Volcanicity is a process whereby magma rises from within the earth’s interior into the earth’s crust or is injected onto the earth’s surface. When the magma rises within the earth’s crust it is called Magma. When it reaches the earth’s surface it is called lava.

TYPES OF VOLCANOES

There are three types of volcanoes. These are:

6. Active volcanoes
6. Dormant volcanoes
6. Extinct volcanoes

• Active Volcanoes

These are volcanoes that are younger and show signs of recent eruptions. An example is mount Cameroon.

• Dormant Volcanoes

Dormant volcanoes are volcanoes that are less active and erupt less frequently with little violence. An example is Mount Katmai in USA.

• Extinct Volcanoes

These are volcanoes that have not erupted for a long time and show no signs of further eruptions. An example is Mount Kenya.

Main Processes of Volcanicity

There are two main volcanic processes. These are:

• Intrusive processes
• Extrusive processes

1. Intrusive Processes

These are processes in which magma cools and solidifies within the earth’s interior.

2. Extrusive Processes

These are processes in which lava cools and solidifies on the earth’s surface.

There are two types of lava. These are:

• Acid lava
• Basic lava

Acid lava is viscous like toothpaste and it flows slowly for a short distance and solidifies.

Basic lava is fluid (watery) and flows faster over a long distance before solidifying.

Structure of a Volcano

A volcano is a hill or mountain that is formed during volcanic activity. It has three main parts:

• A vent or crack through which lava flows.
• The volcanic core which is the hill or mountain.
• The crater which is a circular depression found on a volcanic core.

FEATURES OF VOLCANICITY

1. INTRUSIVE FEATURES

Intrusive features are features formed when magma cools and solidifies internally. These features are:

• Dykes
• Sills
• Laccoliths
• Lopoliths
• Phacoliths
• Batholiths
• Volcanic plug or spine

2. EXTRUSIVE FEATURES

Extrusive features are those formed as a result of the rising and cooling of magma onto the earth’s surface. These features include:

• Lava cones
• Ash and cinder cones
• Composite cones
• Craters and calderas
• Volcanic mountains

1. Lava cones

There are two types of lava cones:

5. Basic lava cones (Shield volcanoes)
5. Acid lava cones (Cumulo volcanoes)

1. Basic lava cones

These are expensive broad-based lava cones with gentle slopes. They are formed when very fluid basic lava constantly erupts gently. The lava flows over a long distance before it solidifies. They are also called lava shield or shield volcanoes. An example is Maura loa in Hawaii.

2. Acid lava cones (Cumulo Lava Cones)

These are narrow based lava cones with steep sides. They have no craters at the sumi. They are formed when highly viscous lava flows over a short distance and solidifies. An example is the Central Highland of France, Itasy Mountain in Madagascar.

Differences between acid lava cones and basic lava cones

• Acid lava cones are formed from highly viscous lava while basic lava cones are formed from very fluid basic lava.
• Acid lava cones flows over a short distance while basic lava cones flows over a long distance.
• Acid lava cones are narrow based lava cones while basic lava cones are broad based lava cones.
• Acid lava cones have steep slopes while basic lava cones have gentle slope.
• Acid lava cones lack a crater while basic lava cones have a crater.

2. Ash and cinder cones

These are small steep sided cones that are made up of mostly rock fragments. These rocks fragments are called ash and cinders. They are formed when lava and rocks are shattered during violent eruptions. The lava cools and solidifies into smaller fragments called ash and cinder. These ash and cinder falls back and accumulate around the vent to form the cone. An example is Mbapit and Nkogham near Foumbot in Cameroon, Likaiyu cone in Kenya.

3. Composite cones

Composite are large steep-sided mountains made up of layers of ash and lava. They are also called Strato-volcano and they are the most common volcanoes.

They are formed by several eruptions which build up successful layers of ash and lava. As cones escape through the sides of the cones, they form parasitic cones. An example is Mount Cameroon, Mount Oku, Mount Bamboutos and Mount Kilimanjaro in Tanzania.

Differences between Ash and Cinders Cones and Acid Lava Cones

Ash and cinder cones are small steep sided cones, made up of mostly rocks fragments while Composite cones are large deep sided mountains made up of layers of ash and lava

4. Craters and calderas

1. A Crater: is a small circular depression with steep slopes found on a volcanic cone. They are formed when violent eruptions off the top of a volcanic cone. This creates a depression called a crater. They may become filled with water to form a crater lake. An example is Lake Barombi, lake Awing and Lake Oku.

2. A caldera: Is a very large basic basin shaped depression with steep walls. It can be formed in two ways.

• When a violent eruption blows off the top of a mountain. Or
• When the top of the mountain collapses inward into the vent. An example is Etoga and Elengum Caldems on mount Manenguba in the Western highlands of Cameroon, Yega caldera on mount Tibesti in the Sahara.