What are the main hazards generated by volcanic activity? Flashcards
(17 cards)
EXPLOSIVE eruptions
- CONVERGENT plate boundaries
LAVA TYPE & CHARACTERISTICS:
- Rhyolite (more acid)
- Andesite (less acid)
- Acidic
- High viscosity
- Low temperature at eruption
STYLE OF ERUPTION :
- Violent bursting of gas bubbles
- Highly explosive
- Vent and top of cone often shattered
MATERIALS:
- Gas
- Dust
- Ash
- Lava bombs
- Tephra
- LOW FREQUENCY
SHAPE :
- Steep sided strato-volcanoes
- Calderas
EFFUSIVE eruptions
- DIVERGENT plate boundaries
LAVA TYPE & CHARCTERISTICS :
- Basalt
- Basic
- Low viscosity
- High temperature at eruption
STYLE OF ERUPTION:
- Gas bubbles expand freely
- limited explosive force
MATERIALS:
- Gas
- Lava flows
- HIGHER frequency
SHAPE:
- Gently sloping sides (shield volcanoes)
- Lava plateaux
PRODUCTS of explosive eruptions:
- strato volcanoes
- minor igneous features
STRATO/COMPOSITE CONE VOLCANOES:
- Formed by layers of ash & acid lava
- Concave symmetrical profile
Complex internal networks of lava flows form minor igneous features:
SILLS ~ approximately HORIZONTAL, minor intrusion of magma surrounding older rock
DYKES ~ approximately VERTICAL, minor intrusions of magma surrounding older rock
PRODUCTS of explosive eruptions:
- acid magma
- Calderas
ACID MAGMA:
- Does not flow easily
- Vents fill with a mass of solidified magma
- enormous pressures build up
- Eventually erupt explosively
CALDERAS:
- Volcanic craters
- 2km in diameter
- Formed when the cone is destroyed & underlying magma chamber is largely emptied
- Unsupported sides of the volcano
collapse
PRODUCTS of effusive eruptions:
- Lava plateaux
- Shield volcanoes
LAVA PLATEAUX:
- Basic lava erupts from multiple fissures
- Cover vast areas with free flowing lava
- Event known as FLOOD BASLATS
- Example ~ Deccan Plateau, Central India ~
covered 500,000km^2
SHIELD VOLCANOES:
- Broad, low profile volcanic landforms
- Eruptions of basic lava result in volcanoes with gently sloping sides
- Successive flows can accumulate forming huge volcanoes
- Example ~ Skjaldbreidur, Iceland ~ 11km
What are hot spots?
- Intra-plate volcanoes
- A fixed area of intense volcanic activity
- where magma from a rising mantle plume, reaches the Earths surface
Examples of hot spots:
HAWAIIAN ISLANDS
HAWAIIAN ISLANDS:
- Lies at centre of Pacific plate, thousands of km from the nearest plate boundary
Pacific plate moves NORTHWEST causing:
- accumulation of basalt on ocean floor, forming the islands
- eventually, EXTINCTION of volcanoes as they lose their source of magma
Mauna Loa & Mauna Kea:
- Active volcanic peaks
- reach over 4000m above sea level
- rise over 9000m from ocean floor
Kauai:
- Older island with no volcanic activity
- weathering and erosion have broken down volcanic rocks into fertile soils
Examples of hot spots:
EAST AFRICA
EAST AFRICA:
- 4000 km long RIFT VALLEY
- Over 30 millions years, crust has been uplifted and stretched, causing tension in the rocks
- Causes RIFTING ~ magma forced to surface forming a line of active volcanoes
Super volcanoes:
Definition, examples & impacts
SUPER-VOLCANO ~a volcano that erupts more than 100km^3 of material in a single eruption event
- The evidence of this activity is the existence of GIANT CALDERAS
YELLOWSTONE SUPER-VOLCANO ~ WYOMING:
- Has a caldera measuring 75km in diameter
Super-volcanoes active in the past 2 million years:
- LONG VALLEY ~ eastern California
- TOBA ~ Indonesia
Most Recent super-volcano eruption:
- Taupo, North island, New Zealand
- 27,000 years ago
IMPACT ~ deduced from extent and depth of ash layers and their impact on plant, insect and animal species
Volcanic Explosivity Index ~ VEI
What is it?
What are the limitations and benefits?
Combines :
MAGNITUDE ~ amount of material erupted
INTENSITY ~ speed at which material is erupted
- Single number from 0 to 8
- Each increase in number represents nearly a TEN FOLD increase in explosivity
Several factors are taken into account when assessing explosivity:
- VOLUME of erupted material
- HEIGHT ejected material reaches
- DURATION
- QUALITIVE descriptions
LIMITATIONS:
- Not useful for effusive eruptions
BENEFITS:
- Suggests relative impacts volcanoes may have at different geographical scales
Lava Flows
- Everything in the path of lava will be either burned, bulldozed or buried
- Destroy infrastructure, property & crops
- Rarely cause injuries or fatalities
Impact is dependent on the TYPE of lava ~
BASALTIC:
- free-flowing
- runs for considerable distances
- Hawaii July 2015 ~ a lava flow extended for 20km
ACIDIC:
- e.g Rhyolite
- Thick and pasty
- Do not flow easily
Pyroclastic flows
- A combination of very hot gases (500 degrees+), ash and rock
- Travelling at very high speeds (100km/h)
- Follow the contours of the ground and destroy everything in its path
- Inhalation of these hot and poisonous gas & ash can cause almost instant death
Example: POMPEII ~ overwhelmed by a pyroclastic flow from Mount Vesuvius in AD 79
Tephra
- Describes ANY material ejected from a volcano into the air
SIZE ~ ranges from very fine ash to large volcanic bombs
HAZARDS:
- bury farmland in layers of ash and destroying crops
- collapsed buildings due to weight of ash
- difficulty breathing for those with respiratory diseases
- The ejected material is HOTTER than surroundings, so rises fast into the stratosphere
Lahars
- Type of mud flow
- Consistency of wet concrete
- Snow and ice melt during an eruption and flow rapidly down the cone and MIXES with rock fragments, ash & soil
SPEED ~ 50 km/h
- everything in their path is either destroyed or buried under thick layers of debris
Example: Nevado del Ruiz eruption, Columbian town of Amero was overwhelmed by lahars, causing 23,00 deaths
Floods
- Eruptions beneath an ICE FIELD or GLACIER cause rapid melting
Example: ICELAND
- several active volcanoes lie under an ice field
- during an eruption, vast quantities of water accumulate until they find an exit from under the ice
Tsunamis
- Caused by a VIOLENT eruption of a ISLAN volcano
Followed by:
- massive displacement of ocean water
- tsunami waves capable of travelling
600 km/h
DEEP WATER ~ height of 1m and wavelength of 200km
APROACH SHORE ~ height rapidly increases and transfers large amounts of energy when they break
Example: KRAKATOA
- Created tsunamis, drowning 36,000 people
Toxic gases
- CO, CO2 & SO2
- Pose deadly and silent threats to human populations
SO2:
- Combines with atmospheric water to produce ACID RAIN
- This damages crops and pollutes surface water and soils
Example: LAKE NYROS CAMEROON
- Deep lake that occupies a volcanic crater
- A leak of CO2 caused 1700 people and al animal life to be ASPHYXIATED
