Each type of volcanic massif (structure) contains lavas, pyroclastic rocks and intrusions, but these differ in proportions and compositions.
The four main types of large volcanic landforms are:
- lava plateaus: extensive relatively flat-lying accumulations of lava sheets which are mainly basaltic in composition (e.g. recent lavas of Iceland).
- shield volcanoes: conical-shaped structures consisting mainly of lava flows (e.g. the volcanoes of the Hawaiian islands).
- strato volcanoes: composite volcanos which contain both lava and pyroclastic deposits and often change their shape as a result of their explosive volcanic activity (e.g. Vesuvius).
- pyroclastic sheet deposits: extensive, relatively flat-lying accumulations of pyroclastic material (e.g. those of the North Island of New Zealand).
These result from numerous eruption of very large volumes of extremely fluid, basaltic lavas, which accumulate as sheet-like flows covering large areas. These Flood Basalt Provinces include the Columbia River basalts of Oregon and Washington states, USA, and the Deccan plateau basalts of India. However, the Siberian basalt province is probably the most important and may have helped in the extinction of up to 95% of all life on Earth at the Permo-Triassic (225 million years ago) boundary.
Large plateau lava fields do not necessarily represent exceptional rates of magma supply. For example, the Columbia River basalts accumulated over 200 000 km3 of basaltic lavas over 10 million years at an average rate of accumulation of 2 km3 per century. Most plateau-type eruptions are fissure eruptions, often associated with volcanic centres. These eruptions tend to be long but narrow and are fed from dykes.
A shield volcano builds up around a volcanic centre so that the lavas thin outwards from the volcano. The Hawaiian Islands are built from shield volcanoes. In a 'Hawaiian-type' eruption, the basaltic lava flows quietly from a crater or fissure over a large distance. Repeated eruptions over a long time develop the characteristic shape of a shield - like cone with gentle outward slopes.
With a slightly more explosive style and a more viscous magma, the shield volcano passes into steeper cones typical of strato volcanos. An in-between stage between types is represented by Mount Etna, which rises from sea level to a height of 3.3 km and is some 40 km across at its base. It has grown during the past few million years by frequent but small eruptions (Mount Etna has erupted 15 times this century).
Strato volcanoes (also known as composite volcanoes) rise up by the accumulation of alternating lavas and pyroclastic deposits. This gives a steep cone with a summit crater. The internal structure of these volcanoes can be very complex, with many generations of dykes, sills, lavas and ash flows and falls. Although andesite is a typical rock in these volcanoes, rhyolites and basalts are also erupted. A well-known strato volcano is Mount St Helens in the state of Washington, USA.
Pyroclastic sheet deposits
In some volcanic provinces, large areas are covered by pyroclastic deposits (e.g. central North Island of New Zealand - the Taupo Volcanic Zone). The volcanic material largely consists of sheets of rhyolitic ignimbrite which cover an area of over 20 000 km2. The Taupo Volcanic Zone is still active with large hot springs and three large andesitic strato volcanoes (Ruapehu, Tongariro and Ngauruhoe) at its southern end. Yellowstone National Park in the USA contains one of the world's largest rhyolite plateaus. The volume of the rhyolite lava erupted over the last two million years is over 6000 km3 (in comparison, Mount Vesuvius contains only 12 km3). As Yellowstone National Park contains many hot springs, it is still active and eruptions may well occur again.
Ondine Evans , Web Researcher/Editor