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Volcanoes and Igneous Activity Overview

This note provides an in-depth look at volcanic formations and their relationships with various lava properties, emphasizing how these factors influence the behavior and characteristics of volcanoes.

Shapes of Volcanoes

Volcano shapes are heavily influenced by the viscosity and composition of the lava that they emit:

• Basalt:

  • Characteristics: Low silica content (typically 45-55% SiO2), resulting in low viscosity (approximately 10-100 Pa·s). This allows for fluid lava flows that can stretch for miles, creating wide, gentle slopes. Commonly associated with Hawaiian volcanoes, which produce extensive lava fields.

  • Example: Mauna Loa in Hawaii.

• Andesite:

  • Characteristics: Intermediate composition between basalt and rhyolite (about 57-63% SiO2); produces moderate viscosity (around 100-1000 Pa·s) leading to more explosive eruptions than basalt. Tends to form stratovolcanoes with steeper profiles due to alternating layers of lava and ash.

  • Example: Mount St. Helens in the United States.

• Rhyolite:

  • Characteristics: High silica content (over 63% SiO2) yields high viscosity (greater than 1000 Pa·s), leading to thick, pasty lava flows that can block vents and cause explosive eruptions. Typically forms lava domes or calderas due to the buildup of lava that cannot flow far from the vent.

  • Example: Yellowstone Caldera in Wyoming.

Types of Lava Flow

• Basaltic Flow:

  • Fast-flowing and can cover vast areas rapidly, leading to the development of features such as lava tubes.

  • Characteristic: Low gas content, resulting in non-explosive eruptions.

• Andesitic Flow:

  • Typically slower than basaltic flow, resulting in steeper hillsides and more violent eruptions.

  • Characteristic: Intermediate gas content, can lead to explosive eruptions when pressure builds.

• Rhyolitic Dome:

  • Lava piles up near the vent, forming a dome that can collapse during eruptions.

  • Characteristic: High gas content and high viscosity; prone to explosive behavior.

• Rhyolitic Spire:

  • A steep, narrow peak formed from thick, highly viscous lava that solidifies quickly near the vent, creating spectacular formations.

Lava Types

• Felsic Magma:

  • Definition: Magma with high viscosity due to high silica content; can lead to explosive eruptions.

  • Composition: Rich in quartz and feldspar minerals.

Types of Lava

• Pahoehoe:

  • Characteristics: Flows quickly (about 10 times faster than aa) and is very hot (over 1000°C), forming smooth, ropy surfaces.

  • Formation: As lava cools, it loses gas and takes on a rope-like texture as it is still flowing.

• Aa:

  • Characteristics: Moves more slowly than pahoehoe, is cooler (around 800-1000°C), acquires a rough, jagged, blocky texture as it solidifies.

  • Formation: Forms when the lava cools and thickens, breaking apart into angular pieces.

Lava Surface Textures

• Pahoehoe Lava:

  • Smooth upper surface allows for unique geological features and creates dramatic landscapes like landforms and collapsed tubes.

  • Can transition to aa if cooled too much or loses gas rapidly.

• Blocky Lava (Aa):

  • Exhibits a rough, jagged texture, indicative of thicker lava flows that solidify quickly and does not flow easily.

  • Creates steep, rugged terrains near volcanic vents.

Pyroclastic Debris

• Tephra: General term for molten material ejected during eruptions. Varies in size:

  • Ash: Less than 2 mm in diameter.

  • Lapilli: Between 2 mm and 64 mm.

  • Bombs: Greater than 64 mm that solidify in the air.

  • Nueé Ardente: Dense clouds of hot gas and ash capable of racing down the volcano’s slopes at hundreds of km/h.

Vesicles

• Definition: Small holes formed from gas bubbles that were trapped in the lava as it cooled; common in basalt boulders, such as those found at Sunset Crater, AZ. These vesicles can provide insights into the gas content and eruption style of the magma.

Volcano Shapes

• Volcano shapes determined by the structure of the vent system:

  • Circular Central Vent: Common in stratovolcanoes, allows for a concentrated eruption of materials.

  • Fissure Eruptions: Occur along linear cracks in the Earth's surface, such as seen in shield volcanoes like Mauna Kea in Hawaii.

Plumbing Systems of Volcanoes

• Central vent systems may contain flank vents, which can lead to more complex eruptions and potential caldera formation, where the volcano collapses into a large depression after an explosive eruption due to evacuated magma.

Types of Volcanoes

• Shield Volcanoes:

  • Characteristics: Broad, dome-shaped with gentle slopes, predominantly formed from low-viscosity basalt lava.

  • Eruptions: Typically non-explosive, allowing lava to flow over great distances, creating their shape.

  • Example: Kilauea in Hawaii.

• Cinder Cones:

  • Characteristics: Small, steep-sided volcanoes formed from ejected lava fragments that accumulate around the vent.

  • Formation: Often from a single eruptive event; usually have a bowl-shaped crater at the summit.

• Lava Domes:

  • Characteristics: Form when viscous lava piles up near the vent, creating steep-sided structures.

  • Example: Mount St. Helens' Lava Dome.

• Stratovolcanoes:

  • Characteristics: Composed of layers of ash, tephra, and lava flows; characterized by a combination of explosive and effusive eruptions.

  • Examples: Mount Fuji in Japan and Mount Pinatubo in the Philippines; often found along subduction zones.

Volcanic Eruptions and Hazards

• Effusive Eruptions:

  • Driven mainly by low-viscosity basaltic lava, leading to long-lasting lava flows.

  • Relatively gentle and predictable.

• Explosive Eruptions:

  • Characterized by rapid gas expansion and fragmentation of magma. Create significant ash clouds and pyroclastic flows.

Volcanic Hazards

• Lava Flows:

  • Generally slow-moving but capable of destroying anything in their path, including buildings and forests.

  • Can last for days to months, depending on the eruption dynamics.

• Ashfall:

  • Can cover large areas, impacting air travel, agriculture, and health by contaminating water supplies and respiratory systems.

• Lahars:

  • Dangerous flows of volcanic ash mixed with water; can occur during or after an eruption.

  • Often triggered by heavy rains or the melting of snow and ice on the volcano.

Specific Hazards

• Debris Avalanches:

  • Large rock masses that break loose from a volcano; occurs during eruptions or landslides.

  • The 1980 eruption of Mt. St. Helens is a prime example.

• Lahar Events:

  • Historical examples include the devastating lahar that buried the town of Armero in Colombia in 1985, resulting in significant casualties.

Summary of Volcanic Hazards

Lahars and debris avalanches can occur even when a volcano is dormant. They pose significant risks that can arise due to geological instability, long after an eruption has ended. The risks presented by these phenomena vary widely based on volcanic activity, geographic location, and environmental conditions. Understanding these hazards is crucial for risk mitigation and disaster preparedness in volcanically active regions.