Plate tectonics
An essay question, answer and assessment
[Time for another Q and A post. This is to highlight the difference between AO1 (Demonstrating knowledge) and AO2 (Applying that knowledge) in the UK examination context.
The geographical context is Plate tectonics – an optional area of content, but a popular one.]
A reminder: for A level Geography, there are two key Assessment Objectives (AOs):
AO1: demonstrate knowledge and understanding of places, environments, concepts, processes, interactions, and change, at a variety of scales.
AO2: apply knowledge and understanding in different contexts to interpret, analyse, and evaluate geographical information and issues.
So, as on previous occasions, here are two versions of the answer.
The first answer is unannotated.
The second answer is in two formats:
(a) AO1 in italics
(b) AO2 in bold.
[Remember my self-imposed limit of 600 words]
All types of plate boundaries generate tectonic hazards for people. To what extent are those at convergent (destructive) boundaries the most damaging?
Tectonic hazards can be primary, such as earthquakes and volcanic eruptions, and secondary, such as tsunami, lahars and landslides. All tectonic hazards have the potential to cause harm to people and their property, leading to injury, deaths and economic losses.
There are several types of plate tectonic boundary, including convergent where plates collide, divergent where plates move apart and conservative or transform where plates slide past each other. It is the case that convergent boundaries are the most damaging, but sometimes earthquakes at conservative boundaries can lead to major disasters, and rare large eruptions at divergent boundaries can lead to damage. Not all convergent boundaries are equally dangerous.
Earthquakes occur frequently along the convergent continent–continent collision zone that has formed the Himalaya Mountains where the Indian and Eurasian plate collide. These include Kashmir in 2005 (killed 80,000+) and Nepal in 2015 (8000 deaths). These earthquakes cause widespread destruction in mountainous terrain, including secondary landslides that often account for 20–30% of all deaths. They happen in low- to middle-income countries (Nepal, Pakistan) which lack preparation and high-quality response in some cases. Therefore, with better management, such as earthquake-resistant buildings, damage could be reduced from future earthquakes.
Subduction zone convergent boundaries are arguably much more dangerous. This is because the largest earthquakes recorded, with magnitudes of 9.0+, have all occurred where oceanic plate is subducted under another plate. These megathrust earthquakes, such as in the Indian Ocean in 2004 and off Japan’s east coast in 2011, cause violent ground-shaking and often have tsunamis as a secondary hazard. In addition, these convergent boundaries are volcanically active, unlike collision zones, because wet partial melting of the subducting plate generates andesitic magma which can erupt as a devastating stratovolcano, such as Mount Merapi in Indonesia in 2010 and Mount Pinatubo in 1991. These eruptions can devastate large areas with pyroclastic flows and lahars. It is the risk from a wide range of tectonic hazards that make subduction zones arguably the most dangerous plate boundaries.
Nevertheless, conservative/transform boundaries can experience very damaging earthquakes. The Gaziantep earthquake on the East Anatolian Fault in Turkey-Syria in 2023 killed over 60,000 people and caused damage estimated at $160 billion. A similar earthquake at Izmit, Turkey in 1999 killed over 20,000 on the related North Anatolian Fault. The damage from these earthquakes is of a similar magnitude to those occurring on convergent subduction zones.
In contrast, divergent margins are rarely as hazardous. This is because at mid-ocean ridges the plate boundary is usually several kilometres below the sea, so volcanic activity is submarine, and plates diverge more gently so earthquakes are rarely above magnitude 7.0. There are exceptions, such as the disruptive eruption of Eyjafjallajökull on Iceland in 2010 which caused air traffic chaos in the North Atlantic but no direct deaths. The eruption of Mount Nyiragongo in the DRC in 2002 killed 300 and made 120,000 homeless, showing that divergent rift valley volcanoes can be deadly, but they are rare.
