. An example of such a correlation is the huge continuous chain of volcanoes, called the Ring of Fire, that circles the Pacific from New Zealand, over the

Fig. 3.15. Distribution of active volcanoes (dots) around the Earth (after Press et al. 2004)

Philippines, Japan, the Kurils, Aleutians, western North and South America. However, there are also a number of volcanoes that occur inside the plates such as the Hawaiian volcanoes.

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  • lyle shaw
    What are terrestrial rocks?
    8 years ago

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