Successive eruptions from a single vent build the mountainous accumulation of
Material known as volcanoes. Three types of volcanoes are generally recognized.
- a) Shield Volcanoes
- b) cinder cones and
- c) composite cones or strata volcanoes.
Each has a characteristic style of eruption and consequently characteristic form.
Shield Volcano: When highly fluidal lava is extruded, the volcano takes the shape of a broad Slightly domed structure called a shield volcano. Shield volcanoes are built primarily of Basaltic lava flows and contain only a small percentage of pyroclastic material. They have A slope of a few degrees at their flanks and not more than 10 degrees near summit. Typical Example of a shield volcano is the Mouna Loa in the Hawaiian Islands which is one of the Largest volcanoes in the world.
Cinder Cones: As the name suggests cinders are built up of ejected rock fragments. Because Unconsolidated pyroclastic material has of a high angle of repose (between 30 and 40 Degrees) these volcanoes have the steepest slopes. Cinder cones are built of chiefly of cinder And ash and are generally circular in ground plan and have the form of truncated fone with A crater at its top. They are quite numerous in the Hawaiian Islands.
Composite cones or Strato: Volcanoes These volcanoes consist of alternating layers of lava Flows and pyroclastic materials. They have structures that are intermediate in form between The steep cinder cones and the flat domes of shield volcanoes. This composite construction Shows that a volcano thus built was at times violently explosive in activity and at other times Quite, swelling up streams of lava. The deposits of successive eruptions form layers, some Of coarser and some of finer materials. Thus a crude layering or stratification is produced, The layers sloping down and out from the conduit of the volcano. The layering is further Accumulated by the sheets of lava which are inter-calated between the beds of fragmental Ejects. Mt mayon in the Philippines is the most active volcano of this type. Other examples Include that sacred mountain of Japan, the magnificent Fujiyama towering 2700 meters Above the surrounding country.
Calderas: some volcanoes have usually large craters, with huge circular pit resulting from An explosive eruption or collapse of a former volcanic cone, they are called calderas. A Name given after La Caldera of the Canary Islands. Most calderas are of great size and Many of them are very wide as, compared with their depth. Two splendid examples of Depressions of this type are the calderas at the summits of Kilonea and Mouna Loa in Hawaii. Both these are formed by the collapses into the partially emptied magma chamber Below. Such collapse is the result of the withdrawal of the lava column Form below the Shield-shaped cone either by eruptions on to the surface or by movements of magma within The crust. The impression is ample proof of the downward movement of the summits of the Cones. Further the steep inner walls of many calderas are marked by concentric and Interweaving fissures marking the slips and faults caused by the down-dragging cone. Here the magma is injected into the concentric fissures and dykes are formed.
Fissure Eruptions: Even though volcanic eruptions from a central vent are the most Familiar, by far the largest amounts of volcanic materials are extruded from cracks in the Earth’s crust called, fissures. Rather than building cone, those long narrow cracks distribute Volcanic materials over a wide area. An extensive area in western India was formed in this Manner, and the numerous fissures extruded very fluidal basaltic lava. It is popularly called Deccan traps. Another famous example of such fissure eruption is the Columbia River Plateau in the North Western U. S. A. Because of the high fluidity of the basaltic lava large Areas were indurated by such lava flows and the terms flood basalts appropriately describes The water like nature of these basaltic lava flows.
Tephra rocks and tephra clumps have different names depending on size.
|Tephra (single)||Pyroclastic rock