Influence of Environment on Magma Properties, Eruption Processes, and Deposits

In this chapter, we consider the properties of the different environments through which magmas pass, namely the Earth’s lithosphere, atmosphere, hydrosphere, and glaciosphere, to assess the influence that these different environments have on eruption processes. The physical properties and conditions of the different environments and the ambient fluids through which magma travels are fundamentally important in influencing the properties of magma, the behaviour of volatiles, the styles of eruption, the dispersal of products, and deposit characteristics. Buoyancy is one of the most important factors driving the rise and the rise rate of magmas to the surface, and most magmas will rise as long as they are less dense than the ambient rocks they are rising through, because they have buoyancy. However, dense basalt magmas rise to the Earth’s surface all the time, even though they are denser than the crust, and even rise through unconsolidated to weakly consolidated sedimentary basin fill successions with much lower densities. This indicates that other factors also drive magma upwards. These include the pressure difference between the mantle or crustal magma source area, where the pressure regime is lithostatic, and the Earth’s surface (atmospheric, hydrostatic, and glaciostatic pressure regimes), upward momentum once upward migration begins, and volatile exsolution and bubble growth as magmas rise, which can significantly reduce the bulk density of magma, making them more buoyant than at the source. Once erupted, the thermal properties of ambient air, water, or ice will interact with the hot magma to determine the final eruption style. In subaqueous and subglacial environments, water/glacial depth and hydrostatic/glaciostatic pressure play fundamental roles in determining eruption and fragmentation processes. These environmental and crustal variables determine if magma reaches the Earth’s surface or stalls in the subsurface, and if it erupts, whether the eruption is effusive and lava-forming, or explosive, and whether or not erupting lavas will be subject to quench fragmentation, an autoclastic process that is unique to subaqueous and subglacial environments. Another critical factor in understanding the behaviour of erupting magma and eruption styles and deposit types in subaqueous settings is the state of exsolved volatiles (supercritical, critical, subcritical, and gaseous fluids), as well as that of the external ambient water that magma comes in contact with, which we discuss in detail in this chapter. The environment has a clear effect on the eruption processes and deposits.