ADVERTISEMENTS:
In this article we will discuss about the elevated forms and depressed forms of extrusive topography that are produced by the entire process of volcanicity.
1. Elevated Forms:
(i) Cinder or Ash Cones:
Cinder or ash cones are usually of low height and are formed of volcanic dusts and ashes and pyroclastic matter (fragmental materials). The formation of cinder cones is initiated due to accumulation of finer particles around the volcanic vent in the form of tiny mound, say ‘ant mount’ which varies in height from a few centimetres to a few metres in the beginning. The size of the cone gradually increases due to continuous accumulation of volcanic materials minus lavas.
ADVERTISEMENTS:
Sometimes, the rate of growth of the cone is so high that it gains height of 100 m or more within a week. The slopes of cinder cones range between 30° and 45°. Larger particles are arranged near the craters and rest at the angle between 40° and 45° and the finer particles are deposited at the outer margins of the cones. Since such cones are formed of unconsolidated larger particles and are seldom compacted by lavas and hence they are permeable to water.
Such cones are on an average less susceptible to erosion and hence they maintain their original forms for hundreds of years provided that they are not destroyed by ensuing violent explosion. The volcanic cones of Mt. Jorullo of Mexico, Mt. Izalco of San Salvador, Mt. Camiguin of Luzon Island of Philippines etc. are typical examples of cinder cones [fig. 9.7(1)].
(ii) Composite Cones:
Composite cones are the highest of all volcanic cones. These are formed due to accumulation of different layers of various volcanic materials and hence these are also called as strato-cones [fig 9.7(2)]. In fact, these cones are formed due to deposition of alternate layers of lava and fragmental (phyroclastic) materials wherein lava acts as cementing material for the compaction of fragmental materials. The cone becomes comparatively resistant to erosion if it is coated by thick layer of lava.
ADVERTISEMENTS:
On the other hand, if the outer layer is composed of fragmental materials, the composite cone is subjected to severe erosion. Most of the highest symmetrical and extensive volcanic cones of the world come under this category e.g., Mt. Shasta, Mt. Ranier, Mt. Hood (USA), Mt. Mayon of Philippines, Mt. Fuziyama of Japan, Mt. Cotopaxi of Ecuador etc.
(iii) Parasite cones:
Several branches of pipes come out from the main central pipe of the volcano when the volcanic cones are enormously enlarged. Lavas and other volcanic materials come out from these minor pipes and these materials are deposited around newly formed vents located on the outer surface of the main cone and thus several smaller cones are formed on the major cone [fig. 9.7(3)].
ADVERTISEMENTS:
These cones are called parasite cones because the supply of lava for these cones comes from the main pipe. These cones are also known as adventive or lateral cones. Shastina cone is a parasite cone of Mt. Shasta of the USA.
(iv) Basic Lava Cone:
Basic lava cone is formed of light and less viscous lava with less quantity of silica. In fact, when the lava coming out of fissure flows is deficient in silica and is characterized by high degree of fluidity, it cools and solidifies after spreading over larger area.
Thus, a long cone with significantly low height is formed. Such cones are also called as shield cones because of their shapes resembling a shield. Since these cones are composed of basaltic lavas, they are also called as basic lava cones. These are also known as Hawana type of cones [fig. 9.7(4)].
(v) Acid Lava Cones:
Acid lava cones are formed where the lavas coming out of volcanic eruptions are highly viscous and rich in silica content. In fact, such viscous lavas have very low mobility and hence they are immediately cooled and solidified after their appearance on the earth’s surface. Thus, high cones of steep slopes are formed. Such cones are very often known as Strombolian type of cones [fig. 9.7(5)].
(vi) Lava Domes:
Lava domes are in fact similar to shield cones in one way or the other. Lava domes differ from shield cones as regards their size. Actually, lava domes are larger and more extensive in size than the shield cones. These are formed due to accumulation of solidified lavas around the volcanic vents.
Based on the mode of origin and the place of formation lava domes are divided into 3 categories e.g.:
ADVERTISEMENTS:
(A) Plug dome (formed of lavas due to filling of volcanic vents),
(B) Endogenous dome (formed of silica rich viscous lavas) and
(C) Exogenous dome (formed of silica-deficient lava with high degree of fluidity.)
