The Influence of Magma Supply and Eruptive Processes on Axial Morphology, Crustal Construction and Magma Chambers

Two-thirds of the surface of the Earth is created at mid-ocean ridges where magmas rise from the mantle and cool to form the oceanic crust. The objective of this Thesis is to examine the influence of magma supply and eruptive processes on axial morphology, crustal construction, and the properties of crustal magma chambers at intermediate and fast spreading ridges. Variations in magma supply on time scales of ~100 Kyr generate along-axis changes in crustal thickness and temperature. Magma sill properties and hydrothermal activity are closely linked to spreading events which occur on much shorter time scales (ca. 10-100 yr) than the longer-term variations in magma supply reflected in along-axis changes in ridge morphology. The seismically constrained depths of ridge crest magma sills (>1-2 km) are considerably deeper than the level of neutral buoyancy (100-400 m). The apparent inverse relationship between magma sill depth and spreading rate suggests that a thermally controlled permeability boundary, such as the solidus horizon, controls the depth at which magma ponds beneath mid-ocean ridges. Recent thermo-mechanical models predict that, at intermediate spreading rates, rift valley and magma sill formation are sensitive to small changes in crustal thickness and mantle temperature. Analysis of gravity at an intermediate spreading ridge shows that small differences in crustal thickness (300-700 m) and mantle temperature (10-15°C) are indeed sufficient to produce major changes in lithospheric strength and axial morphology. A stochastic model for the emplacement of dikes and lava flows with a bimodal distribution of lava flows is required to satisfy geological and geophysical constraints on the construction of the extrusive section. Most dikes are intruded within a narrow zone at the ridge axis. Short flows build up approximately half the extrusive volume. Occasional flows that pond at a considerable distance off-axis build up the remainder of the extrusive section. This Thesis underlines the importance of eruption dynamics in the emplacement of the uppermost volcanic layer of the crust and of the crustal thermal structure in controlling local variations in magma sill depth and ridge morphology.