Download Shield Volcanoes

Volcano Formation
Volcanoes form when hot material from below
rises and leaks into the crust. This hot material,
called magma, comes either from a melt of
subducted crustal material, and which is light and
bouyant after melting, or it may come from deeper
in the interior of a planet and is light and bouyant
because it is *very* hot.
Magma, rising from lower reaches, gathers in a
reservoir, in a weak portion of the overlying rock
called the magma chamber. Eventually, but not
always, the magma erupts onto the surface. Strong
earthquakes accompany rising magma, and the
volcanic cone may swell in appearance, just before
an eruption, as illustrated in this picture. White
arrows in the picture show the volcano getting
bigger as magma rises inside. Scientist often
monitor the changing shape of a volcano,
especially prior to an eruption. The different
reasons why a volcano forms are
•via plumes or hot spots in the lithosphere
•as a result of subduction of the nearby
lithosphere
Shield Volcanoes
Shield volcanoes are the largest volcanoes on Earth that actually look like volcanoes (i.e. not
counting flood basalt flows). The Hawaiian shield volcanoes are the most famous examples.
Shield volcanoes are almost exclusively basalt, a type of lava that is very fluid when erupted.
For this reason these volcanoes are not steep (you can't pile up a fluid that easily runs
downhill). Eruptions at shield volcanoes are only explosive if water somehow gets into the
vent, otherwise they are characterized by low-explosivity fountaining that forms cinder cones
and spatter cones at the vent, however, 90% of the volcano is lava rather than pyroclastic
material. Shield volcanoes are the result of high magma supply rates; the lava is hot and littlechanged since the time it was generated. Shield volcanoes are the common product of hotspot
volcanism but they can also be found along subduction-related volcanic arcs or all by
themselves. Examples of shield volcanoes are Kilauea and Mauna Loa (and their Hawaiian
friends), Fernandina (and its Galápagos friends), Karthala, Erta Ale, Tolbachik, Masaya, and
many others.
Strato Volcanoes
Strato Volcanoes comprise the largest percentage (~60%) of the Earth's
individual volcanoes and most are characterized by eruptions of andesite and
dacite - lavas that are cooler and more viscous than basalt. These more viscous
lavas allow gas pressures to build up to high levels (they are effective "plugs" in
the plumbing), therefore these volcanoes often suffer explosive eruptions. Strato
volcanoes are usually about half-half lava and pyroclastic material, and the
layering of these products gives them their other common name of composite
volcanoes. The lava at strato volcanoes occasionally forms 'a'a, but more
commonly it barely flows at all, preferring to pile up in the vent to form
volcanic domes. Some strato volcanoes are just a collection of domes piled up
on each other. Strato volcanoes are commonly found along subduction-related
volcanic arcs, and the magma supply rates to strato volcanoes are lower. This is
the cause of the cooler and differentiated magma compositions and the reason
for the usually long repose periods between eruptions. Examples of strato
volcanoes include Mt. St. Helens, Mt. Rainier, Pinatubo, Mt. Fuji, Merapi,
Galeras, Cotopaxi, and super plenty others.
Rhyolite Caldera Complexes
Rhyolite caldera complexes are the most explosive of Earth's volcanoes but often don't even look
like volcanoes. They are usually so explosive when they erupt that they end up collapsing in on
themselves rather than building any tall structure (George Walker has termed such structures
"inverse volcanoes"). The collapsed depressions are large calderas, and they indicate that the
magma chambers associated with the eruptions are huge. In fact, layers of ash (either ash falls or
ash flows) often extend over thousands of square kilometers in all directions from these calderas.
Fortunately we haven't had to live through one of these since 83 AD when Taupo erupted. Many
rhyolite caldera complexes, however, are the scenes of small-scale eruptions during the long
reposes between big explosive events. The vents for these smaller eruptions sometimes follow the
ring faults of the main caldera but most often they don't. The origin of these rhyolite complexes is
still not well-understood. Many folks think that Yellowstone, for example, is associated with a
hotspot. However, a hotspot origin for most other rhyolite calderas doesn't work; they occur in
subduction-related arcs. Examples of rhyolite caldera complexes include Yellowstone, La
Primavera, Rabaul, Taupo, Toba, and others.
Monogenetic Fields
Monogenetic fields also don't look like "volcanoes", rather they are collections
of sometimes hundreds to thousands of separate vents and flows. Monogenetic
fields are the result of very low supply rates of magma. In fact, the supply rate
is so spread out both temporally and spatially that no preferred "plumbing" ever
gets established; the next batch of magma doesn't have a pre-existing pathway
to the surface and it makes its own. A monogenetic field is kind of like taking a
single volcano and spreading all its separate eruptions over a large area. There
are numerous monogenetic fields in the American southwest and in México,
including Michoacan-Guanajuato, San Martín Tuxtla, Pinacate, and the San
Francisco volcanic field.
Flood Basalts
Flood basalts are yet another strange type of "volcano." Some parts of the world are covered by
thousands of square kilometers of thick basalt lava flows - individual flows may be more than 50
meters thick, and individual flows extend for hundreds of kilometers. The old idea was that these
flows went whooshing over the countryside at incredible velocities (e.g., like a flash flood). The
new idea is that these flows are emplaced more like flows, namely slow moving with most of the
great thickness being accomplished by injecting lava into the interior of an initially thin flow. The
most famous US example of a flood basalt province is the Columbia River Basalt province,
covering most of SE Washington State and extending all the way to the Pacific and into Oregon.
The Deccan Traps of NW India are much larger and the Siberian Traps are even larger than that
(but poorly understood). The Ontong Java plateau may be an oceanic example of a flood basalt
province.
Mid-ocean ridges
This is a map of the major oceanic
spreading centers. This is sometimes considered to
be one ~70,000 km-long volcano. Here, the plates
are pulled apart by convection in the upper mantle,
and lava intrudes to the surface to fill in the space.
Or, the lava intrudes to the surface and pushes the
plates apart. Or, more likely, it is a combination of
these two processes. Either way, this is how the
oceanic plates are created.