Glasgow University’s Centre for Open Studies organised an excursion tour to Gran Canaria in April-May 2016. One of the many spectacular views that the group had was this one into the caldera of the Tejeda volcano.
The rocks to the lower left of the brightly coloured rocks are shield basalts that belong to the lowest stratigraphic unit of Gran Canaria and are thus some of the oldest rocks on the island.They formed from lava that erupted from the volcano in the Miocene, between about 14.5 and 14 million years ago. Following the eruption of the basalts, a huge volume of ignimbrite (the P1 ignimbrite) was ejected, and this led to the collapse of the volcano’s magma chamber to form the huge Tejeda Caldera, an elliptical structure with dimensions 20 km by 17 km. During the following 6 million years there were repeated cycles of pyroclastic eruptions from ring fractures at the caldera rim, and these produced extensive deposits of tuff both inside and outside the caldera.
The brightly coloured rocks to the right of the basalts are tuffs that were laid down inside the caldera during this prolonged phase of igneous activity. The bright colours are the result of low-temperature hydrothermal alteration caused by extensive flows of liquid in the faulted marginal areas of the caldera. Later erosional processes have removed the basalts that originally lay in front of the caldera and what we are seeing is a cross section through the rocks immediately adjacent to the caldera and into the caldera itself: the sloping junction between the shield basalts and the bright tuffs represents the margin of the caldera.