Nomination for a World Heritage Site 2009 - Phoenix Islands ...

ut may point to either hot spot motion or magmatism caused by short-term local lithospheric
extension as the origin of the Tokelau ridge.
The geological processes that formed the Kiribati Phoenix Islands and associated seamounts
are likely the same as for Howland and Baker. Atoll and reef island development began when
the volcanic foundations were still emergent islands in the Cretaceous to Eocene periods,
followed by subsidence being offset by upward reef growth maintaining proximity to the sea
surface over long time periods.
The summits of Baker and Howland Islands are the smallest of the Pacific remote islands of
the USA, and the highest point on any of these lies just 8 m above sea level. Their tops are all
isolated peaks rising from the deep ocean (3,000 m or greater) with no surrounding ridges or
other connected features. Although the flanks of these islands are very steep with slopes
between 30° and 40° down to a depth of about 1,000 m, both also have small shallow terraces
that probably reflect previous sea level stands. The terraces marked by the areas of low slope,
are present between ~7 and 17 m and between ~90 and 130 m around the islands. Additional
terraces are present at multiple depths off Howland Island. According to bathymetric
information available (see also chapter on bathymetry), it is probable that the PIPA islands
and seamounts, are similarly isolated volcanic peaks extending from deep sea. Seafloor age
adjacent at Baker and Howland is estimated to be 123-124 (Ma).
Larson and Chase (1972) correlated the magnetic anomaly sets near the Phoenix Islands, east
of Japan, and west of Hawaii as part of their analyses of patterns and evolution of sea-floor
spreading in the Cretaceous and Cenozoic periods. This was based on the Phoenix lineation
set of magnetic anomaly data. A magnetic reversal model was derived and applied. This
shows that the Phoenix Islands, Japan, and Hawaii were formed during the Cretaceous and
Cenozoic period.
Magnetic anomaly data around the Phoenix Islands was used to analyze sea-floor spreading
in the Cretaceous and Cenozoic periods. In the 1970s, Pacific Islands Applied Geoscience
Commission (SOPAC) conducted manganese nodule surveys around the Phoenix Islands.
The eastern side of the Phoenix Islands portion of the Kiribati EEZ appeared to have higher
amounts of manganese nodules than the western side. Phoenix Islands manganese nodules
had an average of 1.35 percent Ni content. These nodules (and nodules from all of Kiribati’s
EEZs) were of low grade and appear to have no economic value for commercial harvest in
the foreseeable future.
Several bathymetric surveys have been completed in the Phoenix Islands. Seabed surface
composition was primarily calcareous ooze, siliceous-calcareous clay, and brown clay. Most
of the seamounts on the eastern side of the Phoenix Islands portion of the Kiribati EEZ were
surveyed by the Japanese in the late 1980s.
The substrate of the Phoenix Islands is almost entirely limestone with accumulated organic
matter (Fosberg and Sachet c1976). Garnett (1983a) noted that the soils on Kanton were
composed of coral sand and larger fragments of dead corals and other biogenic materials.
Little or no humic material was in the soil. Soil samples at most sites around Kanton were
primarily sand (GoK 1995).
Darwin has been reported to have used some of the Phoenix Islands as a basis for
development of his theory of coral reef and atoll development.
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