What Occurs at a Hot Spot?

Key Takeaways:

Hotspots are areas where heat rises from the mantle, forming volcanoes on the crust above.
Hotspot volcanoes are caused by relatively stationary mantle plumes, unlike other volcanoes at plate boundaries.
As plates move over hotspots, chains of volcanoes form and are transported away from the plume.
Extinct hotspot volcanoes subside and erode as they move away from the active plume.
Hotspot volcanoes are generally less explosive than volcanic arcs at subduction zones.
Hawaii and American Samoa have long chains of islands formed by hotspot volcanism.

Introduction

Volcanoes are awe-inspiring manifestations of the powerful forces within our planet. But not all volcanoes are created equal. While many form at the boundaries between tectonic plates, others originate from hotspots deep beneath the Earth's surface. So what exactly occurs at these enigmatic hot spots?

This comprehensive guide will analyze the features, causes, and effects of hotspot volcanism. Key questions like where hotspots form, how they create volcanoes, and how they differ from other volcanic activity will be addressed. Scientific research and geological evidence will provide insight into hotspots' inner workings. Readers will come away with an in-depth understanding of what transpires at these unique geological locales.

With volcanoes posing hazards but also creating new land, the dynamics of hotspots have far-reaching implications. Their longevity and internal heat source distinguishes them from other temporary volcanic systems. Whether studying Earth's history or exploring its future, the life and death of hotspot volcanoes is essential knowledge. By evaluating hotspots from formation to eventual dormancy, this article will illuminate what drives these pockets of activity inside our planet.

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Hotspot Volcanism vs Plate Boundary Volcanism?

Hotspot volcanism differs from the volcanism occurring at tectonic plate boundaries in some key ways:

Cause

Hotspots are caused by plumes of hot material rising from deep within the mantle.
Plate boundary volcanoes occur due to processes like subduction and rifting at plate margins.

Location

Hotspots occur at intraplate regions rather than at plate boundaries.
The Hawaiian Islands formed over a hotspot in the middle of the Pacific plate.

Longevity

Hotspots are relatively stationary and can remain active for tens of millions of years.
Plate boundary volcanoes are ephemeral, lasting as long as tectonic movements persist.

Eruptive Style

Hotspot eruptions tend to be less violent and explosive overall.
Subduction zone volcanoes can erupt more catastrophically as magma interacts with water.

Structure

Chains of volcanoes and seamounts trace plate motion over hotspots.
Plate boundary volcanoes form localized ridges or arcs.

So in summary, hotspots constitute long-lived, intraplate volcanism fueled by mantle plumes, exhibiting qualities distinct from plate boundary volcanic systems.

Where Do Hotspots Form?

Hotspots arise from narrow columns of hot mantle material called mantle plumes that originate deep within the Earth. Seismic imaging indicates mantle plumes initiate near the core-mantle boundary, over 2500 km below the surface.

From there, these narrow upwellings, 10–100 km across, make their way up through the mantle. As a plume of hot material rises and approaches the lithosphere, it begins to melt due to adiabatic decompression. This generates basaltic magma that can erupt through the crust to form volcanoes on the surface.

So unlike volcanism tied to plate boundaries, hotspot volcanism is driven by internal heat stemming directly from the underlying mantle. And multiple hotspots can occur, even far from plate boundaries. Hawaii, Yellowstone, Galapagos, and Iceland are among the notable hotspots currently active.

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How Do Hotspots Create Volcano Chains?

As a tectonic plate moves over a mantle plume, the progressive formation of volcanoes produces a hotspot track. The formation of these characteristic chains occurs as follows:

Rising plume melts through lithosphere, generating magma
Volcano forms over plume, beginning chain
Plate motion carries volcano away from plume
Plume melts through to form new volcano
Previous volcano goes extinct without magma supply
Chain lengthens as sequence continues

So the interplay between the stationary hotspot and moving plate results in a line of successively older, extinct volcanoes trailing away from the active plume. The Hawaiian-Emperor seamount chain, extending over 5800 km, provides a classic example.

Why Are Hotspot Volcanoes Less Explosive?

Several factors contribute to the relatively gentle nature of hotspot volcano eruptions compared to those at subduction zones:

Magma composition – Hotspot magma tends to be basaltic, so more fluid and less viscous. This inhibits explosive activity.
Magma temperature – Mantle-derived hotspot magma can be hotter, making it less prone to explosive fragmentation.
Gas content – Degassing occurs readily with fluid basaltic magma, limiting volatile buildup.
Interaction with water – Subduction volcanoes feature magma interacting with water, making them more explosive.

So the combination of smooth basaltic magma at high temperatures, decompressive degassing, and lack of external water produces the more effusive eruptions characteristic of hotspots like Hawaii and Iceland.

However, hotspot volcanoes are not devoid of hazards. Lava flows can destroy property, volcanic gases pose toxicity risks, and landslides and ashfalls are still possible depending on conditions. But the cataclysmic, explosive eruptions driven at subduction zones seem generally absent at benign hotspots.

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What Happens as Hotspot Volcanoes Go Extinct?

As a tectonic plate carries a hotspot volcano away from its underlying plume, the supply of fresh magma is eventually cut off. Without replenishment, the volcano becomes inactive and goes extinct.

Several geological processes ensue after hotspot volcanism ceases:

Erosion – Exposure to wind and water wear the edifice down over time. Islands can be eroded down to seamounts.
Subsidence – With the magma source depleted, the weight of the volcano causes it to gradually sink downward.
Atoll formation – Coral reef structures may surround the sinking island, forming an atoll shape.
Seismic quiescence – Earthquake and volcanic activity dies down without the heat source.

So hotspot volcanoes transition from active to dormant as they depart the mantle plume. While still massive edifices, they erode and submerge over millions of years after their fiery inception at the Earth's core.

How Have Hotspots Shaped Hawaii and American Samoa?

The isolated volcanic island chains of Hawaii and American Samoa provide classic examples of hotspot volcanism in the Pacific Ocean basin:

Hawaii

Formed by the Hawaiian hotspot, currently located under the Big Island
Eight major islands stretch northwest over 5800 km
Oldest island, Kure Atoll, is 29 million years old
Islands get progressively younger toward the southeast
Oahu is 3.7–2.6 million years old

American Samoa

Samoan hotspot underlies Tutuila and Rose Atolls
Samoa hotspot track extends 2,000 km to the Samoa Islands
Tutuila, main island, formed 1.5 million years ago
Youngest islands, Ofu and Olosega, are under 400,000 years old

So both chains reveal the telltale progression in age away from their underlying hotspots, documenting the islands' drift over tens of millions of years. The hotspots continue fueling new island growth as the oldest islands erode away after forming.

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Conclusion

Hotspots constitute unique volcanic systems fueled by mantle plumes arising from deep in the Earth's interior. As tectonic plates traverse over these relatively fixed thermal upwellings, chains of volcanoes form and then go extinct. The geology of Hawaii and American Samoa provides quintessential examples of such hotspot tracks.

Compared to volcanoes at plate boundaries, hotspot volcanoes exhibit gentle effusive eruptions due to magmatic composition and characteristics. While posing less explosive hazards, they can still inflict damage through lava flows, ashfalls, gases, and landslides.

From initiation in the lower mantle to eventual erosion as seamounts, the life cycle of hotspot volcanoes is driven by a deep thermal engine within our dynamic planet. This comprehensive guide illuminated the inner workings of these intraplate volcanic factories that build and bury islands over geological time

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