medium · GMAT Verbal

Passage: Volcanic eruption styles are largely determined by the viscosity of the magma and its gas content. Basaltic magmas, which are relatively low in silica, have low viscosity, allowing gases to escape easily. This results in effusive eruptions characterized by relatively gentle lava flows, typical of shield volcanoes like those in Hawaii. In contrast, rhyolitic and andesitic magmas are high in silica, making them extremely viscous. This high viscosity traps volcanic gases, allowing pressure to build until it is released in a violent, explosive eruption, characteristic of stratovolcanoes like Mount St. Helens. However, the presence of external water—such as from a crater lake or groundwater—can complicate this simple silica-based model. When magma comes into contact with water, it can trigger a 'phreatomagmatic' eruption. The rapid expansion of steam fragments the magma into fine ash, increasing the explosivity regardless of the magma's initial silica content. Understanding these interactions is vital for volcanic hazard assessment, as they suggest that even volcanoes with traditionally 'gentle' basaltic magma can produce devastating explosive events under specific hydrologic conditions. The main idea of the passage is to:

1. Contrast the geological features of shield volcanoes and stratovolcanoes.
2. Provide a historical account of the most significant phreatomagmatic eruptions of the last century.
3. Explain how silica content and external water influence the explosivity of volcanic eruptions.
4. Argue that phreatomagmatic eruptions are more dangerous than eruptions driven by silica content.
5. Describe the chemical process by which volcanic gases are trapped in high-silica magma.

Sign up free to see the explanation and track your rank →