Seismic Waves and Volcanoes
Body waves travel through the body of the Earth
Seismic Wave
and arrive before the surface waves emitted by an
by Jerry Coffey
earthquake. There are two types of body waves: P
waves and S waves.
A seismic wave is the wave of energy caused by
the sudden breaking of rock within the earth or an
The seismic wave known as P waves( primary
explosion. It is the energy that travels through the
waves) can move through solid rock and fluids,
Earth and is recorded on seismographs.
like water or the liquid layers of the earth. It
Seismology is the study of earthquakes and
pushes and pulls the rock it moves through just
seismic waves that move through and around the
like sound waves push and pull the air. These are
Earth. A seismologist is a scientist who studies
the fastest moving seismic waves and are always
earthquakes and seismic waves. There are several
the first to reach a seismic station. An S wave
different kinds of seismic waves. They move in
(secondary wave) is slower than a P wave and can
different ways. The two main types of waves are
only move through solid rock, not through any
body waves and surface waves. Body waves can
liquid medium. It is this property of S waves that
travel through the Earth’s inner layers, but surface
led seismologists to conclude that the Earth’s
waves can only move along the surface of the
outer core is a liquid. S waves move rock particles
planet. Earthquakes radiate seismic energy as both
up and down, or side-to-side–perpendicular to the
body and surface waves.
direction that the wave is traveling.
Surface waves are a type of
seismic wave that only
travels through the crust. S
waves are a lower frequency
than body waves, and are
easily distinguished on a
seismogram because of that.
Though they arrive after body
waves, surface waves are
almost entirely responsible
for the damage and
destruction associated with
earthquakes. There are two
types of surface waves: Love
waves and rayleigh waves.
A Love seismic wave is the
fastest surface wave and
moves the ground from sideto-side. Confined to the
surface of the crust, Love
waves produce entirely
horizontal motion.
The Rayleigh seismic wave
rolls along the ground just
like a wave rolls across a lake
or an ocean. Since it rolls, it
The Earth – Part 1 (ART 7.2)
Seismic Waves and Volcanoes
moves the ground up and down, and side-to-side
Subduction-created magma rises into the lower
in the same direction that the wave is moving.
crust, where it collects and occasionally erupts
Most of the shaking felt from an earthquake is due
vigorously, even explosively. The geometry of
subduction means that volcanoes tend to appear in
to the Rayleigh wave.
long arcs.
Volcanism in a Nutshell
By Andrew Alden
Volcanism is the eruption of material from deep
in the Earth. In many cases, eruptions build up a
pile of material, a mountain that is called a
volcano. But here I'll discuss volcanism, because
most eruptions don't build volcanoes. This is a
highly simplified treatment of an intricate subject.
Four types of magmatic volcanism
The third type of volcanism covers the small
fraction of volcanoes, about 10 percent, in places
that aren't related to divergent or convergent
margins. These are lumped together as hotspot
volcanoes. There are two schools of thought about
hotspots. The majority of geologists consider
hotspots to arise from very deep in the mantle, in
rising plumes of hot material. A minority has a
newer theory involving fracturing of the
lithospheric plates, in which magmas form much
as they do in divergent settings.
The majority of Earth's volcanism happens
underwater at the midocean ridges, or, in platetectonic language, at divergent margins. The crust
is pulled apart, and the hot rock in the mantle
beneath begins to melt as the pressure upon it is
released. Its temperature doesn't change, only the
pressure. The part that melts—the magma—rises
while the rest of the mantle rock stays behind. The
two parts have different compositions: the magma
is basalt and what remains is peridotite, a heavier
rock that is largely olivine.
The fourth type is not occurring today, but has
happened in recent geologic history. It is flood
volcanism, in which enormous amounts of basalt
lava pour out of fissures and cover areas of
thousands of square kilometers. Flood basalts
appear both on land (for example the Columbia
River Basalts and the Deccan Traps) and under
the sea (the Ontong Java Plateau, Kerguelen
Plateau, and more). These are under intense study
but remain a major unsolved problem in geology.
This kind of volcanism is mostly a quiet oozing of
basalt lava out of long cracks in the seafloor.
Seafloor fissure eruptions have been filmed from
research submarines. There are a few divergent
margins on land, and volcanism there is very
different from the oceanic case.
Products of volcanism
The second type of volcanism happens in
association with subduction zones, places where
oceanic plates laden with water and sediment
plunge into the hot mantle. It is responsible for
building most of the world's volcanoes. Fluids that
are driven off the descending plates rise into the
upper, overriding plate where they promote the
formation of magma. Notice that in this case
magma is created by adding water to mantle rock,
not by releasing pressure on it.
The Earth – Part 1 (ART 7.2)
Volcanism recycles the material that goes through
plate tectonics. To a first approximation,
everything that is subducted returns to the surface
Seismic Waves and Volcanoes
in magma. When magma is erupted—that is,
Nonmagmatic volcanism
when magma becomes lava—it returns solids,
There are less well-known types of volcanism that
fluids and gases to the Earth's crust and surface.
don't involve magma: mud volcanism is one. Mud
volcanoes come in two types. On land, hundreds
The solids are igneous rocks, ready to enter the
of them occur in areas where hydrocarbons are
rock cycle. They may be flows of lava that cool
abundant, like Trinidad or Azerbaijan. Under the
into thick layers of hard rock, or shattered
sea, thousands of them occur near subduction
fragments of volcanic ash. Either way, the
trenches, where serpentinite mud is abundant.
minerals of igneous rocks become available to
turn into sedimentary and metamorphic rocks.
Another newly discovered form of volcanism
involves asphalt. Asphalt, or tar, flows were first
The fluids act underground. As they rise magmas
documented on the floor of the Gulf of Mexico in
release water, which incorporates with it dissolved
2003. No one knows how many of these tar
silica, metals and other elements in a chemically
volcanoes there are.
active mix. These magmatic fluids can alter the
rocks around them and deposit bodies of ore and
sulfate minerals.
The gases are what cause lava to erupt. Just as
bubbles form in an opened can of soft drink, so do
sulfur gases, carbon dioxide, and water vapor in
rising magma. The result is that rising magma
expands, and this in turn makes it rise faster. The
gases leave the lava and enter the atmosphere,
where they influence its composition and affect
the global climate in various ways.
The Earth – Part 1 (ART 7.2)