Geology Reference
In-Depth Information
through which an active fl ow can be observed (Figure 5.3b),
or access can be gained to an inactive lava tube. In Hawaii, lava
moves through lava tubes many kilometers long and, in some
cases, discharges into the sea.
Geologists defi ne two types of lava fl ows, both named
for Hawaiian fl ows. A
pahoehoe
(pronounced
pah-hoy-hoy
)
fl ow has a ropy surface much like taffy (
latter are viscous enough to break up into blocks and move
forward as a wall of rubble.
Pressure on the partly solidifi ed crust of a still-moving
lava flow causes the surface to buckle into
pressure ridges
(
Figure 5.5a). Gases escaping from a flow hurl globs of
lava into the air, which fall back to the surface and adhere to
one another, thus forming small, steep-sided
spatter cones
, or
spatter ramparts if they are elongated (Figure 5.5b). Spatter
cones a few meters high are common on lava fl ows in Hawaii,
and you can see ancient ones in many areas.
Many lava flows, especially
mafic ones, have a distinctive
pattern of columns bounded by
fractures, or what geologists call
columnar joints
. Once a lava
flow stops moving, it contracts
as it cools and produces forces
that cause fractures called
joints
to open. On the surface of a lava
fl ow, the joints are commonly po-
lygonal (often six-sided) cracks
that extend downward, thus
forming parallel columns with
their long axes perpendicular to
the cooling surface (
◗
Figure 5.4a). The
surface of an
aa
(pronounced
ah-ah
) fl ow is characterized by
rough, jagged, angular blocks and fragments (Figure 5.4b).
Pahoehoe flows are less viscous than aa flows; indeed, the
◗
Figure 5.6).
Excellent examples of columnar
jointing are found in many areas.
Much of the igneous rock in
the upper part of the oceanic crust
is a distinctive type consisting of
bulbous masses of basalt that re-
semble pillows, hence the name
pillow lava
. It was long recognized
that pillow lava forms when lava is rapidly chilled beneath wa-
ter, but its formation was not observed until 1971. Divers near
Hawaii saw pillows form when a blob of lava broke through the
crust of an underwater lava fl ow and cooled almost instantly,
forming a pillow-shaped structure with a glassy exterior. The
remaining fl uid inside then broke through the crust of the pil-
low, repeating the process and resulting in an accumulation of
interconnected pillows (
◗
An excellent example of the “taffy-like” appearance of pahoehoe.
a
◗
Figure 5.7).
In addition to lava fl ows, erupting volcanoes eject pyroclas-
tic materials, especially
volcanic ash
, a designation for pyro-
clastic particles that measure less than 2.0 mm (
Z
Figure 5.8).
In some cases, ash is ejected into the atmosphere and settles
to the surface as an
ash fall
. In 1947, ash that erupted from
Mount Hekla in Iceland fell 3800 km away on Helsinki, Fin-
land. In contrast to an ash fall, an
ash fl ow
is a cloud of ash
and gas that flows along or close to the land surface. Ash
fl ows can move faster than 100 km/hr, and some cover vast
areas.
In populated areas adjacent to volcanoes, ash falls and
ash flows pose serious problems, and volcanic ash in the
atmosphere is a hazard to aviation. Since 1980, approximately
b
An aa lava fl ow advances over an older pahoehoe fl ow. Notice
the rubbly nature of the aa fl ow.
◗
Figure 5.4
Pahoehoe and aa Lava Flows Pahoehoe and aa were
named for lava fl ows in Hawaii, but the same kinds of fl ows are
found in many other areas.
Search WWH ::
Custom Search