Alex Hoxsie
Whitney Merrill
October 21, 2008
Term Paper: Draft 1
Island Life:
The truth about hot spots and volcanic arcs
From the densely populated and highly industrialized Japan to the tropical paradises of
Hawaii and the rich fishing grounds of the Aleutians, islands support life in every climate we
observe on the Earth. Geological processes related to the constant movement of tectonic plates
create these oases rising from the abyss. The Earth’s crust is comprised of many lithospheric
plates, essentially sheets of cold, rigid rock that make up the planet’s surface as we know it.
There are twelve major plates that float atop the warmer and less rigid asthenosphere. As the
individual plates move, they collide with, rub against, and pull apart from each other. The
motion of the plates and these interactions between them are responsible for shaping the surface
of the Earth and allowing molten rock, or magma, to rise up from far below the surface, creating
volcanoes and eventually island chains. As we explore the birth of volcanoes is it relates to
islands, we will also describe why island chains occur where they do, what specifically they’re
made up of, and how old they are. By the end of this paper we hope to have made sense of the
seemingly random nature of islands and prove that their formation follows a documented and
timeless pattern. But before we can move on, it is imperative that we introduce the two different
types of island chains that exist in the oceans of Earth. First, we will discuss volcanic island arcs
that exist along certain plate boundaries. These islands are directly related to tectonic
interactions. Hot spot island chains, on the other hand, are the result of intraplate volcanism,
magma rising up through the crust away from plate boundaries. Although dependent on plate
movement, these island chains are not products of plate interactions. By the end of this paper,
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the differences between island arc and hot spot chains should be apparent and the reader will
hopefully be able to distinguish between them in the real world thereafter.
Volcanic Arcs
Volcanic island arcs form as tectonic plates collide and one subducts, or sinks, under the
other. “Subduction zones are fundamental sites of melt generation, crustal genesis, and recycling
of material between crust and mantle” (Jicha). In order for an island arc to form, oceanic crust
from one plate has to subduct under another piece of oceanic crust. Let’s take the Aleutian
Islands as an example. As the Pacific plate moves, it collides with the North American plate and
is forced down under it into the warmer mantle (Jicha, 2006). “The modern sinking lithosphere
does not melt in most subduction zones; instead it releases aqueous fluids that trigger the mantle
wedge to melt” in what is called partial melting (Tatsumi and Eggins, 1995). As this partial
melting occurs, magma moves upwards through the upper asthenosphere and then the lithosphere
towards the surface. This magma has higher silica content from the partial melting, becoming
less mafic as it rises towards the surface, eventually erupting as andesitic lava. As the lava
builds up over time, the aquatic volcano gets gradually higher, until it finally breaks the ocean
surface, forming an island. When an aquatic volcano becomes dormant before it reaches the
ocean’s surface, it forms a seamount.
Subduction zones occur around the world, and island arcs are formed whenever these
subduction zones involve two plates of oceanic crust. Currently, such locations are largely found
in the Pacific. Japan and the Philippines are large, widely known island chains, but there are
many other island arcs in southern Oceania all the way up to the Aleutian Islands between
Alaska and Russia. In fact, by tracing the western edge of the Pacific plate, the eastern edge of
the Eurasian plate, and the Philippine plate we can see where the majority of island forming
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subduction occurs today. However, for comparison’s sake we should mention that subduction
also occurs elsewhere. When a section of oceanic crust subducts beneath continental crust, the
same process of partial melting occurs but the volcanoes form on land instead of on the ocean
floor. These volcanic arcs are formed through the same process, but their locations and
compositions are different.
The formation of island arcs begins far below the Earth’s surface when basaltic rock is
turned to magma by the partial melting that we’ve already discussed. Basaltic magma,
characterized by its low silica content, would erupt as a runny, low-viscosity, lava. However,
this is not the type of eruption that we observe on island arcs. “Because melts formed by partial
melting tend to be richer in silica than the rock from which they were derived, partial melting of
ultramafic rock can produce mafic magma” (Marshak 158). This is why mafic rock under island
arcs becomes less mafic (or more felsic) is it rises towards the surface. This less mafic magma is
also more viscous and therefore much more explosive during eruptions. As the lava cools it
becomes largely basaltic andesite and andesite. However, it is important to realize when
studying trends that there are always exceptions. In 2006, “P-wave velocity models developed
from the recent seismic studies have been interpreted to reflectan overall basaltic composition of
the [Aleutian] arc crust, which is considerably more mafic than the crustal structure of other
island arcs” (Jicha, 2006).
Island arcs vary greatly in age. Although some, like the Aleutian Islands, began to form
around forty-six million years ago, others are in their formation stage today. It is less important
to think about when an island arc formed than how long it will last into the future. Weathering,
by wind and water, gradually break down the rocks that make up island arcs, bringing them back
to the seas from which they sprouted. As the basalt that makes up the oceanic crust gets older, it
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sinks and sits lower on top of the mantle. This process gradually brings the island arc that
formed out of the oceanic crust down with it. As island arcs weather and sink, they become
smaller and some cease to be islands all together. They return to the ocean as seamounts and
guyots (flat topped seamounts). When considering the life span of an island arc, it is important
to note its size, composition, and the density of the oceanic crust it is built upon. With these
factors in mind, geologists can trace out what they believe will happen to island arcs as they
travel through geologic time.
