vulcanicity

Volcanoes can be classified based on two main criteria: mode of eruption and period of eruption.

Based on the mode of eruption

1. Central eruption

   This type of eruption is usually violent and explosive. It involves the eruption of lava, volcanic ash, and fragmental materials through a pipe, driven by the impact of violent gases.

   • Examples: Mount Kilimanjaro and Mount Oldonyo Lengai (both located in Tanzania).

2. Fissure eruption

   Fissure eruptions are quieter in nature, where lava flows calmly through fissures, fractures, or faults due to low pressure.

   • Example: Laki fissure eruption of 1783 in Iceland.

Based on the period of eruption

1. Active volcano

   An active volcano erupts frequently, and it is considered active only if it has erupted within the last 500 years.

   • Example: Mount Nyiragongo in the Eastern Republic of Congo.

2. Dormant volcano

   A dormant volcano has infrequent eruptions and is often referred to as a "sleeping volcano". These volcanoes are still capable of erupting but have not done so in a long time.

   • Examples: Mount Kilimanjaro, Mount Meru, and Mount Kenya.

3. Extinct volcano

   An extinct volcano is one that has ceased erupting for a very long time and is not expected to erupt again. Over time, weathering and erosion can significantly alter or remove the volcano.

   • Example: Mount Rungwe.

Volcanic materials ejected during an eruption include:

1. Lava

   Lava is molten rock that has reached the Earth's surface, often accompanied by gases.

2. Vapour, fragmental materials, and ashes

   These materials are also ejected during volcanic eruptions.

3. Magma

   The molten rock beneath the Earth's surface is called magma, and it is known as lava once it reaches the Earth's surface.

Chemical nature of volcanic materials

1. Acidic materials

   • These are highly viscous (sticky) and less mobile due to a high silica content.
   • They tend to be more destructive because they move slowly.

2. Basic materials

   • Basic materials are fluid, less viscous, and more mobile.
   • They have a lower melting point and less silica, allowing them to flow long distances before solidifying.

Classification of lava and magma

Lava and magma can be classified based on the color of the minerals they contain:

1. Felsic lava/magma

   Contains lighter-colored minerals and is more viscous.

2. Mafic lava/magma

   Contains darker-colored minerals and is less viscous.

The chemical composition of volcanic rocks:

Chemical Nature	Silica (%)	Degree of Mobility	Example of Rock Type
Acid	More than 66	Extremely viscous and less mobile. Solidifies rapidly at high temperatures over 850°C	Rhyolite
Intermediate	52-66	Fairly viscous. Unable to flow far before solidification	Trachyte
Basic	45-52	Very fluid and mobile. Able to flow long distances before solidifying	Basalt

Vapour and gases

1. Vapour and steam constitute about 60% to 90% of the total gases emitted during an eruption.

   • The vapour includes:
     • Phreatic vapour
     • Magmatic vapour

2. Gases emitted from a volcanic eruption include gaseous compounds of:

   • Sulphur
   • Hydrogen
   • Nitrogen

Most of these gases are dispersed into the atmosphere.

Role of gases in eruptions

1. The interaction between gases contained in molten magma generates significant heat, which maintains high temperatures within lava pods in the crater.
2. Gases also act as lubricants, and the loss of gases from lava increases its viscosity.

Pyroclastic materials

Pyroclastic materials, also known as solid materials, are ejected during explosive eruptions. These materials can include:

1. Fragments of rock from the blown crust
2. Angular fragments of rock
3. Lava that has cooled in the volcanic pipe
4. Fine materials such as:
   • Pumice
   • Cinder
   • Lapilli
   • Ash
   • Dust

Liquid materials

Liquid materials are one of the most important products of a volcanic eruption, reaching the Earth's surface as lava (molten magma).

The nature of the volcanic form depends on both the nature of the eruption and the lava involved.

1. Acidic lava

   • Contains much silica.
   • Has a high melting point.
   • Is viscous and tends to solidify in the vent, causing recurrent explosive eruptions.

2. Basic lava

   • Poor in silica but rich in iron and magnesium minerals.
   • Has a lower melting point, which allows it to flow considerable distances before solidifying.
   • Tends to produce flat zones of great diameter and is associated with non-explosive eruptions, as seen in shield volcanoes.

There are two major types of features or landforms associated with vulcanicity: intrusive and extrusive features.

