Question

Predict the type of magma associated with Mount St. Helens. a) Andesitic b) Basaltic c) Rhyolitic

Short answer

The type of magma associated with Mount St. Helens is andesitic.

Step-by-step solution

Consider the Volcanic History

Mount St. Helens is part of the Cascade Range, which is known for having explosive eruptions. This suggests a specific type of magma associated with these types of volcanic activities.

Identify Common Magma Types for Explosive Volcanoes

Volcanoes like those in the Cascade Range typically have magma types that are intermediate to high in silica content, leading to more explosive eruptions. This eliminates basaltic lava, which is low in silica and less explosive.

Determine Silica Content and Viscosity

Andesitic magma has intermediate silica content and viscosity, leading to more explosive eruptions compared to basaltic magma. Rhyolitic magma, while even higher in silica, is less common in this environment and extremely viscous.

Examine Historical Eruption Data

Review historical data on Mount St. Helens eruptions, which characteristically include pyroclastic flows and explosive events, typical of andesitic magma.

Conclusion Based on Analysis

Given the volcanic activity history and characteristics, the magma associated with Mount St. Helens is predominantly andesitic.

Key concepts

Andesitic magma

Andesitic magma is a key player in many volcanic eruptions around the world. Found in many volcanically active areas, it forms at convergent plate boundaries, particularly where oceanic crust subducts under continental plates. This type of magma is named after the Andes Mountains, where it is prevalent.

Andesitic magma typically contains intermediate levels of silica, which significantly affects its viscosity. This means that it is thicker and more resistant to flow than basaltic magma but less than rhyolitic magma. Because of this medium viscosity, andesitic magma contributes to more violent and explosive eruptions compared to its basaltic counterpart. Yet, it is not as viscous or explosive as rhyolitic magma, which ranks highest in silica content.

Key features of andesitic magma include:

• Intermediate silica content (approximately 52-63%)
• Thicker than basaltic magma
• Moderate to high explosiveness
• Commonly associated with stratovolcanoes and volcanic arcs

Understanding these characteristics helps explain why Mount St. Helens, a stratovolcano, predominantly features eruptions driven by andesitic magma.

Silica content in magma

Silica content is a crucial component that determines the nature of a volcanic eruption. It refers to the amount of silicon dioxide (SiO₂) present in the magma. This content in magma varies, influencing the magma's viscosity and eruptive potential.

Viscosity plays a significant role as it affects how quickly magma can flow. Higher silica content leads to higher viscosity, making magma thicker and more prone to trapping gases. This trapped gas can increase pressure, leading to more explosive eruptions.

Types of magma based on silica content are:

• Basaltic magma: Low silica content (~45-52%), very fluid
• Andesitic magma: Intermediate silica content (~52-63%), moderately viscous
• Rhyolitic magma: High silica content (~69-77%), highly viscous

The silica content not only dictates the explosiveness of the volcano but also the type of volcanic rocks that are formed post-eruption. This is why understanding silica content is vital for predicting volcanic behavior.

Explosive volcanic eruptions

Volcanic eruptions are nature's grandiose displays, dictated largely by the type of magma involved. When discussing explosive volcanic activity, andesitic magma stands out due to its specific properties.

Explosive eruptions occur when high-viscosity magma traps gases beneath the earth's surface. This pressure accumulates until it explosively releases, propelling ash, gases, and pyroclastic materials high into the atmosphere. Andesitic magma's intermediate silica content contributes to such explosive behavior, as it is capable of both trapping gases and sustaining high pressures.

Characteristics of explosive eruptions include:

• High eruptive columns and ash clouds
• Formation of pyroclastic flows and lahars
• Potential creation of calderas following a massive eruption

These eruptions are much more hazardous than effusive eruptions due to their suddenness and potential destructiveness, posing significant risks to nearby human populations and technology.

Mount St. Helens volcanic activity

Mount St. Helens, located in Washington state, USA, is one of the most famous volcanic landmarks due to its dramatic eruption history. Part of the Cascade Range, this stratovolcano is most noted for its major eruption in 1980, which became a defining moment in volcanology.

This volcano is primarily associated with andesitic magma, which leads to its famous explosive eruptions. The viscosity of this magma creates significant pressure within the volcano's magma chamber, often resulting in violent pyroclastic flows, lava domes, and intense ash clouds.

Key historical highlights of Mount St. Helens include:

• 1980 eruption: Released about 1.3 cubic kilometers of material, reshaping the landscape
• Regular eruptions and seismic activity traced back hundreds of years
• Combination of explosive eruptions and lava dome formations

As part of the Cascade Range, Mount St. Helens provides essential data for understanding the dynamics of stratovolcanoes and continues to be a focal point for the study of volcanic activity.