Презентація на тему «Volcanoes: Eruptions and Hazards»

Volcanoes: Eruptions and Hazards

What is a volcano?
A volcano is a vent or 'chimney' that connects molten rock (magma) from within the Earth's crust to the Earth's surface.
The volcano includes the surrounding cone of erupted material.
vent
cone
magma chamber
conduit

What causes volcanoes?
The Formation of Magma
Mantle rock melts when the temperature increases
or the pressure decreases.

What causes volcanoes?
Where Volcanoes Form
Tectonic Plate Boundaries!!!
~75% world's active volcanoes in Ring of Fire

Plates
Earthquakes
Volcanoes
Volcanoes and Earthquakes are found on places where plates meet

What causes volcanoes?

What causes volcanoes?
Hot Spots

How and why do volcanoes erupt?
Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface.
Same principle as hot air rising, e.g. how a hot air balloon works
When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts.

Large amounts of gas and a high viscosity (sticky) magma will form an explosive eruption!
Think about shaking a carbonated drink and then releasing the cap.
Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruption
Where the magma just trickles out of the volcano (lava flow).
How and why do volcanoes erupt?

The Composition of Magma Determines whether it is explosive or not!
High content
More likely to bettt!!!
High t content
More likely to be ttt!!!
Why?
Silica has a thick, stiff consistency
Flows slowly
Tends to Harden in the volcano's vent
water
silica
EXPLOSIVE
EXPLOSIVE

Pyroclastic material
Rock fragments created by tt eruptions
magma explodes from volcano and solidifies in the air
existing rock is shattered by powerful eruptions
Volcanic blocks
Volcanic bombs
Lapilli
Volcanic ash
EXPLOSIVE
What Erupts from a Volcano?

What Erupts from a Volcano?
Lava can be
thick or thin.
Blocky lava
Pahoehoe
Aa
Pillow lava

How do volcanoes affect the Earth?
Flows and Fallouts
hot ash can flow really quickly
Knock down buildings
Dam rivers (flooding/drought)
Kill crops and livestock
Climatic Changes
Ash & Gases can block sunlight
Drop average global temperature noticeably

Types of Volcanoes
Shield volcano
Cinder cone volcano
Composite volcano

Craters, Calderas, and Lava Plateau
Crater
From explosions of material out of the vent and the collapse of material back into vent
Caldera
Much larger depression that forms when magma chamber empties and its roof collapses
Lava Plateau
Forms when lava erupts from long cracks, or fissures, and spreads out evenly (thousands of km)

Explosive Eruptions
Mt. Redoubt
Explosive volcanic eruptions can be catastrophic
Erupt 10's-1000's km3 of magma
Send ash clouds >25 km into the stratosphere
Have severe environmental and climatic effects
Hazardous!!!
Above: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska

Three products from an explosive eruption
Ash fall
Pyroclastic flow
Pyroclastic surge
Explosive Eruptions
Pyroclastic flows on Montserrat, buried the capital city.

Direct measurements of pyroclastic flows are extremely dangerous!!!

Effusive Eruptions
Effusive eruptions are characterised by outpourings of lava on to the ground.
Hawaii
Courtesy of www.swisseduc.ch

Pyroclastic flow
Lahars/Mud flows
Pyroclastic fall
Lava flow
Noxious Gas
Earthquakes
Volcanic Hazards
Courtesy of www.swisseduc.ch

Pyroclastic Flow
For example, eruption of Vesuvius in 79 AD destroyed the city of Pompeii

Pompeii (79AD)
On August 24, 79AD Mount Vesuvius literally blew its top, erupting tonnes of molten ash, pumice and sulfuric gas miles into the atmosphere. Pyroclastic flows flowed over the city of Pompeii and surrounding areas.

Pompeii (79AD)
Pyroclastic flows of poisonous gas and hot volcanic debris engulfed the cities of Pompeii, Herculaneum and Stabiae suffocating the inhabitants and burying the buildings.

Pompeii (79AD)
The cities remained buried and undiscovered for almost 1700 years until excavation began in 1748. These excavations continue today and provide insight into life during the Roman Empire.

How do pyroclastic flows cause devastation?

Pyroclastic Flow - lahars
Hot volcanic activity can melt snow and ice
Melt water picks up rock and debris
Forms fast flowing, high energy torrents
Destroys all in its path

Pyroclastic Fall
Ash load
Collapses roofs
Brings down power lines
Kills plants
Contaminates water supplies
Respiratory hazard for humans and animals

Lava Flow
It is not just explosive volcanic activity that can be hazardous. Effusive (lava) activity is also dangerous.

Volcano Monitoring
Volcano Observatories are set up on all active volcanoes that threaten the human population. These are designed to monitor and potentially to predict the eruptive behaviour of the volcano in question.

Seismicity
Deformation
Gas Output
(on volcano and remote sensing techniques)
These three things are the most important precursors to an eruption.
Volcano Monitoring

Gas samples are collected from fumaroles and active vents.
Gas levels may also be monitored by remote sensing techniques
Gas Monitoring

In Summary..
Volcanoes are extremely hazardous.
However, the volcano can be studied, monitored and understood.
Each volcano is different, and offers a unique set of dangers
Plans may be emplaced to help control potential damage.