Intrusive volcanic features

When magma is forced toward the surface, only a small amount reaches the Earth's surface. The majority of the magma is intruded into the crust, where it cools, solidifies, and forms various features or landforms. These intrusive features are often exposed at the surface through later erosion. The main intrusive (plutonic) features formed within the crust include:

1. Batholith
2. Sill
3. Dyke
4. Laccolith
5. Phacolith
6. Lopolith

Batholith

1. A batholith is a dome or very large mass of igneous rock formed by the cooling and solidification of magma deep within the Earth's crust, often in the heart of a mountain range.
2. Batholiths have a coarse-grained texture and are commonly granitic in composition.
3. These features are typically exposed at the surface after the overlying material is removed through denudation.
4. Batholiths have irregular shapes, with steeply inclined sidewalls against the host rock.
5. Examples: Sierra Nevada Batholith (USA), Idaho Batholith (USA), Aswan Granite Batholith (Egypt), Cape Coast Batholith (Ghana).

Sill

1. A sill is a sheet of magma formed when igneous material solidifies horizontally along bedding planes.
2. Sills are concordant with the rock structure and can vary in thickness and extend over many kilometers.
3. There are three types of sills:
   • Simple sill – A single layer of magma.
   • Multiple sill – Layers of magma intruded successively.
   • Composite sill – Layers of different magma compositions.
4. Sills are important for ore deposits and minerals like gold, platinum, and chromium.
5. They may form ridge-like escarpments, which, when exposed by erosion, can result in waterfalls and rapids (e.g., Kinkon Falls, Guinea, Three Sisters, South Africa).

Dyke

1. A dyke is a wall-like feature formed when magma pushes upward through cracks in the rock, cooling and solidifying across the bedding planes.
2. Dykes are discordant features, meaning they do not follow the plane of the existing rock layers.
3. Dykes can be vertical, horizontal, or inclined, with most being vertical or steeply inclined.
4. Types of dykes:
   • Multiple dykes – Formed by successive injections of homogeneous material into the same fissure.
   • Composite dykes – Formed by successive injections of different materials into the same fissure.

Laccolith

1. A laccolith is a dome-shaped intrusion formed within or between layers of sedimentary rock.
2. It is created by viscous magma that accumulates in a large mass, bending up the overlying rocks.
3. Laccoliths grow in height and width as more magma is added.
4. If the laccolith is more resistant than the adjacent rocks, it may form an upland after denudation.
5. Example: Univukwe Range, Harare, Zimbabwe.

Phacolith

1. A phacolith is a concordant intrusion of igneous rocks formed after the cooling and solidification of magma near the crest of an anticline or base of a syncline.
2. Phacoliths form along sedimentary bedrock and exert great thickness along synclines and anticlines.

Lopolith

1. A lopolith is a very large, saucer-shaped intrusion lying concordant to the rock strata.
2. Lopoliths form a shallow basin as a result of the greater weight of the overlying strata and deposits.
3. Example: Bushveld Basin, Transvaal, South Africa.

Extrusive volcanic features

Extrusive volcanic activity occurs when volcanic materials are released onto the Earth's surface and later cool and solidify. This activity involves the emission of lava flows, pyroclastic debris, volcanic bombs, ash, dust, and gases such as nitrogen compounds, sulphur compounds, and minor amounts of chlorine and hydrogen. Extrusive vulcanicity is responsible for the formation of many landforms on the Earth's surface. Examples of these landforms include volcanic cones, composite cones, explosive craters, calderas, acidic lava, and shield volcanoes.

Ash and cinder cones (pyroclastic cones)

1. Description: Cone-shaped structures predominantly made of fragmented materials produced by explosive volcanic activity.
2. Formation: Formed when lava is violently ejected to greater heights and falls back to Earth, building a cone-like feature. The slopes of the cone are concave due to the spreading tendency of lava at the base of the cone, while the cinder is steep-sided.
3. Examples:
   • Sarabwe Fileko, Rungwe, Southern Tanzania
   • Jos Plateaus, Nigeria

Composite cones (strato-volcanoes)

1. Description: Stratified volcanoes consisting of both lava flows with concave sides and beds of pyroclastic materials.
2. Formation: Lava often escapes from the sides of the cone, building up small conolets. These volcanoes have steep slopes and are the most common type of volcano.
3. Examples:
   • Mount Kilimanjaro
   • Cameroon Mountains
   • Mount Nyiragongo, Democratic Republic of Congo

Volcanic plug (plug dome)

1. Description: A rigid cylindrical plug formed by very viscous lava forced out during explosive eruptions.
2. Formation: The base of the plug is surrounded by exploded debris.
3. Example: Atakor volcanic area, Hogger Mountains, Algeria

Explosion crater (ring crater)

1. Description: A shallow, flat-floored depression surrounded by a low rim of pyroclasts and local rock.
2. Characteristics: Usually less than 50 meters high and often found in groups. A vent blows through the local rock by a series of gas explosions.

