When lava shreds in the inside of the crater are deposited close to a round or column-shaped vent they are often so hot that they fuse into compact rock. In this agglutinate the original shape of the long lava shreds, which can be metres long, is difficult to make out. By contrast, the slags in the overflow area are often fused to the edge of the crater but the individual shreds are still clearly visible (cf. SCHMINKE, H.-U., 2000: Volcanic activity).

Red sandstone
The geological period linked to sandstone is the oldest part of the Triassic. The colourful sedimentary rocks that were formed at this time are mostly red sandstone and shale as well as gypsum and rock salt. They were formed in continental, dry desert climates and can be found in the Weser mountain region, Harz, Thüringen, Hessen and the Schwarz- and Odenwald; it is used as hard core in road-building or as for stone slabs in building construction.

Signs of current volcanic activity in the Vulkaneifel are, amongst others, the mineral springs that bubble up all over the place in the valleys of this romantic landscape; some are easily seen whilst others are deep within bore holes. These mineral waters in and around Daun are very similar to one another in their concentration and composition; they are “sodium-magnesium-hydrogen carbonate mineral waters”. The Celts called them “thriasan” or in High German “Sauerbrunnen”. In the Eifel dialect you will more often hear the term “Drees”, which comes from the Celtic “draisen”. This has a similar meaning to “flow”.


Erosion is the removal of loose parts of earth from the earth’s surface by water or wind.

An eruption (from the Latin eruptio) refers to an ejection of material similar to an explosion.

The expression “folding” refers in geology to the formation of layers within the earth’s crust.

A geyser (Icelandic geysa- to gush) is a hot spring that discharges water as a jet at regular or irregular intervals.

Lava bombs (at Strohn)
The Strohn lava bomb, weighing almost 120 tonnes, is a special geological feature.

With a diameter of almost 5m it is a unique, impressive reminder of earlier volcanic activity in the Eifel. It was discovered in 1969 during a blast at the Wartgesberg quarry, 15m up in the quarry wall. In the winter of 1980/81, local residents using a bulldozer towed it on an iron plate over the frozen snow to where it stands today.

Its origins were disputed for many years as the volcanic forces in the Eifel were not sufficient to have propelled a ball of this size through the air. Drilling into the core of the ball answered the question: during the eruption of the northern Wartgesberg volcano part of the crater wall became separated and rolled back into the vent.

As it did so, it collected molten lava shreds that stuck to its surface. With the next eruption the ball was hurled skywards once again, and once again it rolled back. This process happened several times until the ball was the size it is today. It then became embedded in the crater wall and was only discovered when it came to be demolished.


(Coal) seam
A seam is a horizontal, relatively thin layer of recoverable minerals (e.g. coal seam, ironstone seam). The coal seams sometimes continue to the surface. This means that at some points the coal appears as black sedimentary rock. When coal was first mined, only the surface was dug for coal. However, this type of coal mining ended in the 18th century as the demand for coal grew quickly. To access the coal seams that lay deeper, tunnels were later used, followed by shafts.

Magma (Greek: “mixture”) is a mass of molten rock that appears in sections in the earth’s upper mantle and the lower part of the earth’s crust. Magma’s flowability is what creates volcanic activity (see also volcano) and is important for rock formation as it is from solidified magma that rock called magmatite or plutonic rock forms.

This is the difference between plutonic rock, where the magma in the earth’s core slowly cools and forms crystals and volcanic rock, when the magma cools quickly when it reaches the earth’s surface (for example, as lava or a pyroclastic flow). An example of plutonic rock is granite, whereas basalt is a volcanic rock. (

The term slag is a general description for a blasted, porous residue.

(Sediment), from the Latin sedimentum (act of settling). Sediment is made up of particles found settled in liquid as a suspension. They can also be called deposits. Particles transported by the wind that are deposited when wind speeds decrease are also known as sediment.

The origin of volcanoes:
For a long time it was thought that volcanic rock only made up a small proportion of the earth’s crust. Today, however, we know that including the sea bed 3/4 of the surface of the earth is made up of volcanic rock. Currently on the earth there are around 550 volcanoes described as active or dormant; the latter could re-awaken at any time, as shown by the powerful eruption of Mount Pinatubo in the Philippines on 15th June 1991.

How does a volcanic eruption happen?
Many volcanic eruptions are similar to the popping of a champagne bottle. The inside of the bottle is under pressure and the carbon dioxide is dissolved in the champagne. When the cork is popped, the pressure drops and the escaping carbon dioxide pushes the champagne upwards. Something similar happens in a volcanic eruption. In a chamber deep below the volcano there is liquid molten rock under pressure. If there is a rupture in a fissure in the earth’s crust then the pressure is released and the magma separates. The gases, mainly water vapour, are released and push the liquid rock up to the earth’s surface. The volcano erupts. However, not all magma reacts like this. It is dependent on the chemical composition of the magma as to which type of volcano is formed and how the magma appears at the earth’s surface.

What types of volcano are there?
Essentially the silicon dioxide and water vapour content of igneous molten rock determines how the magma appears at the earth’s surface and which type of volcano is formed.

A. Shield volcano:

1. If the silicon dioxide and water vapour content of the magma is low then highly fluid lava is created with no large explosions. Most of the time this fluid lava is considerably hotter than viscous lava. It therefore takes a long time to solidify and can flow a long way for a long time. This is how flat shield volcanoes are created with a large diameter and a low profile (e.g. Hawaiian volcanic islands).
2. If the silicon dioxide content is low but the water vapour content is high then the bubbles can rise unhindered through the fluid magma. This is how fiery lava jets are created that are accompanied by small explosions. This will also create shield volcanoes, but ones that extend laterally.

B. Stratovolcanoes:
If, as well as having a high silicon dioxide content, the magma also has a high water vapour content, than viscous magma is created. This stops the bubbles from rising to the top, and creates excess pressure that leads to powerful explosions. In these, the lava and other material is ejected in fine ash particles and hurled several km into the atmosphere. If in the meantime the pressure drops then as well as ash and slag being ejected, magma comes out too in so-called lava flows. It is these alternating strata of ash, slag and lava that create stratovolcanoes or cinder cones with a collapsed summit. These are the most common type of continental volcano. The Quaternary cinder cones in the Eifel belong to this type, e.g. the Mosenberg groups of volcanoes near Manderschied in the Vulkaneifel.

C. Maars:
Maars, on the other hand, are crater-shaped volcanoes and the second-most common continental volcano. They are formed when hot magma and groundwater come into contact in a hydraulically active fracture in solid rock. Powerful water vapour explosions fragment the surrounding rock and propel it upwards. After a while the explosion chamber collapses, forming a maar crater at the surface. The maar tephra deposited on the surface, in a wall around the crater, are made up of 80% rock and 20% volcanic ash and lapilli. This type of volcanic activity is called a phreatomagmatic eruption. In the Eifel, up to 75 maars have been discovered. Most of them are what are called dry maars (Eckfelder Maar etc.). A few maar craters have been partly filled by lakes (Merfeelder Maar, Holzmaar, Pulvermaar etc.), marshes or moors (Hinkelsmaar, Dürres Maar etc.).

Windsborn mountain crater lake
The only mountain crater lake north of the Alps. The water is low in nutrients and 1.7m deep. There were various eruptions around 43,000 – 29,000 years ago.