What is the difference?

The silicic acid and water vapour content have a defining influence on the viscosity of magma and the type of eruption on the earth’s surface.

If there is a high content of silicic acid and water vapour then viscous magma is formed; when this rises, extremely high pressure can build up that is then released in powerful explosions. Lava, ash and slag are ejected, followed by lava flows when the pressure drops. The alternating stratification of these volcanic materials forms strata that can be seen in the cinder cones and stratovolcanoes of the Eifel. The volcanic mountains created tower over the earth’s surface. On the summit of a volcanic mountain, in what used to be the vent, a lake made entirely of rainwater can form – this is called a crater lake. What characterises the Eifel, however, are the maars, maar volcanoes or maar craters, as they are also known. As magma rises, if it meets groundwater, then the water suddenly evaporates causing huge water vapour explosions. This process, where water meets magma, is what geologists called a phreatomagmatic explosion. This causes the rock surrounding where the two meet to burst and it is then forced upwards and ejected. The burst explosion chamber then collapses and a crater remains at the earth’s surface, surrounded by a ring-shaped wall of ejected material. As well as volcanic ash, these maar deposits typically contain a high proportion of the rock that exploded into tiny pieces as a result of the water vapour explosions. The collapsed crater sometimes fills with water, forming a maar lake.