San Miguel

San Miguel and Barranco de la Orchilla

Click on the links to see the photographs from each location or click here to see the Tenerife Set in my photo stream.

During the day it is possible to study rocks produced by increasingly violent eruptions.

1. Road Cutting: Effusive pahoehoe lava

2. Scoria Cone: Strombolian eruption

3. Lava Dome: Viscous, evolved, phonolitic lava

4. Chimiche Quarry: Air fall and Pyroclastic Flow deposits from a Plinian Eruption

5. Barranco del Azucar: Pyroclastic Flow deposits from an exploding Lava Dome

San Miguel de Abona

Next to the car parking spaces for the scoria cone, a vesicular lava flow is exposed in the road cutting. There is no pavement at this point, but with care, it is possible to observe pahoehoe texture, a rubbly top / bottom and lava toes, where lava has flowed down into the underlying rubbly-topped lava flow.

The scoria cone at San Miguel de Abona has been quarried for aggregate, leaving an excellent section for examining the products of a Strombolian eruption. About 15 metres high and over 200 metres long, this section contains breadcrust bombs, volcanic glass shards and fused volcanic spatter. Crude stratification is shown within the cone. The students were very cooperative when it came to taking a team photograph. Click here to see it on Flickr or here to view it using the interactive viewer at 360cities.net.

At the end of the track are three irrigation reservoirs, connected by a stone channel, that now houses a steel pipe. The stone is a local ignimbrite, which is relatively soft and can be easily cut into a U-shape for the channel. Because it contains no grain, this stone could be described as a free-stone. Click here to see the irrigation channel in a non-Flash viewer from 360cities.net. Adjacent to the scoria cone is a series of terraced fields, which can also be viewed as an interactive panorama.

Ex 02 - Calculation of density of scoria

Ex 03 - Estimation of velocity of volcanic bombs

Barranco de la Orchilla

At Barranco de la Orchilla, a phonolitic lava dome is intruded by an olivine basalt dyke. The dyke shows multiple phases of intrusion and xenoliths, as well as exfoliation weathering. Phonolite is a common rock type on Tenerife, but it is not one that is generally taught in introductory courses. The name for the rock comes from the ringing sound it makes when you hit it with a hammer. Phonolite is a highly evolved volcanic rock, with a silica percentage similar to andesite, but much higher levels of alkali metals (sodium and potassium). It is a felsic rock, which is viscous, forming features such as this lava dome, but also causes explosive eruptions associated with thick pyroclastic deposits. We estimated the depth of the barranco beneath the bridge to be 67 metres and the height of the lava dome above the road to be 41 metres, giving a total thickness of 108 metres for the lava dome.

Ex 04 - Calculation of depth of barranco using a pebble

Open the map in Google Maps to see the place markers on a full screen and to use the Google Street View.

View San Miguel de Abona in a larger map

San Miguel to examine a cross section through a scoria cone with volcanic bombs.