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 little-changed 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.
Here are 4 of the volcanoes that comprise the big island of Hawai'i.
They are Mauna Kea (MK), Mauna Loa (ML), Hualalai (H), and Kohala
(K). The photo was taken from near the summit of East Maui volcano
(EM). These are the largest volcanoes on Earth.
This is a vertical air photo of the summit caldera of Mauna Loa
volcano (North is to the left). Notice that the caldera is composed
of numerous smaller "cookie-cutter" collapses which
have coalesced to form the main caldera. Notice also that many
of the lava flows (dark and light are 'a'a and pahoehoe, respectively)
have been truncated by the caldera margin. This is an indication
that they erupted from the volcano summit when the caldera was
full. Collapse since then has produced the present caldera.
In this manner of collapsing and filling, calderas come and go
throughout the active lifetime of a basaltic volcano.
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