Intraplate volcanism challenges our understanding of plate tectonics. It happens far from plate boundaries, inside plates themselves. This unexpected volcanic activity hints at hidden processes beneath Earth's surface, making us rethink how volcanoes form.

Hotspots offer an explanation for intraplate volcanism. They're thought to be fixed spots where hot mantle material rises, creating volcanoes as plates move over them. This idea helps explain chains of islands and gives clues about plate movement over time.

Intraplate Volcanism

Definition and Occurrence

• Intraplate volcanism refers to volcanic activity that occurs within tectonic plates, away from plate boundaries
• Intraplate volcanoes are typically located in the interior regions of lithospheric plates, far from the active margins (continental interiors, oceanic basins)
• The occurrence of intraplate volcanism challenges the conventional plate tectonic theory, which primarily focuses on volcanism at plate boundaries
• Intraplate volcanic activity is less common compared to volcanism at plate boundaries but can still be significant in certain regions (African Plate, Pacific Plate)

Challenges to Plate Tectonic Theory

• Plate tectonic theory primarily explains volcanism at plate boundaries (divergent, convergent, transform)
• Intraplate volcanism occurs far from plate boundaries, suggesting additional mechanisms beyond plate tectonics
• The presence of intraplate volcanoes indicates that the Earth's mantle is more dynamic and complex than initially thought
• Intraplate volcanism requires alternative explanations, such as the hotspot theory, to account for its occurrence and characteristics

Hotspot Theory

Mantle Plumes and Hotspots

• The hotspot theory proposes that intraplate volcanism is caused by stationary mantle plumes rising from deep within the Earth's mantle
• Mantle plumes are narrow, upwelling columns of hot mantle material that originate from the core-mantle boundary or other deep mantle sources
• As a lithospheric plate moves over a stationary mantle plume, the plume melts through the plate, creating a chain of volcanoes or a hotspot track
• The age progression of volcanic islands or seamounts along a hotspot track provides evidence for plate motion and the fixed nature of the mantle plume (Hawaiian-Emperor seamount chain)

Evidence for Plate Motion

• Hotspot tracks exhibit a linear age progression, with older volcanoes located further away from the current hotspot location
• The age progression along a hotspot track matches the direction and speed of plate motion determined by other methods (seafloor spreading rates, paleomagnetic data)
• The bend in the Hawaiian-Emperor seamount chain reflects a change in the direction of the Pacific Plate's motion around 43 million years ago
• Other examples of hotspot tracks include the Yellowstone hotspot track (Snake River Plain) and the Réunion hotspot track (Mascarene Plateau)

Hotspot Volcano Characteristics

Shield Volcanoes and Lava Flows

• Hotspot volcanoes are typically large shield volcanoes characterized by gentle slopes and broad, dome-shaped profiles
• Shield volcanoes are built by successive layers of fluid, basaltic lava flows that emanate from summit vents or rift zones
• The low viscosity of basaltic lava allows it to flow easily, creating thin, extensive lava flows that build the shield-like shape of the volcano
• Examples of hotspot shield volcanoes include Mauna Loa and Mauna Kea in Hawaii, and Piton de la Fournaise on Réunion Island

Associated Volcanic Landforms

• Hotspot volcanoes often have a central caldera, which is a large, circular depression formed by the collapse of the volcano's summit
• Rift zones are linear features on the flanks of hotspot volcanoes where magma is injected and erupted, creating elongated ridges and volcanic cones
• Lava plateaus form when multiple lava flows accumulate over time, creating extensive, flat-lying volcanic landscapes (Columbia River Basalt Province)
• Volcanic islands and seamount chains develop as the plate moves over the hotspot, creating a linear array of volcanoes (Hawaiian Islands, Canary Islands)
• Other features associated with hotspot volcanoes include pit craters, lava tubes, and lava lakes (Kilauea's Halema'uma'u crater lake)

Geochemical Signatures of Intraplate Rocks

Trace Element Enrichment

• Intraplate volcanic rocks often have distinct geochemical signatures that differ from those of volcanic rocks at plate boundaries
• Hotspot lavas are typically enriched in incompatible trace elements, such as rare earth elements (REEs) and high field strength elements (HFSEs)
• The enrichment in incompatible elements suggests that the mantle source for intraplate volcanism is distinct from the depleted upper mantle source of mid-ocean ridge basalts (MORBs)
• The trace element signatures of intraplate volcanic rocks provide insights into the composition and melting conditions of the mantle plumes

Isotopic Compositions and Mantle Source

• The isotopic compositions of intraplate volcanic rocks, particularly those of strontium (Sr), neodymium (Nd), and lead (Pb), provide insights into the mantle source regions
• Intraplate basalts often have higher ratios of 3He/4He compared to mid-ocean ridge basalts (MORBs), indicating a deep, primitive mantle source
• The isotopic signatures of intraplate volcanic rocks suggest the involvement of recycled oceanic crust, sediments, or ancient mantle components in the mantle plume source
• Variations in isotopic compositions among intraplate volcanic provinces reflect the heterogeneity of the mantle and the complex history of mantle plumes

Geochemical Variability and Processes

• The geochemical signatures of intraplate volcanic rocks can vary depending on the degree of partial melting, magma mixing, and crustal contamination
• Lower degrees of partial melting in the mantle plume can lead to higher concentrations of incompatible elements in the resulting magmas
• Magma mixing between plume-derived melts and melts from the surrounding mantle or crust can modify the geochemical signatures of intraplate volcanic rocks
• Crustal contamination, where ascending magmas interact with and assimilate continental crust, can also influence the geochemical characteristics of intraplate volcanism
• Analyzing the geochemical variability of intraplate volcanic rocks helps in understanding the complex processes and interactions occurring within mantle plumes and the lithosphere