Layers This photo was taken on the slopes of the volcano that makes up the heart of the island of Te

LayersThis photo was taken on the slopes of the volcano that makes up the heart of the island of Tenerife, the largest of the Canary Islands. Each of these distinct layers likely represents one eruption of the volcanic system that built the island.The darker black layers are more mafic, made of rocks that contain more iron and magnesium. The lighter layers are more felsic, containing more silica and aluminum. Volcanoes like those on the Canary Islands often alternate, with eruptions of varying composition building their structures. You can also see differences in texture – the darkest units are very thin, suggesting that they were small and explosive eruptions, possibly driven by interaction with water to create ash that draped the landscape. The thicker layers are coarse grained enough that some rocks can be found in them, implying that they are pyroclastic flows that draped down the volcano slopes. You can zoom in and see some layers rich in coarse chunks of pumice thrown out of the volcano in these eruptions. Some layers may even represent different phases in the same eruption – coarse grained pumice in one layer followed by more fine-grained material as the eruption style changes.Interestingly, these rocks violate one basic rule of stratigraphy called original horizontality as, unlike sedimentary rocks deposited in the ocean, these rocks were never horizontal. Look at how the slope of the volcano is parallel to the bed layers – the beds actually were deposited on this slope with this thickness. To get sedimentary layers like these, you have to tilt them upwards by 30 degrees using faults or folds, but that isn’t always required on volcanoes.This photograph also illustrates one hazard for geologists – getting across heavily traveled roads to find neat looking outcrop.-JBBImage credit:http://bit.ly/1XVmGByhttp://bit.ly/238btNj -- source link

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