Quick Summary
- Vulcanicity involves movement of magma from earth’s interior to the surface
- Two main types: intrusive (underground) and extrusive (surface) vulcanicity
- Creates various landforms like volcanoes, lava plateaus, batholiths, and sills
- Associated with areas of mountain building and plate boundaries
- Controlled by lava viscosity and gas content in magma
What is Vulcanicity?
Vulcanicity refers to all processes connected with the movement of molten rock material (magma) from the earth’s interior toward or onto the surface. When you see a volcano erupt in places like Cameroon Mountain or Mount Nyiragongo in the Democratic Republic of Congo, you are watching vulcanicity in action.
The earth’s interior contains extremely hot molten rock called magma. This magma is under tremendous pressure from the weight of rocks above it. When this pressure becomes too great, the magma finds weak points in the earth’s crust and pushes through. Think of it like shaking a bottle of soft drink – the pressure builds up until the cap pops off.
Vulcanicity is most common in areas where tectonic plates meet. These are zones of weakness in the earth’s crust where magma can escape more easily. That is why you find volcanoes along the Pacific Ring of Fire, the East African Rift Valley, and the Mediterranean region.
Types of Vulcanicity
1. Intrusive Vulcanicity
Intrusive vulcanicity happens when magma fails to reach the earth’s surface. Instead, it cools and solidifies inside the earth’s crust. The magma may spread between rock layers, fill cracks, or collect in large underground chambers.
Because the magma cools slowly underground (sometimes over millions of years), it forms coarse-grained rocks with large crystals. Granite is a common example of rock formed through intrusive vulcanicity.
Major Intrusive Features:
Batholith: This is a huge mass of igneous rock formed deep in the earth’s crust. A batholith forms when massive amounts of magma cool and solidify far below the surface. Batholiths are usually made of granite and can be hundreds of kilometers wide. Over millions of years, erosion may expose batholiths at the surface. Examples include the Sierra Nevada batholith in the United States.
Sill: A sill forms when magma pushes horizontally between layers of existing rock. The magma flows parallel to the rock layers and cools into a flat, horizontal sheet of rock. Sills can be a few meters to hundreds of meters thick. The Palisades Sill along the Hudson River in the United States is a famous example.
Dyke: Unlike sills, dykes are vertical or near-vertical sheets of igneous rock. They form when magma cuts across existing rock layers and fills vertical cracks. Dykes can be seen as dark vertical bands cutting through lighter colored rocks. They range from a few centimeters to hundreds of meters wide.
Laccolith: A laccolith forms when magma pushes between rock layers but instead of spreading out flat like a sill, it domes upward. This creates a mushroom-shaped mass of igneous rock. The pressure from below pushes the overlying rock layers upward into a dome shape.
Volcanic Plug (Neck): When magma solidifies inside a volcano’s vent, it forms a plug. Over time, the softer outer layers of the volcano erode away, leaving behind the hard plug as a tower of rock. Shiprock in New Mexico, USA, is a dramatic example of an exposed volcanic plug.
2. Extrusive Vulcanicity
Extrusive vulcanicity occurs when magma reaches the earth’s surface. Once magma comes out onto the surface, we call it lava. This type of vulcanicity creates the dramatic volcanic eruptions and lava flows we often see in news reports.
Extrusive vulcanicity produces various materials including lava flows, volcanic ash, volcanic bombs, dust, steam, and gases like sulfur dioxide and carbon dioxide.
Major Extrusive Features:
Volcanoes: These are openings in the earth’s crust through which lava, ash, and gases escape. Volcanoes come in different shapes and sizes depending on the type of eruption and lava viscosity. Mount Cameroon in Cameroon is West Africa’s most active volcano.
Lava Plateau: When very fluid lava erupts from long cracks (fissures) instead of a central vent, it can spread over vast areas. As the lava cools, it creates a flat or gently sloping plateau. The Deccan Plateau in India and the Columbia Plateau in the United States formed this way.
Caldera: This is a large, bowl-shaped depression that forms when a volcano’s magma chamber empties during a massive eruption. Without support from below, the volcano collapses inward, creating a caldera. If water fills the caldera, it becomes a crater lake. Lake Nyos in Cameroon occupies a volcanic crater.
Volcanic Cone: As lava and ash pile up around a vent over many eruptions, they build a cone-shaped mountain. The shape and steepness of the cone depend on the type of material erupted.
Minor Forms of Vulcanicity
Not all volcanic activity involves dramatic eruptions. Some areas show gentler signs of underground heat:
Hot Springs: These are places where groundwater heated by hot rocks underground rises to the surface. The water can be warm enough for bathing. Hot springs exist in areas with recent volcanic activity.
