Edexcel Specification focus:
‘Causes of intra-plate earthquakes and volcanoes associated with hot spots formed by mantle plumes.’
Introduction
Although most tectonic hazards occur at plate boundaries, some earthquakes and volcanoes happen far from them due to intra-plate activity and mantle hotspots.
Intra-Plate Tectonic Activity
What is Intra-Plate Activity?
Intra-plate activity refers to geological events such as earthquakes and volcanic eruptions that occur within the interior of tectonic plates, away from active plate boundaries.
Intra-plate Activity: Tectonic processes, including earthquakes and volcanism, that occur away from plate boundaries due to stress accumulation or mantle plumes.
This activity often occurs in areas of ancient crustal weakness or where stresses from distant plate boundaries build up over time.
Causes of Intra-Plate Earthquakes
Intra-plate earthquakes are less common than those at boundaries but can still be highly destructive. Their causes include:
Reactivation of ancient faults in stable continental interiors.
Isostatic adjustments, such as rebound following glacial retreat, which can trigger seismic activity.
Stress transmission from plate boundary zones across rigid interiors, concentrating along old fault lines.
Loading effects from large bodies of water (e.g. reservoirs) or sediment accumulation can also increase pressure along weak zones.
One notable example is the 1811–1812 New Madrid earthquakes in the central United States, an area far from any plate margin.
Density-shaded map of 6 057 tremor epicentres in the New Madrid Seismic Zone (1974–2011), showing branches of buried faults. Extra detail: includes very low-magnitude tremors (< M2.5) and opacity layering beyond the basic syllabus requirement. Source
Hotspot Volcanoes
What Are Hotspots?
Hotspots are localised areas in the Earth's mantle where plumes of hot, buoyant rock rise towards the surface, causing melting in the overlying crust and resulting in volcanic activity.
Hotspot: A stationary location in the mantle where heat from a rising plume melts the crust above, forming volcanoes often in the middle of tectonic plates.
Unlike typical volcanic activity at constructive or destructive boundaries, hotspots are often situated in the centre of tectonic plates.
Mantle Plumes and Volcanism
Hotspots are powered by mantle plumes, which are columns of very hot rock that rise slowly from deep within the mantle.
Mantle Plume: A concentrated upwelling of heat and molten rock from deep in the mantle, which causes partial melting of the crust, forming a hotspot.As the tectonic plate slowly moves over the stationary hotspot, magma rises to the surface and creates a chain of volcanoes.
Process of Hotspot Volcanism:
A plume of mantle material reaches the base of the lithosphere.
Heat from the plume causes partial melting of the crust.
Magma rises and erupts, forming a volcano.
The tectonic plate continues to drift over the hotspot.
A chain of volcanoes forms, with the youngest located above the hotspot and the oldest progressively further away.
This process is evident in the Hawaiian Island chain, where each island represents a former volcanic centre that has moved away from the active hotspot.
A geologic cross-section showing the upwelling mantle plume beneath the Pacific Plate and successive shield volcanoes (oldest to the northwest). Extra detail: age labels on individual volcanoes extend beyond the core plume mechanism. Source
Terra MODIS true-colour image (27 May 2003) of the Hawaiian archipelago, with sunglint highlighting ocean surface patterns and a red marker on the active Kīlauea vent. Extra detail: sunglint reveals sea-surface texture not required by the syllabus. Source
Characteristics of Hotspot Volcanoes
Hotspot volcanoes typically exhibit the following traits:
Basaltic lava composition, which is low in silica and therefore very fluid.
Effusive eruptions, with relatively gentle lava flows rather than explosive activity.
Shield volcano formations with wide bases and low profiles due to the runny nature of basaltic lava.
Some hotspots, however, are found beneath continental crust, such as the Yellowstone hotspot in the USA. These can produce far more explosive eruptions due to interactions with thicker, silica-rich continental crust.
Differences Between Hotspot and Plate Margin Volcanoes
Key Contrasts:
Location: Hotspots are located in the middle of plates, while plate margin volcanoes occur at boundaries.
Source of magma: Hotspots derive magma from deep mantle plumes, whereas plate boundary volcanoes are driven by subduction or rifting processes.
Volcano type: Hotspots often form shield volcanoes; destructive margins create composite volcanoes with more explosive activity.
Intra-Plate Earthquakes vs Plate Margin Earthquakes
Key Characteristics:
Rarity: Intra-plate earthquakes are less frequent but can be large and unexpected.
