Extrusive Vulcanicity

Quick Summary

• Extrusive vulcanicity occurs when magma reaches the Earth’s surface as lava
• Creates major features: volcanoes (cinder, composite, shield), lava domes, calderas
• Creates minor features: hot springs, geysers, fumaroles, solfataras
• Type of volcano depends on lava viscosity and gas content
• Examples in Nigeria: Jos Plateau volcanic rocks, Mambilla Plateau

What is Extrusive Vulcanicity?

Extrusive vulcanicity happens when molten rock (magma) from deep inside the Earth breaks through to the surface. Once magma reaches the surface, we call it lava. This process creates volcanoes, lava flows, and other dramatic landforms.

Think of extrusive vulcanicity like a pressure cooker releasing steam. The Earth’s interior is extremely hot and under great pressure. When that pressure becomes too much, molten rock bursts out through weak points in the Earth’s crust.

Extrusive vulcanicity is different from intrusive vulcanicity, where magma cools and solidifies below the Earth’s surface without ever reaching the top. Extrusive features form on the surface where we can see them.

Types of Volcanic Eruptions

1. Fissure Eruptions

Lava flows out through long cracks (fissures) in the ground rather than a single opening. These eruptions are usually gentle because the lava is runny (low viscosity). The lava spreads over wide areas, creating flat or gently sloping landscapes called lava plateaus.

Examples include the Deccan Plateau in India and the Columbia Plateau in the United States.

2. Central Eruptions

Lava erupts from a single pipe or vent. This builds up a cone-shaped mountain over time. Central eruptions can be explosive or gentle, depending on the type of lava. Most volcanoes form from central eruptions.

Types of Lava

Major Extrusive Features

1. Cinder Cones (Ash Cones)

Cinder cones are the simplest and smallest type of volcano. They form when gas-rich lava is thrown into the air during an eruption. The lava breaks into small pieces (cinders or ash) that fall around the vent and pile up in a cone shape.

Characteristics:

• Steep sides (30-40 degrees)
• Bowl-shaped crater at the top
• Usually less than 300 meters tall
• Made entirely of loose volcanic fragments
• Form quickly, sometimes in just a few years

Examples: Sarabwe Fileko in southern Tanzania, Busoka and Bitale in southwestern Uganda, areas south of Lake Turkana in Kenya, and volcanic features on the Jos Plateau in Nigeria.

2. Composite Cones (Strato-volcanoes)

Composite cones are large, steep-sided volcanoes built from alternating layers of lava and ash. They form from repeated eruptions over thousands of years. These are the most common and most recognizable volcanoes.

Characteristics:

• Very tall, often over 2,000 meters
• Steep slopes near the top, gentler slopes near the base
• Layers of hardened lava and volcanic ash
• Can have smaller side cones (parasitic cones) on the flanks
• Explosive eruptions due to thick, acidic lava

How they form: During some eruptions, thick lava flows out and hardens on the slopes. During other eruptions, ash and rock fragments are blasted into the air and settle around the vent. Over time, these layers stack up like a layer cake, creating a massive mountain.

Examples: Mount Fuji (Japan), Mount Kilimanjaro (Tanzania), Mount Kenya (Kenya), Mount Cameroon (Cameroon).

3. Shield Volcanoes (Basic Lava Cones)

Shield volcanoes are broad, gently sloping volcanoes that look like a warrior’s shield lying flat on the ground. They form from runny, basic lava that flows long distances before cooling.

Characteristics:

• Very wide base covering large areas
• Gentle slopes (less than 10 degrees)
• Made almost entirely of solidified lava flows
• Non-explosive eruptions with lava fountains
• Can be extremely tall if measured from the ocean floor

How they form: Basic lava is thin and runny, like honey warmed in the sun. When it erupts, it flows easily down the sides of the volcano, spreading over a wide area. Each eruption adds a new layer, slowly building up a massive but flat volcano.

Examples: Mauna Loa and Kilauea in Hawaii (largest shield volcanoes on Earth).

4. Lava Domes (Plug Domes)

Lava domes form when very thick, sticky lava oozes out of a vent but cannot flow far. Instead, it piles up around the opening like toothpaste squeezed from a tube.

