geology week 9 : Plate Tectonics and Plate Boundary Hazards

What is the ‘equation’ for Risk?

Risk = Hazard x Exposure x Vulnerability

Exposure - how close you are to the risk

vulnerability = how will this risk affect you

Lava flow, composition, and viscosity

Lava viscosity depends on the amount of silica present

High viscosity = large silica → thick, chunky, sticky lava

Low viscosity = low silica → runny, fluid, lava

Basaltic, Andesitic, and Rhyolitic lava flows

Basaltic lava = very hot, low silica, low viscosity

• moves rapidly, fluid

Andelisitc lava = intermediate composition, in-between vicious and silica levels

Rhyolitic = felsic, most viscous lava, most explosive and dangerous

Volcanic Materials

Lava

Gas

• low viscosity lava (basaltic) = gas escapes easy

• high viscosity lava (rhyolite) = explosive, difficult for gas to escape

Pyroclastic debris

• fragmental material ejected from volcano → extremely deadly

Eruptive style - will it flow or blow?

Effusive eruptions (flow) → basalt (low silica)

Explosive eruptions (blow) → rhyolite and adesitic composition (high silica → more challenging for gas to escape)

Relation of volcanoes to plate boundaries and rifts

Volcanoes take place usually occur near convergent plate boundaries (subduction zones)

Volcanic hazards

lava flows

tephra

• ash falls around volcano burying landscape.. very heavy and destructive

Pyroclastic flows

• clouds of rolling ash shot up from the volcanic eruption, exteremly fast and kills instantly (so hot) —> Pompeii

Lahars

• ash + water + debris flood

Active vs. dormant volcanoes

Active = erupted in past 10,000 years and likely to erupt soon

Dormant = has not erupted in past 10,000 years but tectonic activity showcases possible future eruptions

Extinct = never to erupt again

Earthquakes

• occur on plate boundaries (subduction zones) where tectonic plates collide

Earthquake hypocenter and epicenter

Hypocenter = place on the fualt plane where earthquale rupture initaites

Epicenter = ground (on surface) right above hypocenter

shorter the distance from the epicenter = more damage!!

Measurement of earthquakes

P-wave (primary) arrives 1st

S-wave (secondary) arrives 2nd

The 1st arrival of P and S waves vary with distance

• distance increase the farther from epicenter

Intensity and Magnitude of Earthquake

intensity = measure of shaking based on damage (in location)

magnitude = actual energy released during an earthquake

damage can be completely difference even if magnitude is the same/similar because of the intensity (damage) and geographical location of the quake!

• ex: solid bedrock regions do not have extra amplification from earthquake while consolidated sediments have a higher destructive intensity (the waves can move through them easier)

Tsunamis

usually occur on plate boundaries where earthquakes occur (convergent plate boundaries — subduction zones)

trust faulting of tectonic plates lifts a huge area of seafloor at once