Tom Simkin

Caldera Collapse in the Galápagos Islands, 1968: The largest known collapse since 1912 followed a flank eruption and explosive volcanism within the caldera

The summit caldera of Isla Fernandina, a large, uninhabited basaltic shield volcano, was further enlarged by 1 to 2 km3 in June 1968. A small quake and large vapor cloud on 11 June were followed 4 hours later by a remarkable volcanic ash cloud and, after another hour, by a major explosion recorded at infrasonic stations throughout the hemisphere. Seismic activity increased to a peak on 19 June, when more than 200 events per day were recorded by a seismograph 140 km away. Several hundred quakes were in the magnitude range 4.0 to 5.4 mb, but few such events were recorded after 23 June. Unusual lightning accompanied the major cloud, and, during the evening of 11 June, distant observers reported red glow and flashes from the area. Fine ash fell that night and much of the next day to distances at least 350 km from the volcano.

Geochemical and structural studies of the Lamont seamounts: seamounts as indicators of mantle processes

Abstract A chain of seamounts located just west of the East Pacific Rise (EPR) axis at 09°55′N (herein called the Lamont seamounts) have been investigated using Sea MARC I and Sea Beam sonar surveys, “Alvin” submersible dives, deep-sea camera tows and dredging. The most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors. The distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts. Ring fractures could be important in focusing post-caldera collapse eruptions around summit craters and building summit plateaus. Structural and morphological data and the petrology of recovered lavas suggest that calderas in the Lamont seamounts probably formed by collapse associated with magma withdrawal into large, primary vertical magma conduit systems, rather than by repeated inflation/deflation cycles associated with steady-state shallow-level magma chambers. Calderas in the Lamont seamounts have evolved through coalescence of adjoining craters and the plan-view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time. Compared to lavas from the adjacent EPR, the majority of Lamont seamount and cone lavas are more primitive, depleted in incompatible elements and compositionally heterogeneous. Variations in light rare earth elements relative to major elements (e.g. TiO 2 , Na 2 O/(Na 2 O + CaO) ) and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single N-type MORB source. Rather it appears that Lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements. Comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the Lamont seamounts are separate from those along the EPR. In particular seamount magmas appear to reflect near-primary melting events in the mantle whereas EPR lavas are the products of shallow-level crystallization and mixing in crustal magma chambers. Absence of long-lived magma chambers within the Lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain. Furthermore, because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the Lamont chain as a whole, we propose that the scale of mantle chemical heterogeneities is less than 5 km.

Fernandina Volcano's evolved, well-mixed basalts : Mineralogical and petrological constraints on the nature of the Galapagos plume

Fernandina Volcano, the most active of Galapagos volcanoes and the one most directly overlying the Galapagos hotspot, is one of a small number of active, plume-related oceanic island volcanoes with a well-documented recent eruptive record. Whole rock and glass analyses of lava and tephra from all 12 known eruptions from 1958 to the present show them to be evolved (Mg # (= Mg/Mg + Fe2+) of 0.47–0.56), plagioclase-phyric, clinopyroxene- and olivine-bearing tholeiites. Thermodynamic modeling indicates that this phase assemblage erupted at 1100–1130°C and is stable only at low pressures (<0.1–0.2 GPa), thereby representing crystallization within a shallow magma chamber underlying the well-defined, 850-m-deep, 5 by 6.5 km caldera. These tholeiites are remarkably homogenous, as is shown by narrow ranges in Zr/Y (5.7–6.3), (La/Sm)N (1.4–1.7), and (La/Yb)N (3.8–4.3). Prehistoric Fernandina lavas, selected for petrographic and geographic variation over the entire subaerial volcano, are of remarkably similar composition (e.g., Zr/Y is 5.2–6.1, (La/Sm)N is 1.5–1.6, and (La/Yb)N is 3.4–4.1), with a single example of a more primitive lava having an Mg # of 0.63, with 8.5% FeO at 8% MgO. Cr-spinel and glass melt inclusions in plagioclase, together with zoned olivine, give evidence for a range of parental melts in the recent shallow magmatic system, with Mg # as high as 0.62. Extensive, shallow magma mixing is required to explain these variations in mineral and inclusion chemistry, indicating more thermal variation within the shallow magmatic system than is implied by the relatively monotonous whole rock compositions. Apparently, Fernandinas shallow magma chamber has physically blocked passage of more primitive magmas to the surface and homogenized the erupted lavas by extensive mixing. Small variations in lava isotopic and compatible element composition, combined with the need to resupply melt to maintain thermal balance, indicate that replenishment of the magma chamber is semicontinuous, with resupply occurring of the order of a decade or less. Plagioclase-dominated, evolved Fernandina lavas are in sharp contrast to more primitive, olivine-dominated Hawaiian lavas; the total lack of primitive melts in the shallow Fernandina magmatic system implies that substantial fractionation and heat loss occurred during melt transportation from the mantle source to the edifice magma chamber, despite the fact that these melts were transported through relatively warm, young, and thin lithosphere adjacent to the Cocos-Nazca ridge. The diffuse nature of Galapagos volcanism, with 20th century eruptions from eight distinct volcanoes over a 20,000 km2 area, further supports this interpretation and suggests that the Galapagos plume is relatively diffuse, weak, and significantly less thermally intense than the vigorous Hawaiian plume. It may in fact be waning.

Gilliss Seamount: Detailed bathymetry and modification by bottom currents

Abstract A bathymetric survey of the Gilliss Seamount, in the northwest Atlantic Basin, using a multi-beam sonar array system reveals an extremely complex morphologic character of this feature. A new chart provides the most detailed topographic presentation of an Atlantic seamount published to date and highlights the similarity of the Gilliss Seamount with terrestrial strata-volcanoes. Bottom photographs and samples reveal pillow-Iava formation. Seismic profiles show that the volcanic basement is irregularly covered by acoustically transparent deposits that are as much as 668 m thick. Volcanic debris and sediments locally are displaced down the flanks of the seamount. Bottom photographs and cores indicate that the transparent layer has accumulated slowly by deposition from suspensate-rich (mostly clay and planktonic foraminifera) water masses that flow around the mid to lower sectors of this submarine volcano. Bottom-current activity also modifies the abyssal plain turbidite-hemipelagic sequence surrounding the seamount.