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Volcanoes

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Volcano

Volcanoes represent spectacular releases of energy from inside the earth’s crust and upper mantle to the surface. A volcano is a typical landform that denotes an opening or fissure on the earth crust through which hot molten magma (lava), gases, molten rock and ashes are ejected onto the surface. Today, there are about 500 active volcanoes on the earth’s surface that are closely associated with plate boundaries.

Why does eruption happen?
Volcanism is primarily associated to plate boundaries. When immense amount of pressure builds up beneath the crust due to the melting of the subducting plate materials that creates huge amount of magma beneath the surface, volcanoes  erupt to readjust the pressure and to release the developed stress.
Hot-spot volcanism also known as mid-plate volcanism though may not be near to the plate boundary. These are associated to the streams of hot deep layer material from the core-mantle boundary that flows upward as mantle plumes. Fissure eruption takes place along the divergent or constructive boundary marked by the mid-oceanic ridges and rift valleys. These are basaltic runny lava created by the upwelling of the mantle convection current.

Click here to see National Geographic photo collection on Volcanoes

Spatial distribution of volcanoes

Prepare all the topics of this Random Wheel in groups and get ready for the round 

Pacific Ring of Fire

Distribution of volcanoes

Pacific ring of fire

What comes out of a volcano?

GasesSteam of water, carbon dioxide and sulfur dioxides, carbon monoxide, hydrogen sulphide etc
LiquidMolten lava – may be either fluid or viscous (sticky). Acid lavas (on the continents at the convergent or destructive margin, called Rhyolite and Andesite lava) are viscous even at high temperatures; trapped gasses inside the bubble often explode violently. Basic lava or basaltic lava (on the divergent margins of the ocean floor) are fluid. Volcanic muds are called lahars.
Solid
Solid materials of
varying grain size (volcanic bomb to ash) ejected into the atmosphere are called Tephra or pyroclastic materials.		Pyroclastic material are mostly solid volcanic material consist of super hot ashes more than 700 degree C, lapilli or cinder (walnut-sized pieces of volcanic rock), Lava bombs (volcanic rocks larger than 64 mm in size) and pumice which may still have evidence of the bubbles of gas trapped as the rock solidified.
Nuées ardentes or pyroclastic flows are fast moving (500 km per hour) hot ashes and other material thrown out from an erupting volcano. Often incandescent (light emitting)
Active, dormant and extinct volcanoesActive- These volcanoes have erupted in the last 80 years. Dormant- Resting, but may erupt in near future. Extinct- dead, will not erupt again.

Classification of volcanoes is done mainly on the basis of the shape of the volcano and its vent and the nature of eruption.

Major types of volcanoesFeatures
Fissure eruptions (basaltic)Volcanic eruption can take place through long cracks called fissure. When two plates move apart basaltic lava may flow considerable distance over gentle slopes. e.g. Laki eruption in Iceland.
Composite or stratovolcano (andesitic)Steeper slopes volcanoes with high viscosity lava and are more explosive than shield volcano. They are marked by well developed central vent and alternating layers of lava and pyroclastic material e.g. Mt. Etna, Mt. Fuji in Japan, Mt. Vesuvius in Italy.
Shield volcano (basaltic)Low, flat, gently sloping volcanos built from fluid, low-viscosity basaltic lava. Mainly formed on the ocean-floor often at the constructive plate margins or at the oceanic hotspots. Thin lava flow built up over a central vent creating gentle upper slope, e.g. Mauna Loa in Hawaii.
Dome volcano (acid lava/rhyolitic)Explosive with highly viscous acid lava e.g. Mt. Pelee

Creation of a new island in the middle of the Pacific Ocean in the Solomon Islands

Fissure eruption in Hawaii

https://www.youtube.com/watch?v=B8hFAvsknnk

Shield volcano facts
Slope angles GentleSteep
Plate boundary Constructive margins, mid-plate volcanoesDestructive margins with an oceanic plate
Products Mostly lavaLava and pyroclastic materials
Lava viscosity Non-viscousViscous
Type of eruptionContinuous and non-violentExplosive with dormant phases

Lava flows from Hawaii’s Kilauea volcano

 

Crater vs Caldera

CraterCaldera
Circular depression usually less than 1 km in diameter. Composite and shield volcanoes have crater at the summit.Is a huge crater caused mainly due to collapse of a part of the volcanic cone into it’s party empty magma chamber after a huge eruption. Subsidence and widening of the crater enlarge the opening to several kilometers. E.g. Yellowstone caldera in the Yellowstone National Park in the USA is considered an active volcano. It has erupted with tremendous force several times in the last two million years. The caldera lie over a hot spot which produced the latest three super-eruptions from the Yellowstone hotspot. According to the analysis of earthquake data in 2013, the magma chamber is 80 km long and 20 km wide and may create another super-eruption in future.

Case Study: The Eruption of Eyjafjallajoekull in Iceland in 2010

Events/volcanoesPlatesTypes of plate boundary and causes of the volcano
2010 volcanic events of Eyjafjallajökull in Iceland Eurasian and North American plates Iceland lies on the Mid Atlantic Ridge, a constructive plate margin separating Eurasian and North American plates. As the plates move apart, fissure is created and magma fills the magma chamber below due to the mantle convection current.

