Magma viscosity

Definition

Magma viscosity refers to the thickness or resistance to flow of molten rock beneath the Earth's surface. This property plays a crucial role in determining volcanic behavior, influencing how magma ascends, erupts, and shapes volcanic landforms. The viscosity of magma is primarily affected by its composition, temperature, and gas content, which in turn impacts the explosiveness of volcanic eruptions and the characteristics of lava flows.

5 Must Know Facts For Your Next Test

1. Magma with low viscosity, such as basaltic magma, allows gases to escape easily, often resulting in non-explosive eruptions.
2. High-viscosity magma, like rhyolitic magma, tends to trap gases, leading to explosive eruptions due to increased pressure.
3. The temperature of magma significantly affects its viscosity; hotter magma is generally less viscous than cooler magma.
4. Magma composition plays a critical role in determining its viscosity, with silica-rich magmas being more viscous compared to those that are low in silica.
5. The presence of dissolved gases in magma can also alter its viscosity; as pressure decreases during ascent, gases can expand and affect the flow behavior.

Review Questions

• How does magma viscosity influence the type of volcanic eruption that occurs?
  • Magma viscosity directly impacts the explosiveness of volcanic eruptions. Low-viscosity magmas allow gases to escape easily without building up pressure, leading to gentle eruptions. In contrast, high-viscosity magmas trap gases more effectively, causing pressure to increase until it results in explosive eruptions. This relationship between viscosity and eruption style is key to understanding different volcanic behaviors.
• Compare and contrast the characteristics of lava flows produced by low-viscosity versus high-viscosity magmas.
  • Lava flows from low-viscosity magmas tend to be fast-moving and fluid-like, creating broad, expansive flows such as pahoehoe. These flows spread out over large areas due to their ability to flow easily. Conversely, high-viscosity magmas create slower-moving lava that forms thick and steep-sided flows or domes. This difference in flow characteristics affects not only the shape of the resulting landforms but also the potential hazards associated with each type of flow.
• Evaluate how changes in temperature and composition of magma can affect its viscosity and subsequent volcanic activity.
  • The viscosity of magma is influenced by both temperature and composition. As temperature increases, magma becomes less viscous and flows more easily; therefore, hotter magmas are typically associated with less explosive volcanic activity. Composition also plays a significant role: magmas rich in silica have higher viscosities compared to those that are low in silica. This means that a change in temperature or composition can alter how magma behaves as it rises towards the surface, significantly impacting whether an eruption will be effusive or explosive. Understanding these factors helps predict volcanic behavior and associated risks.

Related terms

Lava Flow: A lava flow is a stream of molten rock that moves along the ground during a volcanic eruption, characterized by its viscosity which influences its speed and shape.

Volcanic Eruption: A volcanic eruption occurs when there is a sudden release of magma, gas, and ash from a volcano, with the nature of the eruption influenced by magma viscosity.

Pahoehoe: Pahoehoe is a type of basaltic lava flow that is smooth and ropy in texture, typically associated with low-viscosity magma.