By: Alan V. Morgan
It is important to remember that volcanoes are not scattered randomly over the surface of the Earth. They are usually, but not always, located near the margins of the huge lithospheric plates that makeup the Earth's surface. The shapes of volcanic landforms are a reflection of the composition of the lavas that are extruded by the volcano.
Lava composition also affects the type of activity exhibited by the volcano. Before we return to the types of volcanoes we should revisit the nature of the magma and how lavas reach the surface. Figure 1a illustrates a hypothetical crustal section extending about 10 to 15 km under a volcano. Magma originates under heat and pressure deep beneath the Earth's surface in magma chambers or batholiths (bottom of Fig. 1a). The rock types seen when the magma cools in a batholith might be granite (if the rock type is acidic), perhaps syenite or diorite (if intermediate) or gabbro (if basic) -(see the igneous rock classification elsewhere in this issue).
Magma rising from the batholith makes its way toward the surface. When solidified the rock type (at 1) would be very coarse grained or phaneritic (because of slow cooling and abundant elements). Fragments as well as much larger pieces of the roof of the batholith can be seen subsiding into the magma chamber. Frequently these completely melt or are substantially altered. In some cases smaller fragments can be carried rapidly to the surface in an almost unchanged form where they can tell us about the nature of the rocks at depth. These "strange rocks" are known as xenoliths.
The magma rising to the surface cools more rapidly as it ascends to regions of lower temperatures and pressures. Fluid offshoots from the main feeder work their way into the country rock forming dykes and sills. Sills are long, flat intrusions that run parallel to the "grain" of the country rock. Dykes cut across the grain of the country rock. A transgressive sill, is where a flat intrusion moves up from one level to a different level in the country rock. The area at 2 illustrates a smaller chamber or reservoir where magma might gather below the near-vertical conduit to the surface. Crystals forming here are smaller and may contain isolated larger crystals that have been transported from below. This two-sized igneous rock texture is "hypabyssal". A small offshoot where the magma has forced the bedrock upward into a domed structure is illustrated as a laccolith. The grain-size is medium in rocks that solidify at this level.
Magma continues upward, penetrating cracks and expanding joints in the country rock to a point where it eventually escapes at the surface. This is the position of the volcano. At the surface under relatively low temperature and pressure conditions, the magma degasses and chills rapidly, producing lava with very small (fine-grained or aphanitic) crystals (at 3). The shape of the volcano is reflected by the chemical composition of the lava and this is seen in Figure 1b.
Three volcanoes are illustrated. At the top is Skalbreid, north of Thingvellir in south-central Iceland. This is a typical example of a "Shield" volcano. (So-called, because it resembles an old Viking shield resting on its base). Here the basalt lavas are very fluid and run rapidly across the landscape. These are volcanoes with "Montezuma's revenge", and can be easily compared to humans with "the runs"! Because the lava flows readily, the volcanoes have low slope angles and are not usually explosive. However, they do have spectacular lava fountains and produce prodigious quantities of lava. These are well represented by volcanoes in oceanic areas of the planet, such as Hawai'i and Iceland, but they also exist in continental areas elsewhere.
The middle image is of Ngauruhoe, a large (2291m-high) volcano in Tongariro National Park, North Island, New Zealand. Intermediate volcanoes are commonly seen in areas of plate margin subduction, where oceanic crust descends beneath the continents. The volcanoes of the west side of North America, Central and South America and the Caribbean are often of this type. These volcanoes are steep-sided and, because of their explosive nature, can be very catastrophic. They are represented by some of the Caribbean volcanoes (and many others elsewhere). I often describe these as "constipated volcanoes" and their gaseous and explosive nature can be easily related to human alimentary systems! The lava type is commonly rhyo-dacite (a more-intermediate composition than rhyolite), trachyte or andesite (named from volcanic rocks along the Andes). Ngauruhoe frequently extrudes andesite, and is one of the most active volcanoes in the world.
At the bottom is Beerenberg volcano, on Nord Jan, Jan Mayen Island, North Atlantic. Beerenberg is a classical stratovolcano - a huge volcanic pile that is made up of alternating layers of lava and ash. The volcano is 2277m high, and extends another three kilometres below sea level. This cone also shows that volcanoes can vary in composition through time. Most of the modern eruptions have been basaltic, but Beerenberg also has had a long history of trachytic eruptions. (Also see the Jan Mayen article in this issue).
In summary, igneous rocks when formed at depth have coarse textures (large crystals) but when they are extruded as lavas, the crystal sizes are quite small. Volcanoes, whose position are usually determined by the boundaries of plates, or stresses within plates especially in the vicinity of "hot spots", vary in shape and activity by the chemical composition of the lavas that are extruded.