Volcanoes are mountains, but they are very different from other
mountains; they are not formed by folding and crumpling or by uplift and
erosion. Instead, volcanoes are built by the accumulation of their own
eruptive products—lava, bombs (crusted over lava blobs), ashflows, and
tephra (airborne ash and dust). A volcano is most commonly a conical
hill or mountain built around a vent that connects with reservoirs of
molten rock below the surface of the Earth. The term volcano also refers
to the opening or vent through which the molten rock and associated
gases are expelled.
[Illustration: Fountaining lava and volcanic debris during the 1959
Kilauea Iki eruption of Kilauea Volcano, Hawaii.]
Driven by buoyancy and gas pressure the molten rock, which is lighter
than the surrounding solid rock, forces its way upward and may
ultimately break through zones of weaknesses in the Earth’s crust. If
so, an eruption begins, and the molten rock may pour from the vent as
non-explosive lava flows, or it may shoot violently into the air as
dense clouds of lava fragments. Larger fragments fall back around the
vent, and accumulations of fall-back fragments may move downslope as ash
flows under the force of gravity. Some of the finer ejected materials
may be carried by the wind only to fall to the ground many miles away.
The finest ash particles may be injected miles into the atmosphere and
carried many times around the world by stratospheric winds before
settling out.
Molten rock below the surface of the Earth that rises in volcanic vents
is known as _magma_, but after it erupts from a volcano it is called
_lava_. Originating many tens of miles beneath the ground, the ascending
magma commonly contains some crystals, fragments of surrounding
(unmelted) rocks, and dissolved gases, but it is primarily a liquid
composed principally of oxygen, silicon, aluminum, iron, magnesium,
calcium, sodium, potassium, titanium, and manganese. Magmas also contain
many other chemical elements in trace quantities. Upon cooling, the
liquid magma may precipitate crystals of various minerals until
solidification is complete to form an _igneous_ or _magmatic rock_.
[Illustration: An idealized diagram of a volcano in an oceanic
environment (left) and in a continental environment (right).]
The diagram below shows that heat concentrated in the Earth’s upper
_mantle_ raises temperatures sufficiently to melt the rock locally by
fusing the materials with the lowest melting temperatures, resulting in
small, isolated blobs of magma. These blobs then collect, rise through
conduits and fractures, and some ultimately may re-collect in larger
pockets or reservoirs (“holding tanks”) a few miles beneath the Earth’s
surface. Mounting pressure within the reservoir may drive the magma
further upward through structurally weak zones to erupt as lava at the
surface. In a continental environment, magmas are generated in the
Earth’s crust as well as at varying depths in the upper mantle. The
variety of molten rocks in the crust, plus the possibility of mixing
with molten materials from the underlying mantle, leads to the
production of magmas with widely different chemical compositions.
If magmas cool rapidly, as might be expected near or on the Earth’s
surface, they solidify to form igneous rocks that are finely crystalline
or glassy with few crystals. Accordingly, lavas, which of course are
very rapidly cooled, form volcanic rocks typically characterized by a
small percentage of crystals or fragments set in a matrix of _glass_
(quenched or super-cooled magma) or finer grained crystalline materials.
If magmas never breach the surface to erupt and remain deep underground,
they cool much more slowly and thus allow ample time to sustain crystal
precipitation and growth, resulting in the formation of coarser grained,
nearly completely crystalline, igneous rocks. Subsequent to final
crystallization and solidification, such rocks can be exhumed by erosion
many thousands or millions of years later and be exposed as large bodies
of so-called _granitic_ rocks, as, for example, those spectacularly
displayed in Yosemite National Park and other parts of the majestic
Sierra Nevada mountains of California.
Lava is red hot when it pours or blasts out of a vent but soon changes
to dark red, gray, black, or some other color as it cools and
solidifies. Very hot, gas-rich lava containing abundant iron and
magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava
high in silicon, sodium, and potassium flows sluggishly, like thick
honey in some cases or in others like pasty, blocky masses.
[Illustration: Two Polynesian terms are used to identify the surface
character of Hawaiian lava flows. Aa, a basalt with a rough, blocky
appearance, much like furnace slag, is shown at the top. Pahoehoe, a
more fluid variety with a smooth, satiny and sometimes glassy
appearance, is shown at the bottom.]
All magmas contain dissolved gases, and as they rise to the surface to
erupt, the confining pressures are reduced and the dissolved gases are
liberated either quietly or explosively. If the lava is a thin fluid
(not viscous), the gases may escape easily. But if the lava is thick and
pasty (highly viscous), the gases will not move freely but will build up
tremendous pressure, and ultimately escape with explosive violence.
Gases in lava may be compared with the gas in a bottle of a carbonated
soft drink. If you put your thumb over the top of the bottle and shake
it vigorously, the gas separates from the drink and forms bubbles. When
you remove your thumb abruptly, there is a miniature explosion of gas
and liquid. The gases in lava behave in somewhat the same way. Their
sudden expansion causes the terrible explosions that throw out great
masses of solid rock as well as lava, dust, and ashes.
The violent separation of gas from lava may produce rock froth called
_pumice_. Some of this froth is so light—because of the many gas
bubbles—that it floats on water. In many eruptions, the froth is
shattered explosively into small fragments that are hurled high into the
air in the form of volcanic cinders (red or black), volcanic ash
(commonly tan or gray), and volcanic dust.
[Illustration: During the 1959 eruption of Kilauea Iki, fountaining lava
and volcanic debris completely blocked several of the roads in the
Hawaii Volcanoes National Park.]