(vii) Lava Plugs:
Lava plugs are formed due to plugging of volcanic pipes and vents when volcanoes become extinct. These vertical columns of solidified lavas appear on the earth’s surface when the volcanic cones are eroded away. The lava-filled volcanic piple is called as volcanic neck [fig. 9.7(6)].
Generally, volcanic necks are cylindrical shaped and measure 50 to 60 m in height (above the ground surface) and 300 to 600 m in diameter. Some times diatreme term is used to indicate volcanic neck or pipe filled with breccia. ‘Shiprock which towers 515 metres (1700 feet) over the surrounding flat-lying sedimentary rocks of New Mexico, is an excellent example of a diatreme exposed by the erosion of its enclosing sedimentary rocks’ (fig. 9.8).
2. Depressed Forms:
(i) Craters:
The depression formed at the mouth of a volcanic vent is called a crater or a volcanic mouth, which is usually funnel shaped. The slope of the crater depends upon the volcanic cone in which crater is formed. Normally, a crater formed in a cinder cone slopes at the angle between 25° and 30°. The size of a crater increases with increase and expansion of its cone.
A crater may be differentiated from a caldera on the basis of size and mode of formation. An average crater measures 300 m in diameter and 300 m in depth but there are wide range of variations in craters from the standpoint of their size e.g. craters range from small craterlets having a diameter of a few hundred metres to large craters having the diameter of a few kilometres.
The crater of extinct Aniakchak volcano of Alaska has a diameter of 9.6 km (6 miles) and the side walls are 364 m to 912 m (1200 to 3000 feet) high. If the Crater Lake of the state of Oregon (USA) is accepted as a crater, it becomes one of the most extensive craters of the world, though many scientists consider it as an example of a caldera. When a crater is filled with water, it becomes a crater lake.
When the crater of volcano becomes very extensive and if there are few eruptions of very small intensity after long time, several smaller cones are formed within the extensive older crater and thus several small-sized craters are formed at the mouth of each volcanic vents inside the extensive crater. Such craters or craterlets are called ‘nested craters’ or ‘craters within the crater’ or ‘grouped craters’.
Such craters are formed only when the next eruption is smaller in intensity than the previous one. The craters formed at the mouth of volcanic vent of parasite cones developed over an extensive volcanic cone is called adventive crater. Three smaller craters are found within the extensive crater of Mt. Taal of Philippines. Similarly, three and two craters are found within the craters of Visuvius and Etna volcanoes.
(ii) Calderas:
Generally, enlarged form of a crater is called caldera. There are two parallel concepts for the origin of calderas. According to the first group of scientists a caldera is an enlarged form of a crater and it is surrounded by steep walls from all sides.
The caldera is formed due to subsidence of a crater. This concept has been propounded by the U.S. Geological Survey. It is believed according to this concept that Aso crater of Japan and Crater Lake of the USA are the result of subsidence. The second group of scientists has opined that the calderas are formed due to violent and explosive eruptions of the volcanoes.
Daly, the leading advocate of ‘eruption hypothesis’ of the origin of calderas, believes that the topographic features formed by subsidence are ‘volcanic sinks. According to the advocates of this hypothesis if calderas are formed due to subsidence there should not be any deposit of pyroclastic materials and volcanic ashes related to a particular volcanic cone near the caldera but evidences have revealed that the remains of volcanic materials related to a particular cone are found not only near the concerned caldera but are also found several kilometres away from the caldera.
For example, volcanic materials have been found at the distance of 128 km from the caldera of Crater Lake (USA). The significant calderas of the world are (figures in the brackets denote dimension in kilometres) Lake Toba of Sumatra (50 km x 50 km) in Sumatra, Aira (25 km x 24 km) in Japan, Lake Kutchaio (26 km x 20 km) in Japan, Tarso Yega (20 km x 17 km) in Sahara (Africa), Aso San (23 km x 14 km) in Japan, Alban (11 km x 10 km) in Italy, Crater Lake (10 km x 10 km in USA, Krakatoa (7 km x 6 km) in Indonesia, Kilauea (5 km x 3 km) in Hawaii etc. Smaller calderas housed in a big caldera are called nested calderas or grouped calderas (fig. 9.9).
