1. Mid-Ocean Ridges (MORs):
* Setting: Divergent plate boundaries where new oceanic crust is formed.
* Mechanism: As plates move apart, decompression melting occurs. The mantle, previously under pressure, rises and melts due to reduced pressure. This melting produces basaltic magma that rises to the surface, solidifying as pillow basalts and sheeted dikes.
* Characteristics: High-temperature, relatively low-silica basalts with a tholeiitic composition. They often exhibit high MgO and low FeO content, indicating a relatively primitive source.
2. Oceanic Hotspots:
* Setting: Areas of anomalous volcanism often located within oceanic plates, away from plate boundaries.
* Mechanism: Hot plumes of mantle material rise from deep within the Earth, causing decompression melting and generating basaltic magmas.
* Characteristics: High-temperature, alkaline basalts with a higher silica content than MOR basalts. They can also exhibit a greater range of trace element and isotopic compositions, reflecting the complex interaction of the plume with the overlying mantle.
3. Subduction Zones:
* Setting: Convergent plate boundaries where one plate is forced beneath another.
* Mechanism: Water released from the subducting slab lowers the melting temperature of the overlying mantle wedge, leading to the formation of basaltic magma.
* Characteristics: A variety of basaltic magma types can be generated, including tholeiitic, calc-alkaline, and alkali basalts. These magmas often have a more complex geochemical signature compared to MOR or hotspot basalts, reflecting the influence of the subducting slab.
4. Continental Rifts:
* Setting: Areas where continental plates are pulling apart, eventually leading to the formation of new ocean basins.
* Mechanism: As the continent thins, the mantle rises and undergoes decompression melting, generating basaltic magma.
* Characteristics: Basaltic magmas in continental rifts can range from tholeiitic to alkaline, depending on the degree of mantle melting and the presence of pre-existing continental crust. They often exhibit a higher silica content than MOR basalts.
5. Intraplate Volcanism:
* Setting: Volcanism occurring within tectonic plates, often far from plate boundaries.
* Mechanism: The exact mechanisms are still debated, but they likely involve mantle plumes, localized heating of the mantle, or tectonic stresses that induce melt generation.
* Characteristics: Can range from tholeiitic to alkaline, with variations in trace element and isotopic compositions reflecting the specific tectonic setting.
It's important to note that the tectonic setting plays a crucial role in controlling the composition, evolution, and eruption style of basaltic magmas.