Crater

1. Description: A bowl or funnel-shaped depression at the top of a volcanic cone, typically formed after the plug dome is blown off.
2. Characteristics:
   • Diameter of less than 1.6 kilometers.
   • When filled with water, a crater becomes a crater lake.
3. Formation:
   • Through the outpouring of lava during the formation of the volcano. As lava cools and contracts, it forms a depression.
   • Through a volcanic explosion, which blows a hole in the ground, forming an explosion crater or ring crater.
4. Examples:
   • Embakai, Olmoti, Ngozi (Tanzania)
   • Summit of Mount St. Helens (USA)
   • Marsabit, Ol Donyo Nyoke (Kenya)
   • Mahavura (Democratic Republic of Congo)
   • Vesuvius (Italy)

Caldera

1. Description: A large depression formed when the upper part of a volcano is blown away by violent eruptions or the top of the cone subsides into the crust.
2. Types:
   • Collapse caldera: Formed when the volcano's top collapses.
   • Explosion caldera: Formed by an explosive eruption.
   • Erosional caldera: Formed by erosion over time.
3. Examples:
   • Sunda Strait
   • Krakatau-type or explosion-collapse calderas
   • Glencoe-type or cauldron subsidence calderas (e.g., Mokuaweo-weo Caldera, Mauna Loa)

Volcanic vent

Description: A central opening on the Earth's surface, underlying the summit crater, through which volcanic materials (gases, water, liquid lava, and rock fragments) are emitted.

Acidic lava (cumulo dome)

1. Description: A dome-shaped volcano with convex slopes formed when acidic lava solidifies around the vent.
2. Characteristics: Lava piles near the vent instead of flowing away due to its high viscosity, which results from a high silica content and high melting point.
3. Example: Ntumbi Dome, Mbeya Region, Tanzania

Extensive domed volcano

1. Description: A broad volcano with gentle slopes formed when basic lava is poured onto the surface and spreads to cover a large area.
2. Formation: Lava from the Earth's interior flows through a vent, creating a large dome.

Minor volcanic features are typically formed as volcanic activity approaches its end. These features include solfatara, fumarole, mofette, hot springs, geyser, and mud volcano.

Hot springs

1. Description: Natural outflows of superheated water from the ground, containing mineral substances in solution or suspension.
2. Formation: Hot springs are produced by geothermal heated groundwater that emerges onto the Earth's surface. The water may be heated by shallow magma or may come into contact with heated rocks in active volcanic areas.
3. Significance: Hot springs can influence the production of geothermal power.
4. Examples:
   • Amboni, Tanga (Tanzania)
   • Nanyala, Songwe Region (Tanzania)

Geyser

1. Description: A hot spring that forcefully spouts superheated water and steam into the atmosphere due to pressure.
2. Types:
   • Pool geyser: Spouts water from an open and relatively large pool.
   • Nozzle geyser: Spouts water from a small and contracted vent.
3. Characteristics: Geysers are periodic and can shoot hot water and vapor into the air.

Fumarole

1. Description: A vent through which gases and water vapor are emitted.
2. Additional notes: Fumaroles can also emit smoke.

Volcanic eruptions can have both destructive and constructive consequences for humans and the environment.

Destructive consequences

1. Loss of life

   Some volcanic eruptions cause significant loss of life. For example, the 1883 eruption of Krakatoa caused massive sea waves that drowned 40,000 people in neighboring islands.

2. Damage to property

   Volcanic eruptions can destroy homes, crops, and livestock, leading to substantial economic losses.

3. Pollution

   Gases emitted during eruptions can cause air pollution, which may also lead to soil pollution. This pollution negatively affects both the environment and human health.

4. Vegetation destruction

   Hot molten materials ejected during an eruption can destroy vegetation, leading to deforestation and a loss of biodiversity.

Constructive consequences

1. Fertile soil

   Lava, after breaking down through weathering, forms fertile soil that supports agriculture. A notable example is the southern slopes of Mount Kilimanjaro in Tanzania.

2. Formation of mineral deposits

   Volcanic activity can lead to the formation of valuable mineral deposits such as copper and diamonds (e.g., Kimberley in South Africa).

3. Utilization of hot springs

   Some hot springs are used for heating and supplying hot water to buildings, particularly in cold countries like Iceland and New Zealand.

4. Geothermal power

   Volcanic eruptions under hot springs can be harnessed to provide geothermal power, which is used for electricity generation. Countries like Kenya, Ethiopia, and the USA benefit from geothermal energy.

5. Tourism

   Volcanic features such as hot springs and geysers attract tourists, contributing to the development of the tourism industry.