Roadmap for Answer Writing
1. Introduction
- Brief Definition of Magma: Explain that magma is molten rock beneath the Earth’s surface, containing gases like carbon dioxide, sulfur, nitrogen, and chlorine.
- Purpose of Answer: The answer will explore how different types of magma (basic and acid) influence the shape and explosiveness of volcanoes.
2. Factors Influencing Magma Characteristics
- Composition: Focus on the key factors like silica content, iron, and magnesium.
- Viscosity: Describe how viscosity (thickness) of magma affects lava flow.
- Gas Content: Explain how gas content influences explosiveness.
3. Types of Magma
- Basic Lava (Low Silica):
- Characteristics: Low viscosity, fluid, dark in color (e.g., basalt).
- Effect on Volcano Shape: Forms shield volcanoes with broad, gently sloping sides (e.g., Hawaii).
- Explosiveness: Non-explosive, as gases escape easily. Example: Kilauea Volcano in Hawaii.
- Acid Lava (High Silica):
- Characteristics: High viscosity, thick, light-colored, higher in silica.
- Effect on Volcano Shape: Forms composite volcanoes with steeper sides (e.g., Mount Fuji, Mayon).
- Explosiveness: Highly explosive due to trapped gases. Example: Mount St. Helens (1980 eruption).
4. Explosiveness and Magma Viscosity
- Higher Viscosity (Acid Lava): The thicker the magma, the more pressure builds up as gases are trapped, leading to explosive eruptions.
- Lower Viscosity (Basic Lava): More fluid magma allows gases to escape gradually, reducing explosiveness.
5. Examples
- Shield Volcano Example: Hawaii’s Big Island (Basic Lava) – Non-explosive eruptions forming wide, gently sloping shields.
- Composite Volcano Example: Mount Fuji (Acid Lava) – Explosive eruptions leading to steep, conical shapes.
- Calderas: Formed when explosive eruptions cause the volcano to collapse, such as in Yellowstone Caldera (due to acid lava).
6. Conclusion
- Summary: Restate that magma type directly impacts the shape (shield vs. composite) and explosiveness (non-explosive vs. highly explosive) of a volcano.
- Relevance: Understanding magma’s influence helps in volcanic risk prediction and preparedness.
Relevant Facts to Support the Answer:
- Basic Lava Characteristics:
- Rich in iron and magnesium, poor in silica.
- Fluid with low viscosity, allowing lava to travel long distances .
- Results in shield volcanoes with broad, gently sloping sides. Example: Hawaii .
- Acid Lava Characteristics:
- High silica content, leading to high viscosity.
- Forms steep-sided composite volcanoes. Example: Mount Fuji.
- High viscosity leads to explosive eruptions.
- Explosiveness of Acid Lava:
- High viscosity traps gas, leading to violent eruptions. Example: Mount St. Helens, 1980.
- Explosive eruptions often result in calderas, such as the Yellowstone Caldera .
- Volcano Shapes:
- Shield volcanoes (e.g., Kilauea, Hawaii) are formed by low-viscosity lava flowing easily.
- Composite volcanoes (e.g., Mount Fuji, Mayon) are formed by the accumulation of thick, viscous lava with explosive eruptions.
By organizing the answer into clear sections—types of magma, volcano shapes, and explosiveness—along with relevant examples and facts, you’ll create a well-rounded, informative response.
Model Answer
Volcanoes are shaped and their explosive nature is determined largely by the type of magma involved in their formation. The composition of magma, including its viscosity and gas content, plays a crucial role in both the volcano’s shape and how violently it erupts.
1. Basic Lava and Shield Volcanoes
Basic lava, which is rich in iron and magnesium but low in silica, is highly fluid and flows easily. This type of lava is typically dark in color, such as basalt, and has a lower viscosity, allowing it to travel long distances before solidifying. Due to its fluid nature, it leads to the formation of shield volcanoes, which have broad, gently sloping sides. These volcanoes are less explosive because the lava can flow easily, allowing gas to escape gradually. An example of a shield volcano is the Big Island of Hawaii, which has been formed by the consistent outpouring of basic lava.
2. Acid Lava and Composite Volcanoes
Acid lava, in contrast, is rich in silica, making it more viscous and harder for gas to escape. This leads to a build-up of pressure, which results in explosive eruptions. Acid lava tends to solidify quickly and does not travel far, leading to the formation of steeper, conical-shaped volcanoes known as composite volcanoes. These volcanoes are typically characterized by alternating layers of solidified lava and pyroclastic material. Examples include Mount Fuji in Japan, Mount Rainier in the U.S., and Mayon Volcano in the Philippines. The high viscosity of the magma often causes eruptions to be violent, with eruptions creating loud explosions and even forming calderas—large depressions that can result when a volcano collapses after an explosive eruption. Notable calderas include the Yellowstone Caldera in Wyoming and Long Valley Caldera in California.
3. Viscosity and Explosiveness
In general, the higher the viscosity of the magma, the more explosive the eruption. This is because thicker magma traps gases, increasing pressure until it is released explosively. Therefore, while basic lava leads to non-explosive eruptions, acid lava results in some of the most explosive volcanic activity on Earth.