VOLCANIC ACTIVITIES

EXTRUSIVE VOLCANIC FEATURES

Extrusive volcanic features.

Craters

 Crater is a bowl shaped depression at the top of a volcano caused by past volcanic eruptions. Craters can be thought of as the volcano “blowing its lid” where volcanic materials, such as ash, lava and rock fragments, are released. A volcanic crater is relatively small, usually spanning about a half a mile in diameter or less, and can fill with water to form a crater lake.

Caldera If a volcanic eruption causes the magma chamber to empty, the volcano can implode, forming a larger depression known as a caldera. So a caldera can be defined as a large volcanic crater formed by the collapse of the central part of the volcano.

The term 'caldera' comes from the Latin language and means 'cooking pot.' While a caldera would make an awfully big pot of soup, you can see how the large bowl shape of a caldera, along with the smoky look of the ash cloud rising above it after an eruption, could prompt the name.

Cinder Cone Volcanoes

These are the simplest type of volcano which occusr when particles and blobs of lava are ejected from a volcanic vent. The lava is blown violently into the air and the pieces rain down around the vent.

Composite Volcanoes

Composite or strato volcanoes these volcanoes have a conduit system inside them that channels magma from deep within the Earth to the surface. They can have clusters of vents, with lava breaking through walls, or issuing from fissures on the sides of the mountain. With all this material coming out, they can grow thousands of meters tall. As we’ve seen with the famous Mount Saint Helens, composite volcanoes can explode violently. : Mount Rainier, Mount Fuji, and Mount Cotopaxi, for example.

Shield Volcanoes:

These are large, broad volcanoes that look like shields from above. The lava that pours out of shield volcanoes is thin, so it can travel for great distances down the shallow slopes of the volcano. These volcanoes build up slowly over time with hundreds of eruptions creating many layers. Perhaps the best known shield volcanoes that make up the Hawaiian Islands, especially Mauna Loa and Mauna Kea.

Lava Domes

Volcanic or lava domes are created by small masses of lava which are too viscous (thick) to flow very far. The magma from volcanic domes just pile up over and around the vent. The dome grows by expansion of the lava within and the mountain forms from material spilling off the sides of the growing dome. Lava domes can explode violently releasing a huge amount of hot rock and ash.

TYPES OF VOLCANOES

Types of volcanoes

There are three main types of volcanoes

1. Active volcanoes which are currently or have recently erupted.
2. Dormant volcanoes which have not erupted recently and is expected to erupt at any time.
3.  Extinct volcanoes which has stopped to erupt and will never erupt again.

On earth, volcanism occurs in several distinct geologic settings.

The first place of volcanic activities are  those areas associated with the boundaries of the enormous rigid plates that make up the lithosphere (the crust and upper mantle). The majority of active terrestrial volcanoes (roughly 80 percent) and related phenomena occur where two lithospheric plates converge and one overrides the other, forcing it down into the mantle to be reabsorbed.

A second major site of active volcanism is along the axis of the oceanic ridge system, where the plates move apart on both sides of the ridge and magma wells up from the mantle, creating new ocean floor along the trailing edges of both plates. All of this volcanic activity occurs underwater. Iceland is the best example. The magma that are erupted along the oceanic ridges are basaltic in composition.

INTRUSIVE VOLCANIC FEATURES

What are the feature resulted from volcanic activities?

Volcanic activities resulted into different intrusive and extrusive feature.

Intrusive volcanic features

Dyke

This is sheet of rock that is formed in the fracture in the pre -existing rock. Dyke form when a mass of magma cuts across the bedding planes and forms a wall like structure. Example of dyke is found in Jos plateau in Nigeria.

Sill

This is formed when a sheet of magma lies along the bedding planes of the earth's surface. A sill is therefore a horizontal sheet of rock that solidifies from magma that has been injected concordantly between bedding planes; they may be of any thickness and extend for many square of kilometers. Example of sill is a great Whill sill in great Britain.

Batholith

Is large body of intrusive igneous rock believed to have been crystallized at the considerable depth below the earth's surface. They are the largest type of pluton. Example of batholith is Idaho batholith which covers surface area of over 15500 kilometer square.

Lopoliths

These are saucer shaped features formed beneath the earth's surface through magma intrusion. Lopolith forms great shallow basin when magma solidifies within the crust a good example is bush veld igneous complex of south Africa which is composed of both granite and basic rock.

Laccolith

This is a sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of magma is powerful enough that the overlying strata are forced upward giving laccolith a dome or mushroom like form with generally planar base. Some laccolith can be found in Madagascar and Utah USA where they has been exposed by erosion.

Phacolith

This is lens shaped pluton that occupy either the crest of an anticline or trough of syncline. A good example of phacolith can be seen seen in Corndon hills in Shropshire United Kingdom.

VULCANISM

Vulcanism           

Vulcanism or volcanism, refers to the process and phenomena associated with the discharge of molten materials, pyroclastic fragments, or hot water and steam are either intruded (injected) into the earth’s crust or extruded (ejected) onto the surface.

There is difference between vulcanism and volcanism.

Vulcanism involves both intrusive and extrusive igneous activities where volcanism involves only extrusive igneous activities in which materials forced onto the surface.

The molten materials known as magma when it inside the crust also called as lava when it onto the surface of earth.

Magma is the hot molten rock that builds in pressure and explodes from the volcano as lava.

                                           
                                                     Vulcanism diagram