Easter Island and Sala y Gomez are part of the Sala y Gomez
Ridge, a broad band of high topography and scattered seamounts
extending ESE from the East Pacific Rise. It has been proposed that
the Sala y Gomez Ridge results from the movement of the Nazca Plate
over a fixed melting spot in the mantle. To test this hypothesis volcanic
rocks from Easter Island and Sala y Gomez were analyzed for
their K-Ar ages and major element abundances.
Subaerial Easter Island was constructed in three distinct episodes,
occurring at 2.5 m.y., 0.9 m.y., and 0.4 m.y. ago. The
youngest rocks on the island are the Roiho olivine basalts, and are
probably less than 50,000 years old. Eruptive activity on Sala y
Gomez was essentially contemporaneous with the early volcanism on
Easter Island. No migration of volcanism with time is apparent along
the Sala y Gomez Ridge, thus a major criterion of the melting spot hypothesis is not fulfilled.
Volcanic rocks from Easter Island constitute a tholeiitic differentiation
series; they are chemically similar to those from other
islands situated near mid-ocean rise crests. The wide compositional
spectrum is most likely the result of fractional crystallization from a
basaltic parent liquid, though the data is ambiguous for the highly
silicic differentiates. The youngest basalts possess more alkaline
affinities which are probably not related to fractional crystallization
from the earlier basalts. The alkaline nature of these rocks may be
the result of a downward migration of the fusion zone with time, as
the island moved eastward over a progressively thickening lithosphere.
Volcanic rocks from Sala y Gomez belong to an alkali olivine basalt
series. The fundamental chemical differences between the Easter
Island and Sala y Gomez suites suggest that the two islands were not
derived from a common source, as predicted by the melting spot
hypothesis.
The evidence does not support a melting spot origin for Easter
Island, Sala y Gomez, and the Sala y Gomez Ridge. An alternative
model involving diapiric intrusion and decompression melting of
asthenosphere material along a major fracture in the Nazca Plate
provides a better explanation for the data. Synchronous volcanism
along the eastern extension of the Easter Island transform fault has
given rise to the islands and seamounts on the Sala y Gomez Ridge. / Graduation date: 1975
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28563 |
| Date | 11 April 1975 |
| Creators | Clark, James Gregory, 1948- |
| Contributors | Dymond, Jack |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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