Bishop, Chapter ??

Rocks

Igneous Rocks

The so-called crust of the Earth is about 20 mi thick under the continents but averages only some 4 mi beneath the oceans.  If is formed mainly of rocks of relatively low density.  Beneath the crust there is a layer of denser rock called the mantle which extends down to a depth of nearly 2000 mi.  Much of the molten rock material which goes to make up the igneous rocks is generated within the upper parts of the mantle.  This material, which is called magma, migrates upward into the Earth’s crust and forms rock masses which are known as igneous intrusions.  If magma reaches the Earth’s surface and flows out over it, it is called lava.

Within some lavas, fragments of dense, green-colored rocks are sometimes found which consist principally of olivine and pyroxene.  The fragments (xenoliths) are thought to represent pieces of the mantle, carried upward by the migrating magma.

The great majority of lavas consist of the black, rather dense rock called basalt, and most petrologists consider that the primary molten rock material which comes from the mantle has a composition which is near to that of basalt.  Although basalt is the most abundant of the lavas, granite is by far the commonest of the intrusive igneous rocks.  Granite is mineralogically and chemically different from basalt and for many years geologists have wrestled with the problem of how the two rock types are related.  If basalt is assumed to derive from the mantle, is it likely that granite, which is of quite different composition, could also come from the mantle? In recent years, ideas as to how basalt, granite and, indeed, the whole spectrum of igneous rocks described in the following pages have been generated, has been much influenced by the theory of plate tectonics.  In brief, this all-embracing idea is that the uppermost part of the Earth consists of a layer, up to about 100 mi thick, and comprising the crust and the uppermost part of the mantle, that moves over the underlying mantle.  This uppermost layer, which is known as the lithosphere, comprises a number of rigid ‘plates’ which jostle against each other.  In some places new lithosphere is being generated while elsewhere an equivalent amount is being destroyed. New lithosphere is mostly being formed along the midocean ridges (spreading ridges), which are lines, notably running along the center of the Atlantic and within the Pacific Ocean, where magma rises from deep in the mantle, erupts onto the ocean floor and is injected close to the surface, so forming new lithosphere and hence making the plates bigger.  Although this activity is generally concealed beneath the waters of the oceans, it can be seen, for instance, in Iceland, which is located on the MidAtlantic Ridge.  Along some plate margins one plate is forced downward beneath the adjacent one. At these boundaries, known as subduction zones, the lower plate may be driven hundreds of kilometers down into the mantle, where the high temperatures will cause melting. This new magma will eventually rise toward the surface where it forms a line of volcanoes.  The majority of volcanoes in the world are formed in this way and the many thousands of volcanoes that rim the Pacific Ocean are the result of the edges of the ‘Pacific Plate’ being melted as it is forced down beneath eastern Asia and North and South America.

So, the plate tectonic theory explains where and, in general terms, how igneous rocks are formed.  Returning to the problem of basalt and granite, it should be noted that the rocks found along midocean ridges are mainly basalt, while along subduction zones, although basalt is also present, the bulk of the volcanic rocks comprise a spectrum from andesite through to rhyolite, which are chemically equivalent to granite and its associated rocks.  Numerous, large bodies of granite also occur in these zones, and represent the intrusive equivalent of the rhyolite.  The variety of the igneous rocks occurring above subduction zones reflects the heterogeneity of the melted plates, which are built from most of the rock types found at the surface of the Earth.

Other important places where igneous rocks are produced are ‘hot spots’, which are areas within the plates where heat escaping from the deep Earth is concentrated.  In such areas melting may be sufficient to produce volcanoes.  Such hot spots form many oceanic islands, for example Hawaii, or volcanoes within the continents, such as Kilimanjaro.

Although the melting of heterogeneous plates explains some of the diversity of igneous rocks found along subduction zones, there is another important process at work.  When basalt magma starts to crystallize in the upper mantle, or the lower part of the crust, the overall composition of the crystals is not the same as the overall composition of the magma. This means that the liquid part will have a composition different from that of the original magma, and the further the crystallization process goes the greater will be the difference in composition between the liquid and the crystals.  If the crystals and the liquid should now be separated by some mechanism, then rocks of two types will result, each with a composition different from the original basalt.  This process, called differentiation, is capable of producing a great range of rock types, one of which is granite.

The recognition and naming of igneous rocks involves an assessment of grain size and the recognition and estimation of the relative amounts of the constituent minerals.  Additional information is obtained from color index, texture, structure, and sometimes from field relationships.

