Volcanic Eruptions and Magma Types

Types of Volcanic Eruptions

• Highly viscous, silica-rich magmas may produce explosive clouds of hot ash and transition into plumes termed eruption columns.

Anatomy of a Volcano

• A composite cone volcano can exceed 1km in diameter and are termed calderas.
• Eruptions on the sides (flanks) of volcanoes can produce small features that extrude lava.

Famous Volcanoes

• Mauna Loa and Kilauea are examples of volcanoes.
• Paricutin is a volcano in Mexico.

Types of Calderas

• Crater Lake-Type Calderas, Hawaiian-Type Calderas, and Yellowstone-Type Calderas are three types of calderas.

Yellowstone Caldera

• The map shows the extent of the ashfalls for the last 3 caldera-forming eruptions of Yellowstone.

Flood Basalts

• The Columbia Plateau has an illustration of the extent of flood basalts.

Arizona Palisades Sill

• An example of a sill.

Factors Affecting Volcanic Eruptions

• The role of heat, pressure, and volatiles can affect volcanic eruptions.
• The introduction of water can lower the melting points by as much as 100° C.

Global Distribution of Volcanoes

• The Ring of Fire is located around the Pacific Ocean.
• Volcanoes can also be found in the deep-ocean basin and irregularly distributed on the interiors of continents.
• Convergent Plate Boundaries and Divergent Plate Boundaries can also affect the distribution of volcanoes.

Here are the study notes:

Volcanic Eruptions

• Mount St. Helens Eruption (1980)
  • Largest human-recorded volcanic eruption in North America
  • 59 lives lost
  • Entire north flank of the volcano blown out, lowering elevation by 1350 feet
  • Trees flattened within a 400 km2 area
  • Mudflows flowed 18 miles down the Toutle River
  • Nearly one cubic kilometer of ash and debris ejected
  • Ash cloud reached 11 miles into the atmosphere
  • Ash fallout was over 2 meters deep in the immediate area
  • Measurable ash deposits developed in Oklahoma and Minnesota
  • Crops damaged as far east as Montana

Types of Volcanic Eruptions

• Quietest Eruptions (Kilauea, Hawaii)
  • Fluid, basaltic lava flows
  • Occasional lava fountains
  • Over 180 homes destroyed since 1983
• Violent Eruptions (Mount St. Helens)
  • More silica-rich magma, thicker and more viscous
  • Greater gas content leading to more explosive eruptions

Factors Affecting Eruptions

• Magma Composition
  • Affects viscosity and explosivity
• Magma Temperature
  • Affects viscosity and mobility
• Dissolved Gases
  • Affects viscosity and explosivity

Lava Flows

• Basaltic Lava
  • Hot, fluid, and fast-moving (up to 19 mph)
  • Forms in thin, broad sheets or channel-like flows
• Andesitic/Rhyolitic Lava
  • Cooler, thicker, and slower-moving
  • Shorter flow distances
• Lava Tubes
  • Form when lava flows beneath the surface
  • Allow lava to flow long distances from its source

Gas Content and Volcanic Activity

• Gas Content in Magma
  • Affects explosivity and viscosity
  • Analysis of recent eruptions in Hawaii: water vapor (70%), carbon dioxide (15%), nitrogen (5%), and smaller amounts of chlorine, hydrogen, and argon
• Volcanic Gases
  • Propel magma from a volcano
  • Generate large gas plumes that rise high into the atmosphere

Anatomy of a Volcano

• Shield Volcanoes
  • Broad, gently sloping structures
  • Formed from accumulation of fluid basaltic lavas
  • Examples: Hawaii, Iceland, Galapagos
• Cinder Cones
  • Small, steep-sided structures
  • Formed from ejected lava fragments
  • Examples: Paricutin, Mexico
• Composite Cones
  • Large, symmetrical structures
  • Formed from lava and pyroclastic deposits
  • Examples: Mount St. Helens, Mount Rainier, Mount Fuji

Pyroclastic Flows and Lahars

• Pyroclastic Flows
  • Hot ash, dust, and gases that flow down volcano flanks
  • Can reach speeds of up to 125 mph
• Lahars
  • Volcanic mudflows that form when volcanic debris becomes saturated with water
  • Can form during heavy rainfall or snow/ice melting

Calderas and Volcanic Landforms

• Crater Lake-Type Calderas
  • Form from collapse of the composite cone summit after an explosive eruption
  • Example: Crater Lake, Oregon
• Hawaiian-Type Calderas
  • Form from gradual subsidence as magma drains laterally from the magma chamber
  • Examples: Mauna Loa and Kilauea### Yellowstone-Type Calderas
• 630,000 years ago, Yellowstone erupted 1,000 km3 of pyroclastic material, creating a caldera 43 miles across.
• The eruption deposited ash as far away as the Gulf of Mexico.
• The ash deposit formed the Lava Creek Tuff, which is 1,200 feet thick in some locations.
• Magma accumulation led to pressure on the overlying rock, causing gas-rich magma to produce vertical fractures and a ring of eruptions.
• The center of the ring collapsed, forcing more magma to the surface.
• This process produces the largest calderas.
• Three caldera-forming episodes have occurred in the Yellowstone region over the last 2.1 million years.

Volcanic Activity

• Volcanic activity is not only associated with volcanoes.
• The largest volume of volcanic material is extruded through fractures in the Earth's crust, called fissures.
• Fissures produce low-viscosity basaltic lava that can cover a large geographic area.
• Examples of flood basalts include the Columbia Plateau (western US), Deccan Trapps (India), and Ontong Java Plateau (Pacific Ocean floor).

Volcanic Pipes and Necks

• Volcanoes on land are continually weathered and eroded.
• The rock of the volcanic pipe is more resistant to weathering than the surrounding pyroclastic material.
• This results in the pipe standing above the surrounding terrain, forming a volcanic neck.
• Examples include Shiprock, New Mexico.

Igneous Rocks

• Volcanic eruptions receive most of the media attention, but most magma crystallizes inside the Earth's surface.
• These geologic structures are called plutons.
• Tectonic activity and erosion can expose plutons above the Earth's surface.
• Plutons vary in size and shape and can be classified as tabular or massive.

Dikes and Sills

• Dikes are sheet-like bodies produced when magma is injected into fractures that cut across rock layers.
• Sills are tabular plutons formed when magma is injected along sedimentary bedding surfaces.
• Both dikes and sills can have a significant impact on the surrounding rock.

Igneous Batholiths

• Batholiths are intrusive bodies with a surface exposure of more than 100 km2.
• They are by far the largest intrusive igneous bodies.
• Batholiths are typically discordant and can be hundreds of kilometers long and wide.

Magma Generation

• Seismological evidence indicates that the Earth's crust and upper mantle are composed primarily of solid rock.
• Magma generation occurs through partial melting of existing rocks.
• The melting point of a rock is a function of three factors: heat, pressure, and volatiles.
• The geothermal gradient increases with depth, with temperatures reaching 1,200°C to 1,400°C at 100 km within the Earth.

Distribution of Volcanic Activity

• The global distribution of volcanic activity is not random and falls within three general groups:
  • Ring of Fire (around the Pacific Ocean)
  • Volcanoes of the deep-ocean basin
  • Irregularly distributed on the interiors of continents
• The distribution of volcanoes is connected to plate tectonics.

Plate Tectonics and Volcanic Activity

• Plate motion provides the mechanism necessary to result in the melting of mantle rocks, generating magma.
• Three zones of volcanic activity and plate boundaries:
  • Convergent plate boundaries
  • Divergent plate boundaries
  • Areas not associated with plate boundaries (intraplate activity)