VolcanoesChapter 15 Section 2

What is a volcanoA vent in Earth’s crust through

which melted or molten rock flows.Below ground- magmaAbove ground- lava

Volcanoes can form at plate boundariesConvergent boundaries- denser

plate sinks below the less dense plate.

Divergent boundaries- two plates pull apart especially at the mid-ocean ridge. ( 60 % of volcanoes occur here).

Volcano formation cont.Can also occur at hotspots.Hotspots are located away from

plate boundaries.Hawaiian IslandsGeologists speculate hot spots

occur over rising convection currents.

Oldest volcano far from hot spot

Volcanoes foundThe plate boundaries in the Ring

of Fire ( this basically surrounds the Pacific Plate)

Hotspots◦The Hawaiian Islands◦Yellowstone National Park◦Galapagos Islands

Types of volcanoesShieldCompositeCinder Cone

Shield volcanoesCommon along divergent

boundariesCommon near oceanic hotspotsLarge with gentle slopes

Composite VolcanoesSteep sided volcanoesLargeResult from explosive eruptions

of andesitic lava rhyolitic lavaConvergent boundaries

Mount Ranier

Cinder Cone volcanoSmallSteep-sidedErupt gas rich basaltic lava

Volcanic EruptionMagma chemistry determines

how a volcano will eruptDepends on the amount of water

vapor in the magmaAnd- the amount of silica

content.Difference in silica affect the

magma thickness or viscosityViscosity- the liquids resistance

to flow.

Magma chemistry cont.Low silica content- low viscosity

◦Flows like warm maple syrup◦Forms basalt

High Silica content- high viscosity◦Flows like sticky toothpaste◦Forms andesite and rhyolite

Dissolved gasesAll magmas contain dissolved

gasesAs the magma reaches the

surface the dissolved gases can’t stay dissolved so they bubble or explode like a shaken soda bottle

In high viscosity lavas- the gases can escape so they tend to make explosive eruptions

Effects of volcanic eruptions60 different volcanoes erupt each yearLava flowAsh fallPyroclastic flowMud flows

Lava flowsMove slowlyRarely deadlyCan be damagingOften have to evacuate

Ash fallAsh is pulverized rock and glassCan reach heights of 40 kmDisrupt air travelCan affect air quality and cause

breathing problemsCan affect the climate by

blocking out sunlight causing cooling

MudflowsThermal energy can melt the

snow and ice at the summitThis water can mix with mud and

ash on the mountainThis forms mudflows or lahars

Pyroclastic flowExplosive volcanoesProduce avalanches of hot

moving gas, ash, and rockTravel at speeds more than 100

km/hour

Predicting EruptionsCan be predictedMagma causes ground

deformationSeries of earthquakes usually

precedes eruptionVolcanic gas emission also

increasesGround water near volcano

becomes acidic

Climate changeAsh can block sunlight making

temperatures colder and altering weather

Sulfur dioxide emissions form sulfuric acid droplets which reflect sunlight into space lowering temperatures.