Volcanoes and Volcanic Hazards

Mount St. Helens, located in southwestern Washington, was a symmetrical peak in the Cascade Range.
In May 1980, it erupted violently, destroying its north flank and creating a large crater.
Volcanoes are nature's most spectacular events, characterized by ash clouds and lava flows.
Volcanic eruptions are geological hazards that have caused numerous fatalities.

Volcanoes vary in size, ranging from small hills to large mountains.
The relative sizes of volcano types vary significantly:
- Scoria cones and domes are the smallest, formed by a single eruption.
- Shield volcanoes and composite volcanoes are larger, built over multiple eruptions.
- Shield volcanoes have gentler slopes compared to cinder cones, domes, and composite volcanoes.
Four main types of volcanoes construct hills and mountains:
- Scoria cones: Cone-shaped hills composed of loose volcanic cinders and bombs; basaltic or andesitic.
- Volcanic domes: Dome-shaped volcanoes composed of solidified lava and volcanic ash; felsic or intermediate composition.
- Composite volcanoes: Symmetrical mountains made of lava flows, pyroclastic deposits, and mudflows; intermediate composition (andesite).
- Shield volcanoes: Broad, gently curved slopes composed of basaltic lava flows; contain craters or fissures along the summit.

Eruption styles vary from explosive to calm, influenced by magma chemistry and gas content.
Magma behaviors:
*Explosive: ejects lava and ash into the atmosphere.
*Nonexplosive: lava erupts from a vent and flows onto the surface.
Lava flows: Occur when magma erupts and flows away from the vent.
Lava fountains: High initial gas content in less viscous lava propels lava into discrete pieces.
Ash eruptions: Explosive ejections of volcanic ash, pumice, rock fragments, and volcanic gas; gas launches ash up to 40 km.
Lava domes: Constructed from viscous lava that piles up around the vent.
Ash fall: Volcanic ash jetted into the atmosphere falls back to Earth.
Pyroclastic flow: Hot cloud of ash particles and gas that tumbles down the volcano's side at high speed and temperature (e.g., 100 km/hr, 800°C).

Gases, such as $H_2O$ , can dissolve in magma due to chemical bonding with silica chains.
Water exists as the OH-$ molecule.
Carbon dioxide (CO2 $) exists as the$ CO3 = $molecule when dissolved, reverting to$ CO_2$$ gas when out of solution.
Magma holds more dissolved gas at high temperatures.
Pressure is the most critical control on gas content; high pressure keeps gas in solution.
A drop in pressure allows gas molecules to form bubbles in the magma, increasing its volume and driving explosive eruptions.

Viscosity affects how fast magma moves and how fast crystals and gas move through it.
Less viscous magmas (basaltic) allow gas bubbles to escape easily, leading to nonexplosive eruptions.
More viscous magmas (high silica content) trap gases, causing pressure to build and potentially leading to explosive eruptions.

Eruptions of basaltic magma form various rocks, landforms, and other features depending on gas content and lava solidification.
Cinder cones: Conical-shaped hills composed of basaltic scoria, formed rapidly.
Basalt flows: Lava flows from the base of a cinder cone.
- Pahoehoe: Lava flows with small-scale, billowing folds and a