The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers
The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers
Date
1996-12
Authors
Hooft, Emilie E. E.
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Date Created
Location
Juan de Fuca Ridge
Gorda Ridge
Southern East Pacific Rise
Gorda Ridge
Southern East Pacific Rise
DOI
10.1575/1912/5713
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Keywords
Mid-ocean ridges
Plumes
Structural geology
Plate tectonics
Submarine geology
Plumes
Structural geology
Plate tectonics
Submarine geology
Abstract
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.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1996
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Citation
Hooft, E. E. E. (1996). The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/5713