Topic 5 - Concepts and CS Flashcards
Hazard
Event that has a negative impact on humans and the environment, includes substantial loss of life and damage to property
constructive plate margin
two plates moving away (diverging) from each other
Destructive plate margin
Oceanic x continental
Subduction zone
(eg Pacific Ocean of South America)
Oceanic plate is denser than continental plate and so subducts beneath it. Bending oceanic plate at point of collision forms a deep oceanic trench such as the Peru=Chile trench.
As the plates converge the continental plate is uplifted, compressed and then buckles forming folded mountains like the Andes.
The oceanic plate melts then is fully destroyed, this melting zone is called the Benioff zone. The heat is caused by friction as well as the increasing heat. The friction builds tension which is released as an immediate or deep-focus earthquake.
Destructive plate margin
Oceanic x oceanic
Subduction zone
(eg marine trench, western pacific)
The faster or denser plate subducts beneath the other forming a deep ocean trench
Subducted plate melts due to heat from asthenosphere and friction
Friction causes both shallow and deep focus earthquakes
Magma from subduction is silica-rich andesitic magma with high gas content making it explosive
Explosive magma rises through cracks in the continental plate forming strata volcanoes
Destructive plate margin
Continental x continental
Collision zone
(eg Himalayas)
Continental plates have lower densities than the asthenosphere below them.
Neither plate subducts as they have similar densities, instead they collide and crumple upwards forming fold mountains
No volcanoes as there’s no subduction
Violent earthquakes due to constant friction + build-up of tension
conservative plate margin
two plates moving sideways past each other
No volcanic activity as there is no subduction, no new crust etc. Margin associated with powerful earthquakes. Friction between two moving plates builds tension which is suddenly released, as there is no subduction powerful shallow-focus earthquakes are produced
Volcanic hazards
Explosive eruptions
Associated with destructive margins and formation of composite volcanoes
Magma is viscous due to high silica content and explosive due to gas
Volcanic hazards
Pyroclasts
Large angular fragments + aerodynamic volcanic bombs ejected from explosive eruptions = pyroclasts
Can crush people, buildings and cars
Volcanic hazards
Tephra + Volcanic ash
CS Mt Sinabung
Can rise and form an eruption column
Can be carried by the wind and spread over long distances
Ash can collapse roofs and cover vegetation affecting crops + harvest
CS Aug 2010, Jan 2014 Mt Sinabung erupted, blanketing cities in North Sumatra
Volcanic hazards
Pyroclastic flow
Fuego volcano
Occurs when eruption column collapses
Flow comprised of hot rock, ash and volcanic gases, flows down flanks of volcano, can have hurricane force speeds
Most destructive volcanic hazard
CS Fuego volcano, Guatemala erupted causing flows that killed dozens.
Volcanic hazards
Nuee Ardentes
Mt Pelee
Glowing avalanches, pyroclastic flow
Caused by lateral volcanic eruptions where summit crater is blocked
Contains dense material, only travels 50km from the volcano
they are higher density than pyroclasts
CS Mt Pelee erupted, destroying St Pierre in 1902 killing 29,000 people
Volcanic hazards
Lahars (secondary)
Armero
Dangerous mudflows formed when water from rivers/snow-capped mountains mixes with volcanic ash + fragments
Creates fast flowing hazard as it slides down slopes of volcanoes
CS 20,000 people died in Armero when lahars from an eruption engulfed the town in 1985
Volcanic hazards
effusive eruptions
Mt Kilauea
Associated with constructive plate margins, hot spots and formation of shield volcanoes
Form low-viscosity basaltic lava, flows over long distances
CS Kilauea, Hawaii is the most active volcano. It inundated homes in Kalapana and destroyed over 700 homes in 2018, erupted again in Oct 2021
Volcanic hazards
volcanic gases
Mt Pinatubo
Include hydrochloric acid which condenses with water as well as sulphur dioxide
Sulphur dioxide can cause acid rain, ozone depletion and air pollution. Also forms volcanic smog eventually, which damages vegetation
CS Mt Pinatubo ejected 20 mil tonnes of Sulphur dioxide in 1991
Spatial distribution of volcanic activity
Volcanic activity is common at destructive and constructive plate margins, absent from conservative margins and collision zones. Some volcanoes occur at the centre of plates, like the Hawaiian hot spot. Volcanoes are common along rift valleys.
How can the magnitude of eruptions be measured
Can be measured using the Volcanic explosivity index:
Measures relative explosivity of an eruption
Logarithmic scale
High on the scale = high the volume of ejected material
Measures magnitude of eruption
Frequency of volcanic eruptions
St Maria
Some active volcanoes erupt once every 100 000 years, others erupt every month.
Generally, less frequent eruptions have a greater magnitude and more damaging.
CS - Saint Maria, Guatemala is a frequently erupting stratovolcano
However, it had eruptions ranking 6 on the VEI + lahars during rainy seasons
Randomness vs regularity of volcanic eruptions
Kilauea
Some volcanoes erupt at regular intervals; others may be dominant for thousands of years but randomly erupt multiple times in a quick succession.
Kilauea erupts once every 2-3 years
CS - Mt Merapi, Java
most active volcano in Indonesia, 2010
Most active volcano in Indonesia
Stratovolcano with frequently violent eruptions
Tens of thousands of people farm on the fertile flanks, at risk of its eruption
Oct 2010, after more than 5000 earthquakes were recorded beneath the volcano, evacuation of 20,000 was advised
Powerful pyroclastic flows down the flanks of the volcano followed a series of eruptions
Fires, respiratory failures, and blast injuries caused 350 fatalities
350,000 were displaced as their homes were destroyed and farmland was covered in thick ash. Ash also led to aviation problems across Java
Hazard mitigation
Anything done to reduce severity or impacts of a hazard
spatial distribution of earthquakes
Location of earthquakes is closely associated with plate margins. There is a high frequency of earthquakes along the Pacific great ring of fire, extremely concentrated where the Pacific plate meets the Indo-Australia plate at a conservative and destructive margin.
Earthquakes at destructive margins
Subduction results in deep focus earthquakes, subduction also results in a wider horizontal spread as the oceanic plate sinks at an angle, deep into the asthenosphere. These earthquakes are powerful as compressional forces are greatest in the Benioff zone. They can occur under the sea close to populated coastal zones, making the threat of tsunamis more likely. Tend to have higher concentration of earthquakes compared to conservative margins, eg N and W edge Pacific ring of fire