Characteristics:

• Rounded, dome-shaped mounds
• Steep sides that are unstable
• Made of acidic lava with high silica content
• Often form inside the craters of larger volcanoes
• Can collapse suddenly, causing dangerous avalanches

Examples: Lassen Peak in California, Puy de Dôme in France.

5. Calderas

A caldera is a large, basin-shaped depression that forms when a volcano collapses into the empty magma chamber below. Calderas are much bigger than regular volcanic craters.

How they form:

1. A volcano erupts violently, emptying its underground magma chamber
2. Without magma to support it, the top of the volcano collapses inward
3. A huge circular or oval depression is left behind
4. Over time, the caldera may fill with water, forming a crater lake

Examples: Crater Lake in Oregon (USA), Ngorongoro Crater in Tanzania.

6. Lava Plateaus (Basalt Plateaus)

Lava plateaus are large, flat or gently sloping areas formed by repeated fissure eruptions. Runny lava floods out of long cracks and spreads over hundreds or thousands of square kilometers.

Characteristics:

• Flat or gently rolling topography
• Composed of many horizontal layers of basalt lava
• Cover very large areas
• Formed over millions of years

Examples: Deccan Plateau in India, Columbia Plateau in the USA, Ethiopian Highlands in East Africa.

Minor Extrusive Features

1. Hot Springs

Hot springs are places where naturally heated water flows to the surface. They form when groundwater seeps down into hot volcanic rock, heats up, and rises back to the surface.

Hot springs are common in areas with recent or active volcanic activity. People use them for bathing, heating homes, and generating geothermal electricity.

Example: Wikki Warm Spring in Yankari National Park, Bauchi State, Nigeria (though heated by geothermal activity, not direct volcanic action).

2. Geysers

A geyser is a special type of hot spring that erupts periodically, shooting hot water and steam high into the air. Geysers occur when water trapped underground becomes superheated under pressure and then suddenly explodes upward.

How they work:

1. Water collects in underground chambers
2. Heat from volcanic rock raises the temperature above boiling point
3. Pressure builds up because the water cannot expand
4. Eventually, the pressure becomes too great and water explodes to the surface
5. After the eruption, water refills the chamber and the cycle repeats

Examples: Old Faithful in Yellowstone National Park (USA), Geysir in Iceland (where the word “geyser” comes from).

3. Fumaroles

Fumaroles are openings in the Earth’s surface that release steam and volcanic gases (like sulfur dioxide, carbon dioxide, and hydrogen sulfide). They often occur on the slopes of active volcanoes or in areas with cooling magma below.

The gases may smell like rotten eggs due to sulfur compounds. Fumaroles show that volcanic activity is still occurring underground.

4. Solfataras

A solfatara is a type of fumarole that emits sulfurous gases (mainly sulfur dioxide and hydrogen sulfide) and water vapor. The name comes from an Italian volcanic area called Solfatara.

Sulfur often crystallizes around solfatara openings, creating yellow deposits. Solfataras are found in areas with young or active volcanic systems.

Examples: Kyejo in Rungwe (southern Tanzania), Auvergne in France, Java in Indonesia.

5. Mofettes

A mofette is a vent that releases carbon dioxide gas (CO₂) without much steam or other gases. The CO₂ is usually cool or only slightly warm.

Mofettes are dangerous because carbon dioxide is heavier than air and can collect in low-lying areas, displacing oxygen. Animals and people can suffocate if they enter areas with high CO₂ concentrations.

Examples: Kyejo in Rungwe (Tanzania), Auvergne (France), Java (Indonesia).

Extrusive Vulcanicity in Nigeria

Nigeria does not have active volcanoes today, but there is evidence of ancient volcanic activity in some regions:

Jos Plateau

The Jos Plateau in central Nigeria (Plateau State) contains ancient volcanic rocks. Millions of years ago, volcanic eruptions occurred here, leaving behind granite and basalt formations. Today, these rocks are mined for tin and other minerals.

Ash and cinder cones once existed on the Jos Plateau, though erosion has worn them down over millions of years.

Mambilla Plateau

The Mambilla Plateau in Taraba State shows signs of past volcanic activity. The high elevation and volcanic soils make the area suitable for tea cultivation and livestock grazing.

