9.3 Hotspot Volcanism

Hotspot volcanism occurs within the interior of tectonic plates, far from plate boundaries, and is attributed to localized zones of anomalously high mantle temperatures. The most widely accepted explanation is that hotspots are surface expressions of deep mantle plumes—narrow columns of buoyant, hot material rising from the core–mantle boundary (2900 km depth) or from the upper–lower mantle transition zone (~660 km depth). As a plate moves over a relatively stationary plume, a chain of progressively older volcanoes is created, providing a direct record of plate motion.

Approximately 40–50 hotspots have been proposed worldwide, though the number that are truly sourced from deep plumes remains debated. The most convincing deep-sourced hotspots include Hawaii, Iceland, Réunion, Louisville, and Yellowstone. Others may reflect upper-mantle convective instabilities, edge-driven convection, or compositional heterogeneities rather than classical thermal plumes.

The Hawaiian–Emperor Seamount Chain

The Hawaiian–Emperor chain is the type example of a hotspot track. It extends approximately 6,000 km from the active volcanoes on the Big Island of Hawai'i (0 Ma) to the 80 Ma Emperor seamounts near the Aleutian Trench, where the oldest seamounts are being subducted. The chain records the motion of the Pacific plate over the Hawaiian hotspot for the past ~80 million years.

Age–Distance Relationship

\[ v_{\text{plate}} = \frac{\Delta x}{\Delta t} \approx 83 \;\text{mm/yr} \]

For the Hawaiian chain (post-bend segment), the average plate velocity over the Hawaiian hotspot is approximately 83 mm/yr in a N60°W direction, derived from radiometric dating of volcanic islands and seamounts at known distances from the active vent (Kīlauea/Lō'ihi). This is consistent with independent estimates from the MORVEL and HS3 plate motion models.

Island / Seamount	Age (Ma)	Distance from Kīlauea (km)	Rate (mm/yr)
Kīlauea (Big Island)	0	0	—
Maui (Haleakalā)	~1.3	~180	~100
O'ahu	~3.0	~350	~100
Kaua'i	~5.1	~520	~90
Midway Atoll	~28	~2400	~85
Meiji Seamount	~82	~6000	~73 (Emperor)

The Hawaiian–Emperor Bend (~47 Ma)

At approximately 47 Ma, the chain shows a prominent ~60° bend in its trend, from the near-northerly Emperor chain to the west-northwesterly Hawaiian chain. This bend was traditionally attributed to a major change in Pacific plate motion direction, possibly linked to the initiation of subduction along the western Pacific (Izu-Bonin-Mariana system) and the India–Eurasia collision. However, paleomagnetic data from Emperor seamounts indicate that the Hawaiian hotspot itself drifted southward by ~15° of latitude between 80 and 47 Ma, suggesting the bend records both plume motion and plate motion change.

Volcanic Evolution of Hawaiian Islands

Each Hawaiian volcano passes through a characteristic sequence of evolutionary stages as the plate carries it away from the hotspot. These stages reflect the volcano's position relative to the plume axis and the changing melt production rate.

Stage 1

Shield-Building Stage

The main growth phase, producing ~95% of the volcano's total volume. Erupts tholeiitic basalt at very high rates (0.1–0.2 km³/yr). Builds a broad shield volcano over ~0.5–1.0 Ma. Melt fraction F ≈ 10–20% from decompression melting of hot plume mantle. Summit caldera and rift zones are active. Kīlauea and Mauna Loa are currently in this stage.

Stage 2

Post-Shield (Alkalic Capping) Stage

As the volcano moves off the plume axis, eruption rates decline by 10–100 fold. Lavas transition from tholeiitic to alkalic basalt, hawaiite, and mugearite, reflecting lower degrees of partial melting (F ≈ 3–8%) of a more enriched source. A thin alkalic cap (1–5% of total volume) mantles the shield. Hualalāi and Mauna Kea are in this stage.

Stage 3

Erosional Stage

A quiescent period of 0.5–2.5 Ma with no volcanism. Subaerial erosion, stream incision, and marine wave action cut deep valleys and sea cliffs. Coral reef growth fringing the island keeps pace with subsidence. The islands of Lana'i, Kaho'olawe, and much of O'ahu are in this stage.

Stage 4

Rejuvenated (Post-Erosional) Stage

After a long hiatus, small-volume eruptions of highly alkalic lavas (nephelinite, basanite, melilitite) occur from scattered vents. These represent extremely low degrees of melting (F < 2%) of enriched mantle, possibly triggered by lithospheric flexure from the load of the active shield volcano downstream. The Honolulu Volcanic Series on O'ahu (Diamond Head, Punchbowl) and the Koloa Volcanics on Kaua'i are examples.

Hotspot Swell & Buoyancy Flux

Hotspot volcanoes sit atop broad topographic swells—regions of elevated seafloor extending ~1,000 km across and rising ~1.0–1.5 km above the expected depth for lithosphere of that age. The swell reflects the thermal buoyancy and dynamic uplift generated by the underlying plume.

