This is a preview of the new geologic map of Madeira:
It’s a 1:50,000 scale map, published in 2010, that covers the basic geology for the island of Madeira. This new geologic map presents the geology of Madeira more accurately than any of the other previous geologic maps.
Madeira began to form as a small island about 5 million years ago. From then on, overtime, along a few millions years, layers and layers of volcanic material accumulated forming the large shield volcano of Madeira. New lava flows covered and hid the material that had been deposited in the previous layers. But later, erosion would carve valleys exposing the older rock formations that had been kept hidden below in the center of the volcano (in the cross section “X-Y” you can see that the shape of the volcano is not a perfect shield anymore, due to erosion of the volcanic edifice).
As the bottom of the stratigraphic column shows, the first stage of the island formation belongs to the “Lower Volcanic Complex“. This is the complex where you can find the oldest rocks of the island. You can find it outcropping in the areas of São Vicente and Porto da Cruz. Check orange areas in the geologic map. See this Complex outcropping in the cross section “K-L” too.
In the “Intermediate Volcanic Complex” some of the rock formations are also quite old. They lay on top of the “Lower Complex” so they can be found around the same places where that Complex outcrops. But they can also be found in the bottom of many valleys that have been carved by water streams around the island. Check the purplish and blueish areas in the geological map.
The “Upper Volcanic Complex” is at the top of the stratigraphic column, it means that it is more recent. Most of this Complex is composed of layers of lava that flowed on the top of the volcano. There was a time when they covered all the top of the island. However, these layers have been heavily dissected by water streams and today this Complex does not cover the island completely. But not every lava flows were cut by water erosion. Actually, some lava flows are quite young and occurred after the water have carved the valleys. These lava flows ended up flowing along the valleys that had been already opened by water. A great example of this, is the lava flow that occurred in the valley of Seixal where you can find the lava delta it formed when it reached the sea. Check the darker green areas for the later lava flows in the geologic map above. In this same Complex, associated with these dark green lava flows, there are scoria cones (some of them preserving the volcano shape) and craters. You can find these geomorphological features in the whitish areas of the geologic map.
Silveira, A. et al. (2010). Carta Geológica (folhas A e B) and Notícia Explicativa da Carta Geológica da Ilha da Madeira, na escala 1:50000. PDF .