Geysers: A geyser is a hot spring that periodically shoots jets of hot water and steam into the air. This happens when underground water gets superheated and builds up pressure until it explodes upward. Old Faithful in Yellowstone National Park, USA, is a famous geyser that erupts regularly.
Fumaroles: These are openings that release volcanic gases and steam but no water. They often smell of sulfur and indicate volcanic activity continues underground.
Boiling Mud: In some volcanic areas, hot water mixes with clay and volcanic ash to create pools of bubbling mud.
Factors Affecting Volcanic Eruptions
| Factor | Effect on Eruption |
|---|---|
| Lava Viscosity (Thickness) | Thick, viscous lava traps gases and causes explosive eruptions; thin, runny lava flows easily and causes gentle eruptions |
| Silica Content | High silica = thick lava = explosive eruptions; Low silica = fluid lava = gentle eruptions |
| Gas Content | More dissolved gases = higher pressure = more explosive eruptions |
| Temperature | Hotter lava is more fluid and flows easily; cooler lava is thicker and moves slowly |
| Water Content | Water contact with magma creates steam, increasing explosion force |
Importance of Vulcanicity
Benefits:
- Volcanic soils are very fertile for farming – areas around Mount Cameroon have rich agricultural land
- Volcanic rocks provide building materials
- Geothermal energy from volcanic areas can generate electricity
- Hot springs attract tourists, boosting local economy
- Volcanic areas contain valuable minerals
Hazards:
- Volcanic eruptions can destroy homes and farmland
- Lava flows burn everything in their path
- Volcanic ash damages crops and pollutes water
- Poisonous gases can kill people and animals
- Volcanic mudflows (lahars) can bury entire villages
Common Exam Mistakes
WAEC examiners often report these errors:
- Confusing intrusive and extrusive features: Students list sills and dykes as extrusive features. Remember: intrusive = underground, extrusive = surface.
- Failing to explain: When a question says “explain,” don’t just list features. Say HOW and WHY they form.
- Mixing up lava and magma: Magma is underground molten rock; lava is magma that reaches the surface.
- Poor diagrams: Always label your diagrams clearly with arrows pointing to specific features.
- Not using examples: Support your answers with real locations like Mount Cameroon, Lake Nyos, or Mount Kilimanjaro.
Practice Questions
Multiple Choice Questions
1. Which of the following is an intrusive volcanic feature?
a) Lava plateau
b) Caldera
c) Batholith ✓
d) Volcanic cone
2. The molten rock found beneath the earth’s surface is called:
a) Lava
b) Magma ✓
c) Pyroclast
d) Basalt
3. A hot spring that periodically erupts jets of water and steam is called:
a) Fumarole
b) Hot spring
c) Geyser ✓
d) Caldera
4. Which type of lava causes explosive volcanic eruptions?
a) Fluid lava with low silica content
b) Viscous lava with high silica content ✓
c) Cold lava with no gas
d) Thin lava with low temperature
Essay/Theory Questions
1. (a) Define vulcanicity. (2 marks)
(b) Distinguish between intrusive and extrusive vulcanicity. (4 marks)
(c) Describe THREE intrusive landforms. (9 marks)
Examiner’s Tip: For part (c), describe means give detailed characteristics of each landform including how it forms, its shape, size, and an example. Don’t just mention the name.
2. With the aid of diagrams, explain how the following features are formed:
(a) Sill (5 marks)
(b) Dyke (5 marks)
(c) Batholith (5 marks)
Examiner’s Tip: “With the aid of diagrams” means you MUST draw labeled diagrams. Your explanation should match your diagram. Show the direction of magma movement with arrows.
3. (a) Explain FOUR factors that influence the nature of volcanic eruptions. (8 marks)
(b) State THREE benefits and THREE hazards of vulcanicity. (6 marks)
Examiner’s Tip: “Explain” requires you to say not just WHAT the factor is, but HOW it affects the eruption. For example, don’t just say “gas content” – explain that high gas content creates pressure leading to explosive eruptions.
Memory Aids
Remember Intrusive Features – “Big Silly Dogs Love Playing”
- Batholith (large underground mass)
- Sill (horizontal sheet)
- Dyke (vertical sheet)
- Laccolith (mushroom-shaped dome)
- Plug (solidified vent)
Lava vs Magma – “Magic Magma Makes Lava”
- Magma = Underground (still doing magic inside)
- Lava = Surface (the magic is revealed)
Viscosity Rule: “High Silica = Sticky Situation = Explosive!” (High silica content makes thick lava that explodes)
Related Topics
- Classification of Volcanoes (Active, Dormant, Extinct)
- Plate Tectonics and Volcanic Activity
- Types of Volcanic Eruptions
- Distribution of Volcanoes Worldwide
- Effects of Volcanic Eruptions on Human Activities