Chapter 5: Volcanoes & Hazards

Why some volcanoes boom (explosive) and others ooze (effusive)

Viscosity (stickiness) + gas + temperature decide eruption style

Mafic, hot, low gas

runny & quiet (Hawaii)

Felsic, cooler, gas-trapping

sticky & explosive (Mt. St. Helens)

Viscosity

how thick or runny something is.

Thick lava (high viscosity)

moves slowly and can trap gas → explosive eruptions

Runny lava (low viscosity)

flows easily → gentle/effusive eruptions

Types of volcanoes (shapes = lava style)

• Shield

• Cinder cone

• Composite/Strato

Shield

broad, gently sloped; basalt, runny (Hawaii)

Cinder cone

small, steep, one-and-done; made of loose cinders (Parícutin)

Composite/Strato

tall, steep, layers of lava + ash; explosive (St. Helens, Fuji)

Stuff that comes out of volcanoes

• Lava

• Tephra

• Gases

Lava

• Pahoehoe

• As

• Pillow 

Lava: Pahoehoe

Smooth, ropy

Lava: Aa

rough

Lava: Pillow

Underwater blobs

Tephra (pyroclastic) 

Ash, lapilli, bombs/blocks 

Gases

Mostly H₂0, plus CO₂, SO₂

Volcano anatomy vocab

• Conduit

• Vent

• Crater

• Caldera

• Fumarole 

Conduit

Pipe

Vent

Opening

Crater

<1 km

Caldera 

>1 km collapse

Fumarole

gas vent

Hazards (ranked by danger to people)

1. Pyroclastic flows

2. Lahars

3. Ash fall

4. Lava flows

5. Gas

Pyroclastic flows

(1st) Super hot avalanches of ash/gas (fast & deadly)

Lahars

(2nd) mudflows of water + ash (like wet cement)

Ash fall

(3rd) roofs/planes problems

Lava flows

(4th) wreck property, usually slow enough to escape

Gas

(5th) can suffocate/acidify

Where volcanoes happen (plate tectonics)

• Divergent

• Convergent

• Hot spots

• Rifts 

Divergent 

basalt, pillows (mid-ocean ridges)

Convergent 

andesite/rhyolite, explosive arcs (Ring of Fire)

Hot spots

in the middle of plates (Hawaii, Yellowstone)

Rifts

mixed magmas (East Africa)

How we predict

Watch for earthquake swarms, ground bulge, gas changes, heat. Use tiltmeters, GPS, gas sensors, satellites