Magnificent Maderian Geology... |
Madeira is a popular destination with tourists of many types. I like to explore and this is a place really worth exploring. There is some general information on the Madiera Landscapes page. Here I try to give some more in depth information about the geology to go with the geology pictures. it will stiill be a general introduction because I am not trying to re-create the many excellent on-line and printed resources a selection of which I will give at the end of this page.
Madiera is almost entirely a volcanic island. Almost all the rocks you will see have come to the surface though a volcano as molten rocks which have been either blown out of the top as ash, squeezed out of the top as lava flows or squeezed in between other rocks. According to most interpretations, Madeira Island is a hot-spot originating from a mantle plume, that is where an upwelling of hot mantle rocks stays in one place as the earth's crust moves across it. The classic example of this in most text books is Hawaii. Geological studies indicate that the emerged part of the island was generated during late Miocene times (around 7 million years ago) through to early Pliocene times (around 2.5 million years ago) so it is a very young island in geological terms. What we are seeing is just the tip of a of a huge stratovolcano about 6 km high, resting on 130-Ma-old oceanic crust
The series of pictures in this gallery show sequences of ash and lava deposits cut through by dykes and eroded into incredible cliffs and valleys. On the Northers coast the high cliffs are steep and weather beaten as that is the prevailing wind direction, On the south side the cliffs are barely if at all less spectacular, but they are at least a little more sheltered.
Basalt is the most common rock type on Maderia. It is a fine grained "basic" or mafic igneous rock. Basic in this sense describes the chemistry not the simplicity of the rock because basalts can be very complex checmically. It simply describes a rock that is is rich in magnesium and iron and low in silica Si02 which makes it a rock that flows easily when it is melted.
The Basalts are either extruded (they come to the surface and flow) or intruded (they are squeezed between other rocks). One of the featurea that can help determine which way the rocks were put in place is to look at the way that they were cooled. The Rocks which were intruded will have cooled down more slowly and often have columnar cooling fractures in them. The picture to the right shows columnar style colling patterns in a basalt outcrop with ash layers above it in the Encumeada & Paul da Serra area of Madiera.
As well as the fine grained basalts there are a number of more coarse grained igneous rocks. Some rocks are also porphyritic which is a term used to describe rocks that have at least one set of crystals which are much larger than the surrounding crystals. Porphyritic rocks are formed when a column of rising magma is cooled in two stages.
In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating smaller grains that can be invisible to the eye. The Picture to the left is a block of porphyritic rock on the Ponta San Laurenco penninsular
Tuff is a volcanic rock formed from ash. Chunks smaller than 2 mm in diameter are called volcanic ash, layers of larger particles are typically called breccias. Ash is formed when the hot molten magma has a lot of gas or water in it and it explodes out onto the surface creating huge numbers of small and medium sized fragments. They settle out onto whatever surface exists at the time and become welded in place or are moved down from that position by gravity or the weather (wind and rain). Many Ash layers have fragments of variable size, but some consist of just very fine particles.
The size distribution of the particles will give the ash layer a specific set of properties in terms of hardness and whether water will flow through it and these are then brought out by weathering to provide clear lines in the cliffs. The picture to the right shows ash layers on the Ponta San Laurenco penninsular which is a fabulous place for a geology walk because there is so little rainfall and therefore little vegetation on the rocks.
When the breccia or ash layers are being deposited there can be times when more explosive events send up larger blocks of rock and these then fall into the breccia or ash layers. These are called volcanic bombs and they can be found most easily within the ash layers. Volcanic Bombs are defined as a piece of rock which is over 2.5 inches (64mm) in diameter which was thrown up when still molten and has then fallen to the floor. The picture on the right shows a possible volcanic bomb on the Ponta San Laurenco penninsular. Volcanic bombs can be thrown for long distances in the air so these are evidence of very energetic eruptions, and can injure or even kill people.
Basalt as described above, is a hard rock which forms prominent ridges standing proud from the surface or strong vertical cliffs as shown above, but tuff or ash is a very different and varied rock. Some of the Tuff layers will be welded or cemented into very hard layers, where as some of them will be much weaker either because of a lack of cement or welding or because of some aspect of particle size or the chemical makeup. It is therefore not unusual to see tuff/ash layers have been eroded out as weak layers in the surface or the cliffs as is shows in this picture from the Ponta San Laurenco penninsular. Where they are harder they can be very useful building rocks because they are easier to cust and shape than the basalt. They are also less dense so much easier to transport.
Lava Tubes are created when the outer surfaces of flowing lava cool and the hot molten rocks in the interior keep running downhill. Eventually the Volcano that is feeding the flow runs out, but gravity can pull the remaining lava downhill and leave behind an empty tube. On Madiera there are the The São Vicente Caves which are pictured on the right. These caves were formed around 890,000 years ago from a volcanic eruption in Paul da Serra that ran down to the sea. They were first discovered in 1885, by local people who informed James Yate Johnson an English naturalist in Madeira who explored the caves further. The caves were opened to the public on 1 of October 1996.
Within the tubes you can see shark teeth drop structures where the last of the hot lava was dripping from the roof. Typically you do not find stalactites and stalgmites in Lava Tubes, but there is one very small section within these tubes. The Source of the calcium carbonate (CaCO3) is not clear to me at this time. As as side note the Volcano centre at these caves is very well worth a visit if you are in the area.
As mentioned above some volcanic rocks are squeezed between other rocks. These are called dykes if they cut across other rocks or sills if they have been squeezed in between the layers of other rocks. With a complex geological structure as is found in a stratovolcano is is usual to find both of these structures, but it is easiest to identify the dykes because they stand out. Both the pictures on the right show dykes intruded into the ash and lavas on the Ponta San Laurenco penninsular
Some useful links are given here
https://en.wikipedia.org/wiki/Geology_of_Madeira
https://en.wikipedia.org/wiki/Basalt
https://madeiraislands.net/geology/
The pictures in this section are taken with my E-510 and E-500 Olympus cameras with a selection of lenses. The pictures from the Lava tubes and levada passages are lit with a single FL-36 flashgun.