Cameroon Volcanic Line

Mount Cameroon, an active volcano just across Nigeria’s border in Cameroon, is part of the Cameroon Volcanic Line. This chain of volcanoes stretches from the Atlantic Ocean into Cameroon. Though not in Nigeria, it influences the geology of the border region.

Benefits and Hazards of Extrusive Vulcanicity

Benefits

• Fertile soils: Volcanic ash and weathered lava create nutrient-rich soils excellent for farming
• Geothermal energy: Heat from magma can generate electricity and heat homes
• Mineral deposits: Volcanic activity brings valuable minerals to the surface (gold, copper, tin)
• Tourism: Volcanoes attract tourists, creating jobs and income
• Building materials: Volcanic rock is used for construction and road building

Hazards

• Lava flows: Destroy homes, farms, and infrastructure
• Ash falls: Bury crops, damage lungs, collapse roofs
• Pyroclastic flows: Fast-moving clouds of hot gas and rock that kill everything in their path
• Lahars: Volcanic mudflows that sweep away villages
• Poisonous gases: Can suffocate people and animals
• Climate change: Large eruptions can cool global temperatures by blocking sunlight

Common Exam Mistakes

Mistake 1: Students confuse extrusive and intrusive vulcanicity. Remember: extrusive = exit to the surface. Intrusive = stays inside.

Mistake 2: Students list volcano types without describing characteristics or explaining how they form. WAEC wants you to describe and explain, not just name.

Mistake 3: Students forget to give examples. Always include at least one real-world example (preferably from Africa or Nigeria) for each feature.

Mistake 4: Students write “lava” when they mean “magma.” Magma is underground; lava is on the surface. Be precise with terminology.

Practice Questions

Multiple Choice Questions

1. Which of the following is an extrusive volcanic feature?
a) Batholith
b) Laccolith
c) Shield volcano ✓
d) Dyke

2. Composite cones are formed from:
a) Basic lava only
b) Acidic lava only
c) Alternating layers of lava and ash ✓
d) Fissure eruptions

3. Which lava type flows most easily?
a) Acidic lava
b) Basic lava ✓
c) Viscous lava
d) Silica-rich lava

4. A geyser differs from a hot spring because it:
a) Is colder
b) Erupts periodically ✓
c) Contains no minerals
d) Forms underground

5. Evidence of ancient volcanic activity in Nigeria is found in:
a) Niger Delta
b) Jos Plateau ✓
c) Sokoto Basin
d) Chad Basin

Essay Questions

1. (a) What is extrusive vulcanicity? (3 marks)
(b) Describe three types of volcanoes formed by extrusive vulcanicity. (12 marks)

Tip for (b): Choose cinder cones, composite cones, and shield volcanoes. For each, describe: shape, size, type of lava, how it forms, and give an example. Use diagrams if asked.

2. Explain four minor features of extrusive vulcanicity. (12 marks)

Tip: Choose hot springs, geysers, fumaroles, and solfataras. For each, explain what it is, how it forms, and where it occurs. Include why it happens (leftover heat from volcanic activity).

3. (a) Distinguish between acidic lava and basic lava. (6 marks)
(b) State four benefits of volcanic activity. (4 marks)
(c) State three hazards of volcanic eruptions. (3 marks)

Tip for (a): “Distinguish” means show the differences. Make a comparison: viscosity, silica content, color, eruption type, volcano type formed.

4. Describe the formation of a caldera. (10 marks)

Tip: Use numbered steps. Start with magma chamber, explain eruption, describe collapse, mention crater lake formation. Draw a simple diagram.

Memory Aids

Extrusive vs. Intrusive: EXtrusive = EXits the surface. INtrusive = stays INside.

Lava types: Basic lava = Broad shield volcano. Acidic lava = Angry explosive eruptions.

Volcano types by size: Cinder cones are Cute (small). Shield volcanoes are Spreading (wide). Composite cones are Colossal (tall).

Minor features: Hot springs Heat water. Geysers Go off like alarms. Fumaroles Fume with gases.

Related Topics

• Intrusive Vulcanicity
• Classification of Volcanoes
• Distribution of Volcanoes
• Plate Tectonics and Volcanic Activity
• Lakes Formed by Vulcanicity