Volcanism: Cascades vs. Hawaii

This lecture focuses on two distinct areas of volcanism with National Parks, highlighting two different types of volcanic activity: the Cascade Range (specifically Mount St. Helens and Crater Lake) and the Hawaiian Islands (specifically the Big Island, Mauna Loa, and Kilauea).

Recent Volcanic Activity

Kilauea, Hawaii: A very recent eruption occurred last Friday (September 19, 2024), part of an ongoing eruption period that began in December 2023. Video footage of this event will be shown to compare with other eruptions.
Mount St. Helens, Cascades: Footage from the 1980 eruption will be shown, despite its low resolution, to illustrate explosive volcanism.

Magma Formation Review

Magma formation primarily occurs at plate boundaries.

Subduction Zones (Cascades)

Location: The Juan de Fuca Plate is subducting beneath the North American Plate. This is the correct association for volcanoes directly with plate boundaries in the Cascades.
Mechanism: Magma forms due to the dewatering of the subducting plate. The input of volatiles (water) weakens the bonds in the rock of the overriding plate, causing a melt to form.
Reminder: Reviewing the three causes of magma formation is crucial for the exam. This includes: adding water (flux melting), increasing temperature (heat transfer melting), and decreasing pressure (decompression melting).

Hotspots (Hawaii & Yellowstone)

Location: Hotspots are located under crustal plates. For Hawaii, the hotspot is under oceanic crust. For Yellowstone, the hotspot is under continental crust.
Mechanism: Magma forms at hotspots due to increased temperature from a large mantle plume rising through the mantle. The heat melts the rocks.

Magma Characteristics and Viscosity

Cascades Volcanism (Subduction Zones)

Magma Evolution: As magma forms and rises, it cools. During cooling, mafic minerals (rich in magnesium and iron) crystallize first and are lost from the melt.
Chemical Change: The remaining magma becomes progressively more felsic (rich in silica, aluminum, potassium, sodium).
Viscosity: More felsic and cooler magma is stickier and thicker, meaning it has a higher viscosity. Viscosity is the resistance to flow.
Impact: This high viscosity directly influences the type of volcanism seen in the Cascade Range.

Hawaii Volcanism (Hotspots)

Magma Source: The magma originates from a mantle plume under relatively thin oceanic crust.
Chemical/Thermal Nature: The magma does not travel as far or cool as much as in subduction zones. Therefore, it remains chemically more mafic and generally hotter.
Viscosity: This mafic and hot magma is runnier and less viscous, meaning it has a low viscosity.
Impact: Low viscosity magma leads to effusive, flowing eruptions characteristic of Hawaiian volcanoes.

Volcano Types and Eruption Styles

Cascade Range: Stratovolcanoes (Composite Volcanoes)

Characteristics:
- Large, cone-shaped mountains with steep slopes (e.g., Mount St. Helens, Mount Rainier).
- Formed by successive eruptions of viscous lava flows and pyroclastic material.
Eruption Style:
- Explosive eruptions are common due to high-viscosity magma trapping large amounts of gas (similar to Yellowstone's eruption style).
- These eruptions involve steam, ash, gases, and pyroclastic flows.
Examples: Mount St. Helens, Mount Rainier, and Mount Mazama (which formed Crater Lake).

Mount St. Helens Eruption (May 18, 1980)

This was a pivotal event in US geological history:

Pre-Eruption Signs: Witnessed opening of a huge, several-mile-long fracture on the south face, followed by the north half of the volcano vibrating while the south half remained still.
Landslide: Triggered by a $M_w 5.1$ earthquake, the entire northern slope collapsed in the largest recorded landslide in history.
- It covered $20$ square miles ( $52~ ext{km}^2$ ) of the Toutle River Valley to an average depth of $150$ feet ( $46~ ext{m}$ ).
Eruption Mechanism: The flank collapse effectively unroofed a body of magma and a jacket of superheated groundwater that had accumulated inside the volcanic edifice.
- This sudden relief of pressure caused the water in both bodies to flash to steam in a violent explosion.
Lateral Blast: A direct, sideways blast of gas and rock fragments, traveling at speeds upwards of $600$ mph ( $966~ ext{km/h}$ ), moved ahead of the debris avalanche. It appeared like a