FORMATION OF VOLCANIC ISLANDS AND HABITABLE LAND
São Vicente in the past and today
São Vincente is a so called stratovolcano, which is composed by many layers created by different lava flows and pyroclastics (Fig. 1).
Fig. 1: Cross-section of a stratovulcano
The alternating layers are interrupted by magma that is pushed through these pre-existing rocks, called dykes (black lines in Fig. 2). The oldest known rocks on São Vicente formed 9 million years (Ma) ago (pink shaded areas in Fig. 2). At Sao Pedro they are 5.5 – 4.5 Ma years old (dark blue layers). The youngest rocks are 0.3 Ma in Praia Grande and Calhau (yellow).
Figure 2: Cross section of the present landscape of São Vicente (modified after Ancochea et al.,2010 – more information)
Layer formations and dyke intrusions – what it looks like in the field
Layers of porous, less dense volcanic rocks (a) are found alongside dense intrusions called dykes – fine grained solid rock (b), formed when magma intruded through cracks in existing rocks
Hydrothermal alteration and weathering processes
The erosion to rock layers caused by wind and rain is known as weathering. But before weathering processes change the composition of the rocks, they could also change due to the interaction of hot water fluids, dissolving elements from the surrounding rock. The reddish color is created by iron, when it came in contact with oxygen. Dykes are denser and more resistant to weathering.
Building on an active volcano: Fogo, Cabo Verde
On the island of Fogo, 40% of buildings are constructed of solidified lava (scoria) [Jenkins et all 2017] due to its strength, abundance, and high thermal resistance.
The figure highlights the unique design features of a funco-style house on Fogo, constructed after the 2014/2015 eruption.
Jenkins, S.F., Day, S.J., Faria, B.V.E., & Fonseca, J.F.B.D. (2017). Damage from lava flows: insights from the 2014–2015 eruption of Fogo, Cape Verde. Journal of Applied Volcanology 6:6. https://doi.org/10.1186/s13617-017-0057-6
(constructed post-2014/2015 eruption)
Meghan Troup (TOSST), Allison Chua (TOSST), Manuel Fortes (Uni CV), Jacqueline Bertlich (HOSST)