|
Igneous
Processes and Volcanism
Igneous activity
on Earth is concentrated in three places due to the operation of plate
tectonics. Both plutonism and volcanism occur 1) along the spreading
centers at oceanic and continental divergent plate boundaries, 2) in the upper
mantle and crust above subducting slabs, and 3) in hot spots. The igneous
processes that can occur in these environments are described in Magmatic
Differentiation.
Divergent
boundaries Volcanism occurs along divergent
plate boundaries to form new oceanic crust. The convection of the ductile
asthenospheric mantle near the oceanic ridges and at continental rifts results
in the partial melting of the upwelling mantle peridotite. Partial melting
of mantle peridotite generates basaltic magma. These basaltic magmas may
form magma chambers at depth and partially crystallize there or continue upwards
to erupt near or on the surface of the ocean floor along the ocean ridges.
Rarely,
stratigraphic sections of the oceanic crust and adjacent upper mantle
crystallized at the oceanic ridges, are tectonically emplaced onto land.
Obduction of sections of oceanic crust and mantle is believed to have occurred
in Oman and Cyprus. The exposed sequences consist of a basal peridotites,
stratified magma chambers composed of ultramafic rocks and gabbros, a sheeted
dike complex (steeply-inclined, tablular-shaped igneous intrusions, altered
basalt lava flows and pillow basalts (rounded masses of frozen basalt lavas
quickly cooled with glassy rims formed by submarine eruptions), and capped with
a layer of ocean sediments.
As this page is
written on March 1, 2000, Hekla, a volcano located 70 km
east of Reykjavik, Iceland, is erupting. Hekla, believed to be one of the
two gateways to Hell during the Middle Ages, is one of several active volcanoes
on Iceland. Iceland is located on the Mid-Atlantic Ocean Ridge and
is dominantly constructed of fissure basalt flows produced at the oceanic ridge
(see diagram below). Smaller quantities of rhyolitic lava and tephra also
occur on Iceland. Rhyolitic pillows (as in pillow lavas) the size of large
cars form when rhyolitic lavas are erupted under glaciers (note that the heat of
the eruption causes glacier melting and in some cases, catastrophic
floods). Erosion has exposed the coarser-grained slower-cooled plutonic
igneous rocks in the magma chambers located beneath old extinct volcanic
cones. The volcanic and plutonic igneous rocks produced on Iceland and
along other spreading ridges are characterized by increasing iron and silica
content over time. This compositional evolution produces a series of rocks
called sub-alkaline (formerly tholeiitic).
Hekla
is the most active and the most atypical of the Iceland volcanoes.
Most of the
volcanic rocks produced where continents split apart to form new oceanic crust,
such as along the East African Rift, are also basaltic lavas with minor
rhyolitic material. Extremely alkaline igneous rocks called carbonatites
also occur as both plutonic rocks and very rarely volcanic lavas.
Convergent
boundaries The volcanic activity taking place at
subduction zones differs in composition and type. The volcanism is
typically explosive and partial melting of both upper mantle and continental
crustal rocks is important. Pyroclastic volcanic rocks and tuffs, composed
of glasses, ashes, and volcanic bombs blown out of steep-sided stratovolcanoes
are typical of this environment. The lavas produced by subduction are
typically andesites, that is they contain more silica,
aluminum, sodium and less calcium, magnesium, and iron than the dominantly
basaltic and gabbroic rocks produced at spreading ridges. These rocks make
up an alkaline series because of the marked increase in alkali elements (sodium
and potassium) with increasing silica in the volcanic rocks with continued
crystallization over time.
The volcanism
occurring at subduction zones can occur along an island
arc on an oceanic plate or along a linear volcanic arc on a continental
plate. Pacaya, a Guatemalan volcano, is one of the Andean chain of
continental volcanoes produced by eastward subduction of the Pacific plate under
South America. Mount Mayan is part of a chain of volcanoes composing the
Philippine island arc. The Philippine island arc is the volcanic
expression of the westward subduction of the oceanic Pacific plate under the
oceanic Phillippine plate.
Hot
spots Hot spot volcanism is typically not
located at the boundaries of tectonic plates. The volcanic islands of
Hawaii, the Azores, and Galapagos, and Yellowstone National Park in Wyoming are
hot spots located some distance from plate boundaries.  According to the
United States Geological Survey, more than a hundred hotspots beneath the
Earth's crust have been active during the past 10 million years. Hot spot
activity is identified by volcanism that has occurred for very long period of
time from small localized sources of high heat. Some of the volcanism
occurring on Iceland may be due to hot spot activity. Hot spot volcanism
typically produces basaltic lavas marked by high alkali contents that increase
with time (alkali basalts).
Igneous
minerals There are a number of gemstones that
crystallize from magma or as a result of magmatic processes. Diamonds are
found in kimberlite, a mixture of gas-charged, highly serpentinized and altered
porphyritic peridotite, containing fragments of mantle and crustal rocks
incorporated during the super-sonic ascent to the Earth's surface (In
Australia, diamonds are contained in lamproite, a potassium- and magnesium-rich
mafic volcanic rock). Diamonds contain glass and mineral inclusions that
record their magmatic origin but they did not crystallize from the host
kimberlite or lamproite magmas. Based on the isotopic composition, the
carbon in some diamonds may have been recycled back into the mantle as a result
of sediment subduction. The diamonds occurring in South African eclogites
formed through metamorphic processes. Diamond formation is complex and the
best resource on the web is The
Nature of Diamond at the American Museum of Natural History.
Olivine and pyrope
(magnesium- and aluminum-rich garnet) crystallize from mafic (compositionally
rich in magnesium and iron) magmas at high temperature and pressure in the upper
mantle. Zircon, monazite, tourmaline, beryl, and topaz crystallize in
water- and  volatile-rich granitic pegmatites. Pegmatites are course-grained rocks,
formed through the crystallization of the last residual melts that are rich in
volatiles such as water and fluorine and the incompatable elements (boron,
lithium, beryllium, niobium, tantalum, uranium, thorium, and the rare
earth elements) that do not fit in the crystal structures of minerals
crystallized at higher temperatures. The separation and expansion of
volatile gases in a near surface magma, lava, or hot volcanic ash deposits
during the last stages of crystallization may produce open void spaces.
Beautiful well-formed crystals of topaz, red beryl, spessartine (manganese-rich
garnet), and other minerals crystallize in miarolitic cavities in pegmatites or
void spaces in rhyolitic volcanoclastic rocks from volatile-rich gases.
|
