Notes On Module 6 – Natural Hazards

Resource Description

Notes for Module 6 Hazards. Covers all the main hazards, some with case studies.

Ash Eruptions
Ash is a product of explosive volcanic eruptions – made of tiny fragments of jagged rock, minerals and volcanic glass.
It is hard, abrasive and does not dissolve in water.
Can be powdery and sand-like.
Plumes of volcanic ash can spread over large areas of sky, turning daylight into complete darkness and drastically reducing visibility.
Volcanic ash and gases can sometimes reach the stratosphere (upper layer of atmosphere).
Reflects incoming radiation and creates cooling temperatures.
“Volcanic winters” – in extreme cases the debris blocks out so much sunlight and global temperatures can cool eg. 1883 Krakatoa eruption lowered global temperatures by 1.2°C for 3 years.
Ash can:
Destroy/disrupt machinery
Melt inside aeroplane engines
Contain carbon dioxide and fluorine
Lead to crop failure and animal death
Ascend hundreds of km into atmosphere and stay for years
Pyroclastic flows: explosive eruptions can also create an avalanche of ash, volcanic gases and rock.
These can be impossible for humans to outrun.
They are capable of razing buildings and uprooting trees.
Up to 1000°C making it frictionless and can move up to 700km/hr.
Blast and burn everything in their way.
A benefit: Andisol is a type of soil formed from volcanic ash. Andisols are generally very fertile and support extensive agricultural development.

Lava Flows
Lava flows are the least hazardous process and vary depending on their silica content, temperature, extrusion rate and slope of the land.
They cover roughly 70% of the Earth and are very common on other terrestrial planets, covering 90% of Venus and 50% of Mars.
Pahoehoe: Smooth and continuous lava. Lower effusion rate = slower. Low viscosity. Basaltic lava only. Thin flows (1-3m)
Not very dangerous BUT very destructive. You can walk away from it but you cannot stop what it destroys.
A’a: Rubbly and chunky lava. High effusion rate = faster. Generally basalt. Thicker flows (3-20m)
Composition of pahoehoe and A’a can be the same: pahoehoe lava can turn into A/a lava if it speeds up. A’a cannot become pahoehoe.
Blocky: Even more chunky. Even faster. Very thick flow (20-100m). Can be andesitic, tephritic. Needs more silica.
Pillow lava: Usually basaltic or andesitic. Always in water. Rounded curved shapes.

Lahars
Lahars are a type of flooding that happens in volcanic eruptions when the volcano has snow on it, it is raining or if there are rivers/dams nearby.
1984 Columbian volcano Nevada de Ruiz – 50m deep. Hit the town of Amero 100km from volcano at night. 23,000 people killed in minutes.