Description: Convection in an isolated planet is characterized by narrow downwellings and broad updrafts—consequences of Archimedes’ principle, the cooling required by the second law of thermodynamics, and the effect of compression on material properties. A mature cooling planet with a conductive low-viscosity core develops a thick insulating surface boundary layer with...

Description: From a structural and morphological point of view, the continental margin of southwestern Africa is a middle-aged "pull-apart" or "passive-type" margin which was initially created during the breakup of the supercontinent of Gondwanaland in the Mesozoic. The most important objective of the Leg 40 expedition was the attainment of strata laid down when the newly formed ocean was very young and only a few hundred kilometers wide. Site 361 was drilled at the fall of continental platform into the Cape Basin, to a sub-sea floor depth of 1314 meters and yielded sediments that range in age from upper Eocene to lower Aptian and possibly slightly older.

Description: Sites 595 and 596 of DSDP Leg 91 are located in one of the most poorly studied tracts of seafloor in the world at the time of the survey. During a preparation survey done by the R/V Melville a particular question of geologic regional significance was the lithologic nature of the "reverberant layer" detected in the Southwest Pacific Basin, especially to the north of the site. The drilling also beared on two important aspects of the oceanic crustal composition: the chemistry and petrology of the lavas erupted at rapidly spreading ridges, and the nature and degree of alteration experienced by those lavas over the 100+ m.y. of their posteruptive history.

Description: The deposits from the DSDP Site 424 hole are divided into two major categories according to the relative proportions of the green clay-rich fractions and the presence of Fe-Mn concretions. The Fe-Mn concretions are associated with interlayers of pelagic foraminiferal nannofossil ooze from the Upper Pleistocene. This carbonate ooze is rather enriched in Mn by comparison with the green clay-rich fractions. Representative carbonate ooze samples have been chemically analysed by Cambon P. and Etoubleau J. from COB-CNEXO (Ifremer) using X-ray fluorescence.

Description: Ocean Drilling Program Hole 735B was extended to 1508 m below the sea floor during Leg 176, atop a shallow bank near Atlantis II Fracture Zone on the very slowly spreading Southwest Indian Ridge. All the drilling was in gabbro, and recovery averaged nearly 87%. The drill penetrated a series of stacked plutons consisting mostly of olivine gabbro, but some with troctolite. Each pluton is some 200–500 m thick, each has its own internally coherent stratigraphy, and each apparently represents an individual event of significant magma inflation and addition to the crust. The entire column was extensively deformed along inclined zones of distributed shear before it was completely frozen, this marking the onset of unroofing of the rocks and their ascent to high rift mountains. The deformation mobilized late-stage melts into flow patterns which led to concentration of ilmenite and magnetite in hundreds of seams of oxide gabbro along or near zones of strong crystal–plastic deformation, the highly differentiated melts overall being concentrated by buoyancy forces toward the top of the section, especially in one zone nearly 70 m thick. However, upward flow was ultimately blocked or deflected by zones of impermeable rock resulting either from downward freezing or grain-size reduction during shear. A melt lens probably did not form at the base of sheeted dikes, as it does at the East Pacific Rise. Despite this, the rocks are cumulates, and most are adcumulates, with very low residual melt porosities. Cumulate theory based on stratiform, layered intrusions does not entirely apply to these rocks. Instead, all gabbros, including the oxide gabbros, crystallized in a dense crystal mush in patterns dominated by fractures, channelized flow, and intergranular porous flow. Most gabbros are not layered; weak modal layering of uncertain origin is present in 〈2% of the rocks. Porosity reduction leading to formation of adcumulates at all stages was extremely efficient. This occurred in the course of synkinematic differentiation, also called differentiation by deformation, and involved compaction under conditions of lithostatic loading and shear, emplacement of crystal mushes along inclined, possibly curving faults, dissolution and reprecipitation of minerals along grain boundaries, and pressure solution, which was the final agent in porosity reduction. As the block was lifted from beneath the rift-valley floor, crystal–plastic deformation gave way to brittle fracture, and the now dominantly subsolidus metamorphism shifted from high-temperature assemblages characterized by amphibole near the top of the section, to low-temperature assemblages with smectite–chlorite and zeolite near the bottom. The rocks acquired their stable magnetization during formation of amphiboles and secondary magnetite during this metamorphism. The entire body of rock is reversely stably magnetized at a consistent inclination, and it is rotated to the south, perhaps along a curving detachment surface, away from the ridge segment where it formed, by about 20°. It has an intensity of magnetization sufficient to account for the magnetic anomaly observed over the site. Drilling did not reach ultramafic rock, but dredging indicates that peridotite is probably within a few hundred meters of the bottom of the hole. Seismic Moho, however, is placed at 5 km beneath the summit of Atlantis Bank. Much of the rock between Moho and the bottom of Hole 735B must therefore be partially serpentinized peridotite.