Depth: These events can be deep-seated and difficult to detect in advance.
Risk: Populations in these areas are often unprepared due to low perceived risk, increasing potential impact.
Examples:
Gujarat Earthquake (2001) in India: A major intra-plate event causing widespread destruction in a region with limited prior seismic activity.
Meghalaya Plateau (India) and Tennessee Valley (USA) also show signs of stress accumulation within stable plates.
Summary of Key Processes and Terms
Intra-plate Earthquake Processes:
Reactivation of ancient faults
Isostatic rebound
Stress transfer from plate margins
Induced seismicity (human activities such as fracking, dam building)
Hotspot Volcanism Processes:
Mantle plume rises
Partial melting of lithosphere
Magma erupts through crust
Formation of volcanic island chains
Terms to Remember:
Intra-plate activity
Hotspot
Mantle plume
Shield volcano
Basaltic lava
Effusive eruption
Explosive eruption (for continental hotspots)
Understanding intra-plate and hotspot activity provides insight into why tectonic hazards are not confined to plate boundaries and helps explain hazard patterns in areas traditionally considered geologically stable.
FAQ
Hotspot volcanoes are typically located in the interior of tectonic plates, far from any plate boundary. They often form linear chains of volcanic islands or seamounts, with only one active volcano located directly above the hotspot.
Volcanoes at plate boundaries are found at convergent or divergent margins and are usually part of broader volcanic arcs or mid-ocean ridges.
Hotspot volcanoes tend to erupt basaltic lava and form broad, low-profile shield volcanoes, while boundary volcanoes—especially at destructive margins—tend to be steeper and more explosive.
Yes, hotspots can exist beneath continental crust. A key example is the Yellowstone hotspot in the United States.
Because continental crust is thicker and more silica-rich than oceanic crust, the rising magma interacts more intensely, resulting in:
Higher viscosity magma
Greater gas content
More explosive eruptions
This contrasts with oceanic hotspots like Hawaii, which produce fluid basaltic lava and gentle, effusive eruptions.
Curved chains result from changes in the direction of plate movement over time.
As the tectonic plate slowly shifts over a stationary mantle plume, any change in direction is recorded in the orientation of the volcanic chain. For example:
The Hawaiian–Emperor seamount chain displays a noticeable bend.
This bend reflects a major shift in Pacific Plate movement about 47 million years ago.
Thus, curved island chains serve as historical records of plate motion.
Once a volcano moves off the hotspot, it becomes extinct because it no longer receives a supply of magma.
Over time, these volcanoes:
Cool and subside
Are eroded by weathering and wave action
May eventually sink below sea level, forming seamounts or guyots
A guyot is a flat-topped underwater volcano that was once an island, eroded before submerging.
Yes, intra-plate earthquakes are generally harder to predict.
Key reasons include:
Occur less frequently, so there’s limited historical data
Often happen on ancient, buried faults not well mapped
Lower levels of seismic monitoring in stable continental interiors
This makes assessing risk and issuing early warnings much more challenging than for boundary zones, where seismic networks are denser and fault lines better understood.
Practice Questions
Question 1 (2 marks):
Define the term hotspot and state where one is found.
Mark Scheme:
1 mark for correct definition:
Hotspot: A stationary location in the mantle where heat from a rising plume melts the crust above, forming volcanoes.1 mark for accurate named example:
e.g. Hawaii, Yellowstone, or Iceland (if referring to a hotspot beneath a divergent boundary).
Question 2 (6 marks):
Explain how mantle plumes lead to the formation of a chain of volcanoes in the middle of a tectonic plate.
Mark Scheme:
Award up to 6 marks based on the following indicative content:
Mantle plumes are upwellings of hot material from deep within the Earth's mantle (1 mark).
These plumes cause partial melting of the crust, forming magma that rises to the surface (1 mark).
The hotspot remains stationary while the tectonic plate moves over it (1 mark).
As the plate moves, new volcanoes form above the hotspot while older ones become extinct (1 mark).
This results in a linear chain of volcanoes, with the youngest above the hotspot and the oldest furthest away (1 mark).
Example enhances the answer but is not required (e.g. the Hawaiian Islands) (1 mark for relevant development).
Level-based mark scheme:
1–2 marks: Basic description with limited explanation.
3–4 marks: Clear explanation with some reference to processes.
5–6 marks: Detailed, accurate explanation showing good understanding of volcanic chain formation.
