Intra-Plate Tectonic Activity, Mantle Plumes, and the Hawaiian Hot Spot

Intra-Plate Tectonic Processes and Earthquakes

• Intra-Plate Phenomena:

  • Tectonic processes and earthquakes can occur near the middle of tectonic plates (intra-plate), far away from active plate boundaries.

• Ancient Fault Lines:

  • Intra-plate earthquakes are associated with ancient faults that formed when the solid crust cracked during its long journey over millions of years.

  • Example: The Rhine Rift Valley.

• Newer Fault Lines:

  • Newer large fault systems can also form within plate interiors and have the potential to eventually evolve into new plate boundaries.

  • These faulting situations are typically associated with smaller magnitude earthquakes.

  • Example: The East African Rift valley.

• Crustal Fracturing from Distant Collisions:

  • Collisions of tectonic plates may fracture the crust well away from the actual plate boundary zone.

  • Movement along these interior faults can yield large magnitude earthquakes over a long period of time as the collision slowly takes place.

  • Example: The large collision zone encompassing the Himalayas and the Tibetan Plateau.

Hot Spots and Mantle Plume Mechanisms

• Definition and Core Principles:

  • Volcanic hot spots are specific points where molten material from the mantle breaks through the rocks of the Earth's crust to reach the surface.

  • Hot spots exist independently of plate margins and occur both in oceanic environments and on land.

    • Oceanic Example: The Hawaiian Islands.

    • Land Example: The volcanic region around Rotorua in the North island of New Zealand.

• Plume Mechanism and Plate Interaction:

  • Hot spots are caused by convectional plumes of very hot magma in the mantle that burn through crustal rocks to reach the surface, in a manner similar to burning through metal with a blowtorch.

  • Stationary Plume Principle: The underlying mantle plume is stationary. The point where hot material breaks out of the mantle and onto the surface stays in the exact same place.

  • Surface Displacement via Plate Motion: As tectonic plates move over time above the stationary hot spot (as described by plate tectonics theory), the location where magma breaches the surface continuously shifts relative to the plate.

  • Volcanic Lifecycle: A volcano forms and grows directly above the hot spot until plate movement carries it away from the magma source. Once carried away and no longer sitting over its hot spot, the volcano loses its supply of magma and becomes extinct. A new volcanic cone then begins to form on top of the stationary hot spot.

• Origins and Geological Causes of Hot Spots:

  • Core/Mantle Boundary Upwelling: Upwelling of hot molten material originating deep at the core/mantle boundary (e.g., Hawaiian and Tristan da Cunha hot spots).

  • Sub-Crustal Mantle Plumes: Magma arising from the top of a large mantle plume located just beneath the crust (e.g., Nyiragongo and Cape Verde Islands on the African Plate).

  • Meteorite Impacts: Large meteorite impacts can potentially create symmetrical hot spots on opposite sides of the planet when colliding with the Earth.

Case Study: Hawaiian Islands and the Hawaii-Emperor Seamount Chain

• Geographic Chain Structure and Age Distribution:

  • The Hawaiian Islands extend for around $2,400\,\text{km}$, forming a chain that becomes progressively older moving from the Southeast end to the North West end.

  • The chain includes islands such as Niihau, Kauai, Oahu, Molokai, Maui, Lanai, Kahoolawe, and Hawaii (Big Island).

  • The islands are formed by volcanic activity despite the nearest plate margin being $200$ miles away.

• Plate Dynamics and Hot Spot Dimensions:

  • The Pacific Plate moves in a north-westwards direction over the plume at a rate of about $10\,\text{cm}$ per year.

  • The Hawaiian hot spot is believed to be around $200\,\text{miles}$ across, with narrow channels feeding the different volcanoes.

  • The hot spot has existed for at least $70\,\text{million years}$.

  • The Hawaii-Emperor seamount chain veers north-westwards away from the Big Island of Hawaii.

  • A sharp bend in the chain appears approximately $2,200\,\text{miles}$ to the northwest of Hawaii, demonstrating that a change in the direction of plate movement occurred around $43-45\,\text{million years}$ ago.

• Volcanic Morphology and Lava Properties:

  • The volcanoes are typically very wide with gently sloping sides comprising many thin ($1\text{ to }5\,\text{metres}$ thick) basaltic lava flows, categorized as shield volcanoes.

  • Canives typically have slopes of only $2-3^\circ$.

  • Because the magma source is in the mantle, the plume brings new materials onto the surface in the same way as along constructive margins.

  • The type of lava erupted is identical to constructive margin eruptions: very hot, runny basalt at temperatures around $1,100-1200\,^\circ\text{C}$.

  • Although these lava flows can do immense damage to buildings and crops in their path, they are only occasionally life-threatening.

• Key Volcanic Structures:

  • Mauna Loa:

    • An enormous active shield volcano on Big Island.

    • Rises $9,000\,\text{m}$ from the floor of the Pacific Ocean to its summit.

    • Summit reaches $4,170\,\text{m}$ above sea level.

    • At its ocean base, it measures over $90\,\text{km}$ wide.

  • Kilauea:

    • A subsidiary crater situated on the lower slopes of Mauna Loa.

    • Has been active for over $150\,\text{years}$.

    • Regular eruptions and magnificent firework displays happen on average every $3\,\text{years}$, serving as a major tourist attraction.

    • Recent activity has taken place from two long fissures: the Great Crack and the East Rift.

  • Lo'ihi:

    • The next island to appear in the Hawaiian chain.

    • Currently sits $975\,\text{metres}$ below sea level.

    • Estimated to emerge above sea level within the next $10,000\text{ to }100,000\,\text{years}$.