Hot Spots
The Hawaiian Islands and the Galápagos Islands, along with many other island chains
throughout the oceans of the world, all have one thing in common, their creation. These islands
were formed by hot spots. Currently, there are about 44 different hot spots (Lin, 1998) that have
been identified throughout the world, and about 50 to 100 active hot spot volcanoes. Jason
Morgan, an American geologist, suggested the current accepted model of how hot spots are
created. His model proposed that deep in the mantle a plume, which consists of solid rock, forms
from the heat of the core and rises towards the surface of the Earth as the rock becomes less
dense. Eventually, the mantle plume reaches the lithosphere, where it partially melts to create
magma, which rises and eventually turns into a pool of magma in the Earth’s crust. The pool of
magma then erupts at the surface forming a new volcano. The volcano over the hot spot erupts
over time, and eventually the magma builds enough up to peak through the surface of the ocean,
to create the first sign of a new island. Sometimes however, a hot spot volcano never reaches the
surface of the ocean, it is categorized as a seamount (Lin, 1998) (Marshack, 2008).
The creation of a hot spot island chain is one that takes place over a long period of time.
First, the hot spot volcano underneath the surface of the ocean builds up magma to create a
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seamout, if magma build up continues, the seamount will turn into an island. This island will
continue to grow as volcanic eruptions continue. However, due to tectonic plate movement, the
life of a hot spot volcano eventually comes to an end. As the plates move and change through
the process of subduction at convergent plate boundaries, and the birth of new crust at mid-ocean
ridges or divergent boundaries, the hot spot remains in its fixed location in the lithosphere.
Eventually, the old volcano becomes inactive, and the movement of the plate causes the birth of
a new volcano, and the process begins again. This process continues, and often an island chain is
born. The shape of the island chain also gives insight into how fast and in what direction the
plate moved over time, since the volcanoes migrate with the moving plate. The age of individual
hot spots can be determined by oldest, and most likely inactive, volcano amongst the chain. As
the islands volcanoes move further away from the hot spot, they grow older in age, with the
youngest island volcano or seamount being directly over the hot spot (Marshack, 2008).
Island arc formation differs in many ways from hot spot island formation. The most
obvious difference between the two is the location in which each island formation appears.
Island arcs always exist along a subduction zone, near two plate boundaries. Hot spots,
conversely, occur in random locations throughout the oceans and continents. Again, island arc
volcanoes are formed from the partial melting of the subducting crust of one plate under the
other, where as hot spot magma is created from a rising plume which melts parts of the
lithosphere. Another identifiable difference between the two island formation types would be the
time in which the volcanoes and islands appear. With a hot spot, one island will always be older
or younger than the other due to the movement of the plate the hot spot sits under. An island arc,
though, has the potential to have several synonymous volcano and island births.
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While the majority of hot spots exist in the oceans, a handful of hot spots also exist on the
continents; volcanoes in parts the western United States are formed from a continental hot spot
called Yellowstone. This same hot spot also creates the very famous Yellowstone geysers. The
majority of hot spots lie in the interior of plates as opposed to plate boundaries. However, a few
occur at mid-ocean ridges. When this occurs much more magma is produced, the best example
of this is the island of Iceland. Some of the most well known island chains were created by hot
spots, such as the Easter Islands, the Hawaiian Islands, and Galápagos Islands (Lin, 2998).
The composition of the magma that erupts at the sight of a hot spot varies. Usually, the
composition of oceanic hot spots is mafic, in the form of basalt, from the partial melting of
peridotite from decompression at the base of the lithosphere. Continental hot spots often erupt
with mafic magma. Sometimes, the heat from the magma partially melts the continental crust,
which produces felsic magma. This magma then forms rhyolite (Marshack, 2008).
Volcanoes are very important in the process of shaping continents and other bodies of
land. Without the tectonic interactions between two plates, like convergent boundaries, which
create subduction zones, island countries like Japan would not exist. Hot spots are not only
valuable in their creation of new islands, but they also provide scientist with more information on
how fast plates move and change. Furthermore, the questions about how hot spots form are not
questions that have been answered completely. A hot spot like the one that created the Samoan
Islands, puzzles scientists because the progression of age does not match up with the established
movement of the plates (Koppers et al, 2008). Overall, island arcs and hot spot islands give
scientists another glimpse into the Earth’s past.
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Reference List
Jicha B.R., Scholl D.W., Singer B.S., Yogodzinski G.M., Kay S.M., Revised age of
Aleutian Island Arc formation implies high rate of magma production. Geology, 34, No.
8, pp. 661–664, 2006.
Koppers, A.A.P, Russell, J.A., Jackson, M.G., Konter, J., Staudigel, H., Hart, S.R., Samoa
reinstated as primary hotspot trail, Geology, 36 No. 6, pp. 435-438, 2008.
Lin, J., Hitting the hotspots: critical interactions between hotspots and mid-ocean ridges,
Oceanus, 41 No. 2, pp. 34-37, 1998.
Marshak, S. Earth, Portrait of a Planet, Third Edition. Boston: W. W. Norton &
Company, Incorporated, pp. 85-180, 2008.
"Seamounts along California's continental margin." Submarine Volcanism: Continental margin
seamounts. 2003. Monterey Bay Aquarium Research Institute. 22 Sept. 2008
<http://www.mbari.org/volcanism/seamounts/seam-contlmarg.htm>.
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