Siebengebirge - Geowanderung: Weilberg

At the quarry wall, two types of rock can be seen: tuff and basalt. Tuff is formed during explosive volcanism, when fine-grained ejecta from an eruption cloud deposits on the slopes of the volcano. Accordingly, the yellowish-brown trachyte tuff, visible in the upper part of the quarry wall, shows distinct, oblique layering. It is overlain by unlayered latite tuff at the very top. The lower part of the wall consists of basalt. The basalt lava did not flow out onto the Earth's surface here, but instead got stuck in the trachyte tuff during its ascent and spread out horizontally as a so-called dyke. During the intrusion of the over 1000°C hot melt, the trachyte tuff at the edge turned reddish "fritted" into basalt - a process similar to the firing of clay bricks. Furthermore, the intrusion of the basalt into the dyke caused the trachyte tuff to bulge, leading to the formation of extension fractures. Later, another basalt melt intruded along a approximately 1 m wide, vertical dyke through basalt and tuff. It widens tulip-shaped near the upper limit of the quarry. This smaller amount of basalt did not have enough heat content to frit the trachyte tuff. The remarkably flat terrain surface, into which the quarry was cut, represents a level surface that formed during the erosion of the Siebengebirge volcano and into which the present-day valleys, such as the Rhine Valley, have been incised due to the uplift of the Rhenish Massif.