Los Gigantes cliffs in Tenerife

The highest cliffs in the volcanic island of Tenerife, in the Atlantic Ocean, are found on its western side. These majestic geological features are known as 'Los Gigantes' (the Giants), a term which perfectly befits their massive presence.

Acantilados de Los Gigantes, Tenerife

The cliffs are (like much of the island) composed of 'alkali' basalt, a volcanic rock which contains high proportions minerals like feldspar, olivine and iron oxides. These mineral accretions are sometimes large enough to be seen with the naked eye:

feldspar (white crystals) and iron oxides (brown aggregate) in basalt

feldspar (white crystals) and iron oxides (brown aggregate) in basalt

olivine (greenish crystals) in basalt

The basalt is found in layers which are texturally highly varied, each layer indicating a different episode or process in the island's violent volcanic history. For example, the breccia-like ignimbrite forms when a stream of lava partly melts the substrate upon which it is flowing and drags the pieces of hot rock along, subsequently welding the fragments together into a pudding-like composition, in this case containing fragments of widely disparate size:

ignimbrite

Another rock which stands out visually is red ochre, which forms when hot lava passes over well-developed soils. It can be seen as a thin reddish line in the following photograph:

red ochre stratum

Over millions of years, boulders of basalt tumble down the cliffs and are rounded by wave and wind action.

boulders on the shore

As a rough habitat largely characterized by strong currents and abrasive basalt gravel, the interface between the sea and land is largely devoid of macrofauna, though some usual suspects make an appearance. Most notable of these are two Macaronesian endemics - the limpet Patella piperata Gould, 1846 and the periwinkle Littorina striata King & Broderip, 1832. Closer to mean sea level, Patella tenuis crenata d'Orbigny, 1840 makes an appearance. The third species of limpet that may be encountered on Tenerife is Patella ulyssiponensis Gmelin, 1791, though none of these were seen during this trip. 

Littorina striata King & Broderip, 1832

Littorina striata King & Broderip, 1832

Patella piperata Gould, 1846

Patella piperata Gould, 1846

Specimens of Osilinus sauciatus (Koch, 1845) were found in small clusters, mostly using deep cracks as shelter.

Osilinus sauciatus (Koch, 1845)

Dissertation - A student’s guide to the geology of the Maltese Islands

The citation and abstract to my B. Ed. (Hons) dissertation have been added to the links on the right-hand side of this blog. The page can also be accessed through here.

The Maltese flint

The silicate mineral quartz is, in addition to calcite, one of the commonest minerals on the earth’s crust. An amorphous (non-crystalline) form of this mineral, called variously chert (highly impure) or silex or flint (relatively pure), is found in certain strata of the Maltese rocks.

In a pure state, silicon dioxide is of a transparent colour. This can be seen in crystalline specimens of quartz found in volcanic rocks which are termed ‘rock crystals’. In Malta, nodules or blocks of this mineral contain many impurities mainly consisting of calcium carbonate and other salts; this depends on the parent rock in which they form and results in different colours and varying degrees of translucence.

In the Upper Coralline Limestone Formation, nodules of chert are highly calcareous, opaque and microcrystalline in nature. The impurities in these nodules impart a layered dark brown or charcoal colour. Such nodules are common at the Upper Coralline Limestone of Xemxija and Mistra.

Middle Globigerina Limestone outcrops in Baħrija and Mtaħleb contain usually large blocks of light greyish or khaki chert that intergrades with the limestone and fractures very easily (picture above). Very thin sections of this material allow light to pass through. The parts remaining embedded in the bedrock protrude out and are visible as slightly darker patches against the white limestone.

Middle and Lower Globigerina outcrops in north-central Malta (Mrieħel to Msida) also contain chert, albeit in a the very different form of small to medium-sized nodules. These are somewhat darker in colour, and correspondingly, the silicate material found here is of a light brown to reddish colour.

Crystal clear

These images show the mineral selenite, which is an aggregate of calcium sulphate crystals locally found in the Blue Clay Formation. Photo from Mġarr, Għawdex.