The erupted volcano comes in different forms. These forms are given as under:
(1). Ash Cone:
These are also called Tuff Cones. When heated rapidly by lava, water flashes to steam and expands violently, fragmenting huge amounts of lava into plumes of very fine grains of ash. This ash falls around the volcanic vent, creating an ash cone. Over time, the ash weathers into a rock known as tuff.
(2). Silicic Lava Dome:
When there is high level of silica in the magma, it causes very strong viscosity in the it. This viscosity is either be achieved by the level of quantity of silica or by degassing of fluid lava. When the silica rich magma is ejected, it starts depositing in the form of dome on coooling. The viscous and silica rich magma in the form of a dome is known as Silica Lava Dome.
(3). Basaltic Lava Shield:
Basaltic lava flows erupt primarily from shield volcanoes, fissure systems, scoria cones, and spatter cones. These fluid lava flows can be subdivided into two end-member structural types, based primarily on the nature of lava flow surfaces:
⦁ Pahoehoe Lava — Surfaces are smooth, billowy, or ropy.
⦁ A’a lava — Surfaces are fragmented, rough, and spiny, with a “cindery” appearance
(4). Secondary Cone within old Crater-ring:
Secondary cone comes into form as a result of re-eruption of volcano with in a pre-existing crater. This is usually smaller than the ring of the crater, and forms in the center of the ring.
(5). Composite Volcano:
They are also known as composite cones or Strato-volcanoes. They comes in the form of a conical volcano built up by many layers (strata) of hardened lava and tephra.The composite volcanoes are in the shape of large mountains. In cross section, these look layered. These layers are alternating layers of lava and cinder.
The empty chamber of a volcano formed after the completion of eruption and thus the interior assumes a hollow shape. Caldera is a steep-walled cirucular volcanic basin formed by the collapse of a volcano whose magma chamber empties out. It can also result from a powerful eruption that flows off the peak and crater of a volcano. According to H.J de Blij, “A volcano’s lava and pyroclastics come from a subterranean magma chamber, a reservoir of active molten rock material that forces its way upward through the volcanic vent. When the magma reservoir ceases to support the volcano, the chamber may empty out and the interior of the mountain may become literally hollow. Left unsupported by the magma the walls of the volcano may collapse creating a caldera. Such an event can occur quite suddenly perhaps when the weakened structure of the volcano is shaken by an earthquake. A caldera can also result from a violent eruption which destroys the peak and crater of volcano.”
Some of the world’s largest known calderas are:
⦁ Krakatoa Caldera in Indonesia
⦁ Mount Aso Caldera in Japan
⦁ Morogoro Caldera in Tanzania
(7). Volcanic Plug or Neck:
A volcanic plug, which is also known as volcanic neck or lava neck, is a volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of pressure if rising volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption.
(8). Crater Lake:
Mouth of a volcano having a width of 1.5 kilometer or less is known as the crater. It is mostly found at the top of the volcanic cone. A crater becomes a Crater-lake, when a volcano for a long period of times remains dormant or extinct. The rain water starts being collecting in the crater, thus defining it as a Crater Lake. In simple works, when a crater is filled with water, it is known as the Crater Lake.