Volcanoes are one of the planet's major sources of rocks and minerals. Here we describe the kinds of volcanoes which bring molten rock to the surface. The following pages will describe the types of rocks which these volcanoes produce.

Volcanoes differ from other landforms because they are built up from liquid lava ejected to the surface and then solidified. Also, unlike land built up imperceptibly by continental drift, the changes produced by volcanoes can be sudden and dramatic. The form which a volcano takes largely depend on the type of lava and the composition of the rock it is being forced through. The best known forms of volcano are described here.

Lava Volcanoes

Lava Shields

Shield volcanoes have very fluid lava composed almost entirely of basalt. The lava which arrives at the surface is hot and little changed since the time it was generated. Shield volcanoes are the largest volcanoes on Earth. The basaltic lava can pile up producing huge "shields" with gentle slopes and convex outlines. Explosive eruptions are rare, since 90% of the volcano is lava rather than pyroclastic material (for eaxmple gasses at explosive pressures). The Hawaiian shield volcanoes (Kilauea and Mauna Loa) are the most notable examples of shield volcanoes.

Drawing representation of a Lava Shield

Lava Dome

Unlike shield volcanoes, the lava in dome volcanoes is very thick and viscous often because it is rich in silica and/or andesite. As a result it does not travel far. During eruptions, thick lava piles up on top of the crater, cools down and hardens. Because a dome grows largely by expansion from within, pressure from below shatters the outer hard skin of cooled lava spilling loose fragments down the sides of the volcano as the lava piles over and around its vent. Volcanic domes commonly occur within the craters or on the flanks of large composite volcanoes. A very typical example of a lava dome is the nearly circular Novarupta Dome that formed during the 1912 eruption of Katmai Volcano in Alaska. Novarupta Dome is 800 feet across and 200 feet high.

Lava Cones

Lava cones are among the simplest volcano formations. They are built by fragments - ejecta, thrown up from a volcanic vent - piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and size of the fragments ejected during the eruption. Typical types are spatter cone (molten lava ejected from a vent), cinder cone (loose volcanic fragments called cinders), ash cone (magma combined with water produces explosive steam).

Lava Mounds

Some basaltic volcanoes have no sign of a crater, but are gently sloping mounds, such as Mt Cotterill, Victoria. These extinct volcanoes may owe their shape partly to erosion, although they probably never had very pronounced craters but had lava welling right to the brim before solidification. Such volcanoes, distinguished from cones by their lack of crater, are termed lava mounds by analogy with scoria mounds.

Fissure eruptions

Fissure eruptions are very much a feature of Hawaiian volcanic eruptions. In contrast to the point-source, centralized eruptions that typify most volcanoes, fissure eruptions are initiated along a linear fracture of the earth's crust. Regional fracture systems can appear where the Earth's crust is broken and pulled apart by tensional forces. If these regions are underlain by reservoirs of basaltic magma, this low-viscosity melt will utilize the fractures and ascend through the crust to generate a fissure eruption.

Pyroclastic Volcanoes

Scoria Cone

Scoria cones, also known as cinder cones, are the most common type of volcano. Scoria cones are generated by so called Strombolian eruptions. Strombolian eruptions are discrete explosions (called Strombolian explosions, Strombolian bursts etc.) of relatively fluid lava from a single vent. It is thought that they originate when large gas bubbles rising within the conduit burst at the partly solid surface of the magma column inside the vent. The term comes from eruptions originally observed in the Stromboli volcano in south Italy which has been active for at least 2000 years.

Stromboli erupts every 10-20 minutes with a spectacular display which last for about 10-20 seconds. The locals nickname Stromboli the "Lighthouse of the Tyrrhenian Sea" since the frequent explossions can be seen from far away, especially at night.

Scoria cones are among the smallest of volcanes, generally less than 300 meters in hight. They occur as discrete volcanoes on a basaltic lava field or as nested volcanoes (see below). Many scoria cones erupt only once, unlike Shields or Stratovolcanoes.

Scoria Mound

Some scoria volcanoes have no apparent crater and may be termed scoria mounds to distinguish them from normal scoria cones. The Anakies and Victoria volcanoes are examples of such scoria mounds.

Nested Scoria Cones

These are often generated by eruptions through the sides of shield volcanoes and stratovolcanoes. They are also termed parasitic cones.

Drawing representation of a Scoria Cone

Littoral cones

These have a hydrovolcanic origin, although they are not true hydrovolcanic vents. (The word 'littoral' means beach, which gives a clue to there these volcanoes are often found.) They are composed of basalt tephra that accumulates as ejected debris from the explosive interaction of moving lava and seawater. The irregular cracks in cindery blocks of cooling lava allow water to penetrate into the interior of the hot flow where it then flashes to steam explosively.

Basalt exiting from lava tubes can also generate littoral cones from episodic explosions due to disruption of the lava stream by incoming waves or swells.

Maars

Maars are shallow, flat-floored craters that scientists believe have formed above diatremes as a result of the violent expansion of magmatic gas or steam (diatremes are volcanic pipes filled with broken rock, such as the one which formed the Kimberly diamond mine in South Africa). All maars probably have a diatreme not too far below them. Maars range in size from 200 to 6,500 feet across and from 30 to 650 feet deep.

Most maars are commonly filled with water which forms a natural lake and have low rims composed of a mixture of loose fragments of volcanic rocks and rocks torn from the walls of the diatreme. An excellent example of a maar is Zuni Salt Lake in New Mexico, a shallow saline lake that occupies a flat-floored crater about 6,500 feet across and 400 feet deep.

Composite Volcanoes

Stratovolcanoes

Stratovolcanoes, also known as composite cones, are the most picturesque and the most deadly of volcanoes. Their lower slopes are gentle, but they rise steeply near the summit to produce an overall morphology that is concave in an upward direction. The summit area typically contains a surprisingly small summit crater. Typically, stratovolcanoes have a layered or stratified appearance with alternating lava flows, airfall tephra, pyroclastic flows, volcanic mudflows (lahars), and/or debris flows. The composition of stratovolcanoes may vary from basalt to rhyolite in a single volcano; however, the overall average composition of stratovolcanoes is andesitic.

Most stratovolcanic eruptions are highly explosive, also known as Plinian eruptions (after the Roman natural scientist Pliny, who was killed in one.)These eruptions are often associated with deadly pyroclastic flows composed of hot volcanic fragments and toxic gases that advance down slopes at hurricane-force speeds. Stratovolcanoes are polygenetic, which means that they are characterised by multiple eruptions.

There are many examples of stratovolcanoes including: Mt. St. Helens, Mt. Fuji in Japan, Mt. Mayon in the Philippines, and Vesuvius, the volcano which destroyed Pompeii. Stratovolcanoes are also particularily abundant along the rim of the Pacific Ocean, a region known as the Ring of Fire. Most active stratovolcanoes worldwide appear to be 100,000 years old, although some, like Mt. Rainier, may be more than 1 million years old.

Drawing representation of a Stratovolcano

Supervolcanoes

This is not a scientific term, but is sometimes used to describe volcanoes which throw so much material into the atmosphere that all life is wiped out for thousands of miles around, and affects weather and living conditions across the planet. Supervolcanoes do not happen very often (about once or twice in a million years). In the event of a supervolcano eruption, rock collecting will be the last thing on most people's minds!