Grain Size

Grain size refers to the size (average) diameter of the mineral grains comprising the rock.  Some rocks have large crystals set in a ground mass of smaller grains (see below); in these rocks only the groundmass minerals are taken into account; the large crystals, no matter how obvious, are ignored.  Excluding the glassy rocks, three broad grain size categories are recognized:  fine-grained, in which the grains are generally below the limit of resolution of the naked eye (less than about 0.004 of an inch); medium-grained, in which the grains are recognizable with the naked eye, but minerals hard to identify (0.004 to 0.08 of an inch); and coarse-grained, in which the mineral grains can be identified by the naked eye (coarser than 0.08 of an inch).  The coarsest rocks, in which the mineral grains have diameters of several inches or more, are referred to as pegmatites.  Glassy (or vitric) rocks consist essentially of glass.  If magma, or lava, is chilled very rapidly the potential minerals are unable to crystallize and grow, and glass results.  The best known such glassy rock is obsidian.

Mineralogy

This is the most important single feature to be considered when naming igneous rocks.  Although magma is a complex silicate melt, most igneous rocks are composed of a few essential minerals belonging to a few mineral groups, namely quartz,  the feldspars and feldspathoids (the light colored or felsic minerals), and the pyroxenes, amphiboles, micas and olivine (the dark colored of mafic minerals). Minor constituents are grouped as accessory minerals. All the groups listed are silicates and except for quartz are, within limits, variable in chemical composition.  Once the grain size has been decided, rock names are assigned according to the kind and proportions of the constituent minerals.  It may be necessary sometimes to know the approximate chemical composition of one particular mineral, but this usually requires at least a microscope and so cannot be determined in the field.  The table above summarizes the mineral content and the interrelationships of most of the igneous rocks described in the following pages.

Color Index

The color index of a rock is the proportion of dark minerals iti contains on the scale 0 to 100.

Texture

Texture refers to the shape, arrangement and distribution of the minerals of the rock.  The following descriptive terms are often used.

In a granular texture, (equigranular) (page 158) all grains are of about the same size and roughly of equant shape.  Poikilitic texture (page 166) refers to large grains of one mineral enclosing smaller grains of other minerals. If pyroxene encloses plagioclase, as in many gabbros and diabases, the texture is called ophitic.  In a porphyritic texture (page 172) some large grains (phenocrysts) (or insets) are set in a finer grained or glassy matrix (groundmass). ‘Porphyritic’ is a common adjectival prefix; for example porphyritic granite, porphyritic basalt.

Flow or fluidal texture (page 170) refers to tabular or elongate crystals aligned by flow in the magma, in much the same way as logs in a river.  In glassy rocks flow is marked by swirling lines, and often by trains of bubbles.

Structure

The structure of rocks refers to the broader features of rock masses rather than those which depend on the interrelationships of the grains.

In a layered or banded structure (page 164) the rock comprises layers of contrasting mineral composition that appear on a surface as bands differing in color or texture.  A rock with a vesicular structure (page 177) contains cavities (vesicles) produced by the expansion and escape of gases. Vesicles, which frequently occur in lavas, may be spherical, elliptical or tubular.  When the vesicles are filled with secondary minerals they are referred to as amygdales, and the structure as amygdaloidal.  Xenoliths, or ‘inclusions’, (page 163) are fragments of other rocks included in igneous rocks.  They may vary greatly in shape and size.  Joints are cracks or fissures in rocks along which there has been no displacement. Lava flows sometimes show columnar jointing (page 174) in which the rock has broken on cooling into parallel hexagonal columns roughly perpendicular to the cooling surface.

Field relationships

Igneous rocks can be divided conveniently into three major groups: volcanic (extrusive) rocks are largely glassy and fine-grained and form lava flows, tuffs and agglomerates; hypabyssal rocks are largely medium-grained and occur as minor intrusions (sills, dykes); and plutonic rocks are largely coarse-grained and form major intrusions.

Igneous intrusions are described according to their shapes and their relationships with the rocks they intrude (the country rocks).

Minor intrusions

Dykes are sheetlike intrusions which are vertical or nearly so and which cut sharply across bedding (see sedimentary rocks).  Dykes range from a few inches to hundreds of feet in width.  Sills are sheetlike intrusions which are essentially horizontal and usually follow bedding or foliation. Like dykes they range from a few inches to hundreds of feet in thickness.  Veins are irregular intrusions which sometimes form a complex network.

Major intrusions

Batholiths are large, cross-cutting intrusions, usually of granitic rocks, having steeply dipping contacts and no apparent floor. Exposed batholiths may cover hundreds of thousands of square miles.  Stocks are smaller than batholiths but otherwise similar.  They occupy areas of a few square miles to tens of square miles.

Volcanic rocks

Volcanic cones of volcanoes form when lava and accompanying pyroclasts (lava fragments) are ejected from a vertical pipeilke vent.  Lava may, however, flow from a fissure from which it may travel for considerable distances forming a lava plateau.  Lava which flows into water chills rapidly and may give rise to distinctive pillow lavas (plate 174).