12
4.
VOLCANIC ACTIVITY AND RELIEF
The type of eruption depends mainly on the
viscosity
of the
magma
that the
volcano ejects
6
. Viscosity measures the resistance of magma to flow. This not only
determines the violence of the eruption, but also the type of materials ejected and
the shape of the volcanic cone.
4.1.
Types of eruptions, materials ejected and associated
landforms
In volcanoes that eject very fluid magma, gases escape easily without causing
explosions and the lava spills far from the crater, causing wide volcanic cones that
are not very steep
7
.
In contrast, in volcanoes that eject very viscous (thick) magma with a lot of gases, it
is more difficult for the lava to come out to the surface. This causes an increase in
the pressure and causes violent explosions that pulverise the magma into ash. This
results in narrow cones.
The materials that volcanoes eject are classified by their physical state:
❚❚
Volcanic gases
: the most abundant are carbon dioxide and water vapour.
❚❚
Lava
: degassed magma that has come out to the exterior. It can be type
aa
, when
its surface is fragmented and rough, or
pahoehoe
, also called ropey lava, which
has a smooth surface. Type aa have a higher viscosity magma than pahoehoe.
❚❚
Pyroclastic materials
are solid materials, classified according to size into
bombs
(bigger than 64 mm),
lapilli
(between 2 and 64 mm) and
ash
(less than 2 mm).
Besides the small cinder cones, there are three main types of volcanic activity and
associated cones.
Hawaiian eruption
Vesuvian or Plinian eruption
Vulcanian or Pelean eruption
Shield volcano
These eject very fluid, pahoehoe lava.
The cones are enormous with a convex
profile and very flat.
Composite volcano or stratovolcano
These are the most frequent in volcanic
belts. Layers of lava alternate with
pyroclastic materials. Its profile is thin and
concave.
Small lava domes and pyroclastic
cones
These eject very viscous lava that solidifies
in the chimney or on the crater. They
cause big explosions and burning clouds
of pyroclastic flows.
Scale
cinder cone
composite of stratovolcano
shield volcano
Pahoehoe or ropey lava
Apply
28.
Which type of lava runs
further before solidifying,
pahoehoe or aa?
Volcanic rocks, in general, are not very resistant to
erosion. That is why the typical shapes, such as
cones
or
lava streams
, are only recognisable in areas of
recent volcanic activity. With time, erosion usually
eliminates the materials ejected near the chimney. The
filling formed by more resistant rocks remains in the
landforms in the Canary Islands called
roques
.
6
eject
:
to throw out by force
7
steep
:
having a high gradient
