4. VOLCANIC ACTIVITY AND RELIEF

Talking book 

The type of eruption depends mainly on the viscosity of the magma that the volcano eject⁶. 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

Talking book 

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 steep⁷.

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.

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.

4.2. Volcanism in Spain

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In the Iberian Peninsula there are areas of recent volcanism (8 million years ago) in Cabo de Gata and Campo de Calatrava, and very recently (< 1 million years) in La Garrotxa, but the Canary Islands are the only volcanically active areas. The last submarine eruption on the islands took place in 2011 off the coast of El Hierro; on the surface, the last eruption was the Teneguía volcano on the island of La Palma, in 1971. The Canary Islands are located far away from the plate boundaries. Their volcanic origin appears to be due to the rupture zone running from east to west that periodically opens and allows magma to rise. Successive volcanic activity over the last twenty million years has caused the islands to emerge from the ocean floor.