The most famous volcano on Iceland is is
located on a WSW-ENE fissure that is the surface expression of the mid-oceanic
spreading ridge in the north Atlantic Ocean. Hekla is a 5 km long and 1.5
km high ridge dominantly composed of the products of repeated fissure eruption
and slightly smaller amounts of tephra. Unlike other Iceland volcanoes, it
is a stratovolcano (composed of alternating layers of lava and tephra (pyroclastic
material) abet an elongated statavolcano. Stratavolcanoes are
characteristic of subduction zone volcanism, not spreading ridge volcanism.
Hekla is the only volcano on Iceland
that erupts calcalkaline lavas. Calcalkaline lavas are characteristic
products of subduction zone volcanism; they are very uncharacteristic of
spreading ridge or hot spot volcanism. The lavas and pyroclastic materials
produced by the eruption of Hekla since the Ice Age can be described as two end
members of a series, one highly silicic (rhyolitic), the other andesitic.
The andesitic end member was once known as icelandite (at one time there were
over 300 igneous rock names in use, emphasizing small compositional or
mineralogical differences in the rocks. This practice of over-naming is no
longer encouraged by the International Union of Geological Sciences, the
professional organization responsible for naming igneous
rocks. Instead, reporting of chemical analyses of volcanic rocks
has become standard practice). Intermediate magmas between the rhyolitic
and andesitic end members may result from magma mixing.
The magma chamber below Hekla is at a
depth of about 8 km (5 miles) blow the volcano. Hekla has had a number of large
eruptions, producing vast amounts of tephra (volcanic ash and coarser-grained
volcanic which repeatedly covered up to two thirds of Iceland with rhyolitic
tephra. One eruption occurring in the year 1104, was tremendously
explosive and produced 2.5 km3 (0.6 miles3) of tephra.
Explosive eruptions producing any tephra are rare in this tectonic environment
(i.e. spreading ridges). Typically, Hekla erupts both lava and tephra,
hence the composite nature of the stratovolcanic cone.
Photographs of the February-March, 2000
Hekla eruption are provided by the Nordic
Volcanological Institute and a Iceland