In conclusion, convergent plate boundaries do have the potential to be the most damaging type, especially in subduction zones because all major tectonic hazards are present. However, as the Himalaya example shows, continent–continent collision can also cause major earthquakes with very high death tolls, but the Himalaya is the only example of this. In contrast, subduction zones are widespread, for example the Pacific Ring of Fire. Conservative boundary earthquakes, which are usually shallow so shaking is intense, can also be very damaging but earthquakes and tsunamis are not present so overall risk is lower. Divergent boundaries rarely generate major hazards, and the examples given are exceptions.
All types of plate boundaries generate tectonic hazards for people. To what extent are those at convergent (destructive) boundaries the most damaging?
Tectonic hazards can be primary, such as earthquakes and volcanic eruptions, and secondary, such as tsunami, lahars and landslides. All tectonic hazards have the potential to cause harm to people and their property, leading to injury, deaths and economic losses.
There are several types of plate tectonic boundary, including convergent where plates collide, divergent where plates move apart and conservative or transform where plates slide past each other. It is the case that convergent boundaries are the most damaging, but sometimes earthquakes at conservative boundaries can lead to major disasters, and rare large eruptions at divergent boundaries can lead to damage. Not all convergent boundaries are equally dangerous.
Earthquakes occur frequently along the convergent continent–continent collision zone that has formed the Himalaya Mountains where the Indian and Eurasian plate collide. These include Kashmir in 2005 (killed 80,000+) and Nepal in 2015 (8000 deaths). These earthquakes cause widespread destruction in mountainous terrain, including secondary landslides that often account for 20–30% of all deaths. They happen in low- to middle-income countries (Nepal, Pakistan) which lack preparation and high-quality response in some cases. Therefore, with better management, such as earthquake-resistant buildings, damage could be reduced from future earthquakes.
Subduction zone convergent boundaries are arguably much more dangerous. This is because the largest earthquakes recorded, with magnitudes of 9.0+, have all occurred where oceanic plate is subducted under another plate. These megathrust earthquakes, such as in the Indian Ocean in 2004 and off Japan’s east coast in 2011, cause violent ground-shaking and often have tsunamis as a secondary hazard. In addition, these convergent boundaries are volcanically active, unlike collision zones, because wet partial melting of the subducting plate generates andesitic magma which can erupt as a devastating stratovolcano, such as Mount Merapi in Indonesia in 2010 and Mount Pinatubo in 1991. These eruptions can devastate large areas with pyroclastic flows and lahars. It is the risk from a wide range of tectonic hazards that make subduction zones arguably the most dangerous plate boundaries.
Nevertheless, conservative/transform boundaries can experience very damaging earthquakes. The Gaziantep earthquake on the East Anatolian Fault in Turkey-Syria in 2023 killed over 60,000 people and caused damage estimated at $160 billion. A similar earthquake at Izmit, Turkey in 1999 killed over 20,000 on the related North Anatolian Fault. The damage from these earthquakes is of a similar magnitude to those occurring on convergent subduction zones.
In contrast, divergent margins are rarely as hazardous. This is because at mid-ocean ridges the plate boundary is usually several kilometres below the sea, so volcanic activity is submarine, and plates diverge more gently so earthquakes are rarely above magnitude 7.0. There are exceptions, such as the disruptive eruption of Eyjafjallajökull on Iceland in 2010 which caused air traffic chaos in the North Atlantic but no direct deaths. The eruption of Mount Nyiragongo in the DRC in 2002 killed 300 and made 120,000 homeless, showing that divergent rift valley volcanoes can be deadly, but they are rare.
In conclusion, convergent plate boundaries do have the potential to be the most damaging type, especially in subduction zones because all major tectonic hazards are present. However, as the Himalaya example shows, continent–continent collision can also cause major earthquakes with very high death tolls, but the Himalaya is the only example of this. In contrast, subduction zones are widespread, for example the Pacific Ring of Fire. Conservative boundary earthquakes, which are usually shallow so shaking is intense, can also be very damaging but earthquakes and tsunamis are not present so overall risk is lower. Divergent boundaries rarely generate major hazards, and the examples given are exceptions.