Buoyancy Flux

The strength of a plume can be characterized by its buoyancy flux B, which measures the rate at which buoyant material is supplied:

\[ B = \rho \, C_p \, \Delta T \, Q \]

ρ = mantle density (~3300 kg/m³)

C_p = specific heat capacity (~1250 J/kg·K)

ΔT = plume excess temperature (~200–300 K for Hawaii)

Q = volume flux of plume material (m³/s)

For Hawaii, B ≈ 4–10 Mg/s (Sleep, 1990), making it the strongest hotspot on Earth. In practice, the buoyancy flux is estimated from the topographic swell dimensions and plate velocity: B ≈ ρ_m·g·α·ΔT·w·h·v_plate, where w and h are the swell width and height.

Hotspot	B (Mg/s)	Swell Width (km)	Swell Height (km)	Setting
Hawaii	6.6–8.7	~1200	~1.4	Oceanic intraplate
Iceland	1.4	~1000	~2.0	Hotspot on ridge
Réunion	1.6–2.0	~800	~1.0	Oceanic intraplate
Yellowstone	1.5	~600	~0.5	Continental intraplate
Cape Verde	1.0–1.6	~800	~1.5	Oceanic, slow plate

Notable Hotspot Systems

Réunion & the Deccan Traps

The Réunion hotspot is linked to the Deccan Traps flood basalts (65 Ma) via the Chagos–Laccadive Ridge and Mascarene Plateau. The model proposes that arrival of the Réunion plume head beneath the Indian plate at ~65 Ma triggered the massive Deccan eruptions (~1.5 × 10&sup6; km³), while the subsequent plume tail produced the hotspot track as India drifted northward. Piton de la Fournaise on Réunion is one of the most active volcanoes on Earth today.

Iceland: Hotspot on a Ridge

Iceland is unique: a hotspot centered on the Mid-Atlantic Ridge. The plume's excess temperature (~150–200°C above ambient mantle) produces anomalously thick crust: 20–40 km compared to the normal 6–7 km oceanic crust. The elevated topography of the Iceland plateau reflects both thermal buoyancy and excess melt production. Seismic tomography reveals a low-velocity anomaly extending to at least 400 km depth, with some studies tracing it to the core–mantle boundary.

Yellowstone: Continental Hotspot

The Yellowstone hotspot has produced a ~700 km track of silicic calderas along the Snake River Plain over the past ~16 Ma, as the North American plate moves WSW over the plume at ~25 mm/yr. Three VEI 8 supereruptions occurred at 2.1 Ma (Huckleberry Ridge Tuff, 2500 km³), 1.3 Ma (Mesa Falls Tuff, 280 km³), and 0.64 Ma (Lava Creek Tuff, 1000 km³). Present-day activity includes a large magma reservoir imaged at 5–15 km depth with ~5–15% melt fraction.

Galápagos

The Galápagos hotspot sits near the Galápagos Spreading Center, creating a complex interaction between plume and ridge volcanism. The archipelago shows a westward age progression, with the youngest volcanoes (Fernándina, 0.07 Ma) in the west and older islands (San Cristóbal, ~4 Ma) in the east. The Carnegie Ridge records the plume's track on the Nazca plate.

Hotspot Reference Frame & Age Progression Test

Hotspots have been used to define an “absolute” plate motion reference frame. If hotspots are fixed (or nearly fixed) relative to the deep mantle, then the velocity of any plate can be determined by measuring the age progression along its hotspot track. The HS3-MORVEL56 model (Gripp & Gordon, 2002) uses hotspot data to derive angular velocities for all major plates in this frame.

However, the age progression test reveals that not all proposed hotspots show clear, linear age progressions. Several issues complicate the hotspot reference frame:

Plume drift: Paleomagnetic evidence shows the Hawaiian hotspot moved southward at ~40–50 mm/yr between 80 and 47 Ma, indicating that plumes are not perfectly fixed but are advected by large-scale mantle flow.
Inter-hotspot motion: Geodetic and paleomagnetic constraints indicate that Atlantic and Pacific hotspots have moved relative to each other at rates of 10–20 mm/yr over the Cenozoic, challenging the notion of a single fixed reference frame.
Non-plume hotspots: Some volcanic chains (e.g., Cameroon Line, parts of the Canary Islands) lack clear age progressions, suggesting they may result from lithospheric processes (edge-driven convection, delamination) rather than deep plumes.
Small-scale convection: Numerical models show that sublithospheric small-scale convection can produce volcanic chains that mimic hotspot tracks without requiring deep mantle plumes.

Key Takeaways

Hotspot volcanism occurs within plate interiors and is attributed to mantle plumes
The Hawaiian–Emperor chain records Pacific plate motion at ~83 mm/yr, with a ~60° bend at 47 Ma
Hawaiian volcanoes evolve through four stages: shield, post-shield, erosional, and rejuvenated
Hotspot swells are ~1000 km wide; Hawaii's buoyancy flux (~7 Mg/s) is the highest on Earth
Réunion plume arrival at 65 Ma may have triggered the Deccan Traps flood basalts
Iceland's hotspot on the Mid-Atlantic Ridge produces anomalous crustal thickness of 20–40 km
The hotspot reference frame assumes near-fixed plumes, but inter-hotspot motions of 10–20 mm/yr exist
Not all proposed hotspots show clear age progressions; some may not be deep-plume sourced

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