Download The Origin of Galapagos Island Seamounts and the Impact of Their

The Origin of Galapagos Island Seamounts and the
Impact of Their Discovery on Accepted Tectonic and Speciation Models
by
Jane Doe
Submitted in partial fulfillment of
requirements for GEOL-100
Spring semester, 2006
April 27, 2000
The Origin of Galapagos Island Seamounts and the Impact of Their Discovery
on Accepted Tectonic and Speciation Models
ABSTRACT
The currently-accepted determined ages of the emergent Galapagos Islands are not
old enough to allow for the speciation of the terrestrial life present upon them, including
marine and terrestrial iguanas. The discovery of drowned and subducted seamounts off
the coast of Costa Rica and magnetic signature data from these seamounts suggest that
they were once emergent portions of the Galapagos Island chain. If terrestrial biota
existed upon the seamounts while they were emergent and before the present Galapagos
Islands emerged, the amount of time possible for speciation is increased to an amount
within the margin of error for the genetic distance between the two iguana species.
The Origin of Galapagos Island Seamounts and the Impact of Their Discovery
on Accepted Tectonic and Speciation Models
The emergent Galapagos Islands are around three million years old, which is ample
time for the speciation of the thirteen species of finch found there by Darwin. This is not
enough time, however, to explain the divergence of the Galapagos marine and land
iguanas from a single South American species, based on the calculated genetic distances
between the two. The discovery of seamounts off the coast of Costa Rica, thought to be
representative of drowned islands, could be a possible explanation for the speciation of
the iguanas. These seamounts have been dated between 5 and 14.5 million years old,
which would be ample time for this speciation, assuming that the iguanas did originate
from a single South American species. Geologic data show that the drowned archipelago
remained over the Galapagos hotspot longer than the present emergent Galapagos
Islands, and therefore existed as an island chain for a longer amount of time, allowing
longer time for speciation of life on the islands. (Werner et al., 1999; Christie et al.,
1992)
The Galapagos Islands (or the Colon Archipelago) are located about 1050 km off the
western coast of South America near the equator. The chain consists of fifteen large and
several hundred small islands. The islands are all of volcanic origin and many are
mountainous and contain volcanic craters, some of which are still active; of the 21
emergent volcanoes, 13 have erupted in the Holocene (White et al., 1993). These are
basaltic shield volcanoes rising from the submarine Galapagos Platform (Nordlie, 1973).
The origin of Galapagos volcanism is the Galapagos hotspot, a 90 million-year-old
mantle plume of Mid-Cretaceous origin (Hauff et al., 1997).
The Galapagos Archipelago is situated upon the Nazca plate, south of the Galapagos
spreading center (Fig. 1). The Galapagos spreading center accretes crust asymmetrically
to both the Cocos and Nazca plates (Hey et al., 1977). The Galapagos hotspot, along
with the Galapagos spreading center, has created two hotspot tracks, the Cocos and
Carnegie Ridges. The Cocos Ridge lies on the Cocos plate while the Carnegie Ridge lies
on the Nazca plate. These ridges extend from the Galapagos Islands to the trenches along
Central and South America, where they are being subducted (Werner et al., 1999).
Figure 1. Map of the Galapagos Islands and Galapagos spreading center showing
the Cocos and Carnegie Ridges. From Feighner and Richards, 1994.
Many techniques have been used to date the present emergent Galapagos islands.
For many years, it was believed that the islands were very old, in order for the biota
present to have become so differentiated and speciated. Bailey’s (1976) potassium-argon
ages date the islands at a maximum of 3 million years old. Hall’s (1983) dates from
Espanola Island are more than 3 million years old, more than twice as long as previously
calculated. Based on stratigraphy and paleontological evidence, Hickman and Lipps
(1985) concluded an age of at least 3 million years for the islands.
Drowned seamounts have been discovered off the coast of Costa Rica that are
believed to have once been emergent parts of the Galapagos Island chain (Fig. 2). Rocks
dredged from these seamounts show rounding and other evidence of sea-level erosion and
weathering. This implies that the seamounts were once emergent islands, and therefore
capable of sustaining terrestrial life. (Christie et al., 1992; Werner et al., 1999)
These seamounts are located near the convergent boundary between the Cocos and
Caribbean plates just off the coast of Costa Rica (Fig. 2). The subduction of the Cocos
plate beneath the Caribbean plate has uplifted the mountain ranges in southern Costa Rica
and has led to the extinction of volcanism in the area during the last 5 million years
(Barckhausen, et al., 1998). Questions were posed by many investigators (von Huene et
al., 1995; Hinz and Block, 1992) as to whether other seamounts have been subducted
beneath Costa Rica along with the Cocos plate and whether they remain intact beneath
the overriding Caribbean plate. Magnetic signature data has shown that the seamounts
have subducted, causing furrows in the continental slope, and that the seamounts retain
their magnetization, indicating that they remain physically intact for some time after
subduction (Barckhausen et al., 1998).
Figure 2. Map of seamounts off the coast of Costa Rica. Points 1 through 3 indicate
seamounts upon the oceanic crust. Points 4 through 8 indicate areas where seamount
subduction is supposed. Modified from Barckhausen, et al., 1998.
The drowned seamounts have ages ranging from 5 million years old (Christie et al.,
1992) to 14.5 million years old (Werner et al., 1999). A small 9 million-year-old
seamount at 85 degrees 40’ W 2 degrees S was found to be composed mostly of glassy
basalt and to have several wave-cut terraces, indicating its extrusive, subaerial origins and
that it was once emergent (Christie et al., 1992). The discovery of a 14.5-million-yearold seamount at the oldest preserved portion of a continuous hotspot track suggests that
there have been emergent islands present above the Galapagos hotspot for at least 14.5
million years (Werner et al., 1999).
Based on genetic distances (calculated values based on DNA analysis indicating the
amount of time that would be needed for one species to evolve into another related
species) this date of 14.5 million years is within the margin of error range (15-20 million
years) for the land and marine iguanas to have diverged from a common mainland
ancestral species (Wyles and Sarich, 1983). Their discovery suggests that the iguanas’
species divergence may have occurred on these drowned islands and subducted
seamounts while they were emergent (Carson, 1992). The emergence of the seamounts
would have significantly increased the time possible for speciation of terrestrial life such
as the iguanas. It is thought that this evidence will inspire biologists to configure new
speciation models based on the emergence and disappearance of island chains forming
over hotspots (Carson, 1992).
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