Mount St. Helens eruption 2008

Mount St. Helens is a stratovolcano in Washington, the Pacific Northwest region of the United States.		Sits on the plate boundary between Juan de Fuca and the North American plates Part of the 'Ring of Fire' volcano. Ring of fire is a horseshoe-shaped seismically active belt of earthquake epicentres and the volcanic chain surrounding the Pacific Ocean. It is marked by destructive or convergent plate boundaries.
Mount Vesuvius Eurasian and African tectonic plates.Vesuvius is located on the west coast of Italy near Naples and is the only active volcano on mainland Europe. As part of the Campanian volcanic arc, it formed over the subduction zone created by the collision of the Eurasian and African tectonic plates. African plate is sinking underneath the Eurasian plate creating a convergent/ destructive plate boundary.

The cataclysmic eruption of Mount Vesuvius in 79 AD  (Pompeii) was one of the most spectacular and destructive events in ancient history. Click here to see the pictures of the ancient roman city of pompeii and the impact of the eruption. 

https://www.youtube.com/watch?v=Y1HCkKrQLn0

Location of Mt. Vesuvius

Hazards associated with tectonic events

Earthquake and related hazards– ground displacement, landslides, liquefaction, tsunami, destruction and loss of lives and property.
Volcano and related hazards– explosive blast, lava flows, ash flows and ash falls, mud flows, lahar, hot avalanches or Nuées ardentes, ground displacement, landslides, climate change, destruction, loss of lives and property etc.

Why do people live near volcanoes?

More than 500 million people (approximately 7% of the world population ) live close to volcanoes and in areas at risk from volcanic hazards. Major cities like Mexico City in Mexico is only 50 miles away from the active volcano of Popocatapetl.

Fertile soilVolcanic rocks are rich in minerals, but when the rocks are fresh the minerals are not available to plants. The rocks need thousands of years to become weathered and broken down before they form rich soils. The slopes of Vesuvius in Italy have very productive soils. The area is intensively cultivated and produces grapes, vegetables, herbs, flowers and has become a major tomato growing region.
Mineral MiningMost of the metallic minerals mined around the world, particularly copper, gold, silver, lead and zinc are associated with rocks found deep below extinct volcanoes. Most of sulfur mine is located around active volcanoes.
TourismVolcanoes and related features like hot springs, bubbling mud pools and steam vents and geysers are always popular tourist attractions, such as Old Faithful in the Yellowstone National Park, US attract millions of visitors every year. Tourism creates jobs and supports local economy.
Geothermal energyIt is derived from the Earth’s internal heat. Tapping of naturally occurring hydrothermal convection to generate electricity is very common in Iceland and in New Zealand. It is commonly used for heating and cooking.
OthersOvercrowding and shortage of land, Poverty, inertia to relocate and boding to the soil etc.

What can be done to minimize the risk from volcanoes?

Lava flow diversionCanal or lava channel is dug to divert lava flow.
Cooling of lava to stop flowingSalty water is sprayed to cool down and solidify lava
Mudflow barrierWalls built across valleys to trap mudflows to protect settlements and property.
Volcano monitoringResearch aircraft, satellites and remote sensing, observation borehole measures composition of escaping gases, changes in temperatures and swelling of ground. Magnetometer measures changes in local magnetic fields.
Hazard mapping and planningPast pattern of eruption is projected to predict the future prediction. High risk areas are identified to increase community preparedness and mitigation responses. Mitigation is the effort to reduce loss of life and property by lessening the impact of disaster.

Dealing with volcanic eruption in Hawaii

Supervolcano
A super volcano is capable of producing exceptionally large volcanic eruptions of ejected mass greater than 1 quadrillion kilogram (means 10 to the power 15 = 1,000,000,000,000,000). Yellowstone National Park and Old faithful geyser in the U.S. state of Wyoming

 

Types of VolcanoShield Volcano with basaltic magmaComposite or stratovolcano with andesitic magmaLava dome with rhyolitic magma
LocationConstructive margin at the ocean ridges
Hot spots		Destructive margin with an oceanic plateOn the continental crust
StructureBroad, gently slopingGiant steep sided, symmetrical coneSteep sided cone, irregular shape
Type of eruptionRegular and mostly non explosive, very little pyroclastic material eruptedExplosive with dormant phases,Mainly rhyolitic
Lava typeUsually Basaltic, quiet eruption of fluidlavaMostly Andesitic (acidic), explosive eruption with viscous lava and pyroclastic flowHigh silica content, very viscous lava, often explosive, quickly solidifies on exposure to air
ExampleMauna Loa in
Hawaiian Islands		Mt. Fuji in Japan, Mt. Vesuvius in Italy, Mt. St Helen in USA
Pacific ring of fire also called Andesite line (75% of world volcano)		Lassen peak, California
Mt.Pele in French Caribbean Island of Martinique.

Drone inside a crater

Caldera: Yellowstone Supervolcano

Super volcanoes (Understanding VEI): Documentary

Yellowstone National Park in Wyoming

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