Interesting geological features on La Gomera     [Home]         Übersicht / Overview [click]                         

Interessantes zur Geologie und Verwitterung     Wollsackverwitterung / Spheroidal Weathering [click]




Benchijigua valley with Roque Agando and trade wind clouds over the ridge


Dike near La Laja (district of San Sebastián)


Basaltic dike in a tuff environment, near Los Almácigos (district of Alajeró)


Phonolitic dike (road to Erque); note the adjacent "baked" tephra


Volcanic bomb, or just a lava-lifted ancient boulder? (El Cedro)


Breadcrust-type volcanic bomb (Isola di Vulcano, Italy)



Volcanic bomb, exfoliating (upper Valle Gran Rey)


This area near Arure is locally known as Las Pelotillas - i.e. The Little Balls


Las Pelotillas, slowly evolving from the lapilli tuff (eventually, they roll down)


One of those tuff balls, from recrystallization of calcite and zeolites


Plagioclase decay leads to calcium carbonate deposits


Druse of calcite / hematite / analcime (?) upon ignimbrite



Amphibole and pyroxenes (appr. 1.5 cm long)


Unweathered olivin beneath black obsidian


The bluish sheen of maghemite (incipient weathering of basalt)


This bluish chip from the rock in picture (15) is magnetic


Fractured basaltic scoria with bluish spots of maghemite


Porphyritic andesite with pyroxene phenocrysts



Picritic basalt with augite, oxidized olivin, and calcite


Vesicular dolerite with plagioclase (feldspar) phenocrysts


Xenoliths (foreign rocks) embedded in basaltic lava


Phonolite showing freshly broken face and weathering clefts


Same specimen (22), with manganese oxide dendrites on a cleft surface


Ignimbrite (welded tuff) of oxidized ash and lapilli



Heat-metamorphed soil, weathered to hematite-rich clay


White: silicates of Al, Mg, ...; yellowish: Fe-hydroxides; red: the heavier Fe-oxides


Typical repetitive vertical profile



La Caldera (a flank crater) and Calvario (an eroded laccolith)



Nightfall in Valle Gran Rey


Nightfall in Valle Gran Rey



Weathering of mafic and intermediate rock (Verwitterung von basischen magmatischen Gesteinen)



Porphyritic basalt with a brownish, Fe- oxide-hydroxide-rich weathering rind


Trachybasalt: rind going from feldspar- rich grey to a soft tan top layer (goethite & smectites)


Trachyte pebble with whitish weathering rind (softer brownish outside eroded away)


Vesicular dolerite with phenocrysts and red, hematite-rich weathering rind


Basaltic ignimbrite (welded tuff) with red weathering rind


Basaltic ash tuff with weathering rind



Trachyte with white weathering rind (mostly kaolinite & halloysite)


Porphyritic andesite-trachyte


Phonolite, weathered outside and fresh fractures; from the dike in picture (4)


Phonolite with layered structure


Ferrous hydroxide mud in a brook


Basalt columns



Shrinkage during solidification creates hexagonal columns


Weathering attacks the edges of cracked columns and creates rounded corestones



Weathering column, as seen from above, with a guess of the former hexagonal shape


Weathering dolerite (roadcut exposure; the red coating is oxidized volcanic ash from farther uphill)


Pyroclastic material "farther uphill", rich in iron oxide, which is being washed down when it rains


La Fortaleza de Chipude - an eroded trachytic laccolith



Concentric 'peeling' of trachyte (Fortaleza de Chipude)


A roadcut revealed this typical 'onion skin' disintegration (east of Degollada de Peraza)


Diabase pillow lava, somwehat hidden in a steep roadcut (near La Laguna Grande)...


...and more yet, closer to the top (near La Laguna Grande)


Is this rock about to lay an egg? ... ;-)

(found near Epina)


... and is this a piece of eggshell? ... ;-)

(from near Las Hayas)



A neatly spherical corestone (Pedro Cojo, near Arure)


The same as (55),

close up


Erosion made these boulders roll down


The front one from (57), close-up (El Barro, near Arure)


Here's where such a boulder had been resting (near Arure)


Agglomerate and the scree slope of grus (near Chijeré)



Mafic pyroclastic agglomerate (near Chijeré)


Saprolitified corestones, still embedded ...


... and here fallen down (near Chijeré)


Corestones with 'weathering rindlets'


What once was an agglomerate (near Chijeré) ...


... lies now, exposed by erosion, upon weathering grus



Grossly reduced volcanic bomb or block (near Chijeré)


Fe2O3-biased rindlets (near Chijeré)


Broken corestone, showing unweathered porphyritic basalt



Grus and the core remains of onion skin weathering


 Spheroidal shapes in gabbro-grus (near Chijeré)


Corestone, modified to saprolite by acidic water from overlying soil (La Quintana / Arure)



Phonolitic saprolite strata beneath a stratum of soil (La Quintana / Arure)



Fossilized plant roots (near Presa de Las Rosas)


Saprolitic basalt stump (La Cancela, above Hermigua)


Saprolite (possibly a pyroclastic surge deposit, above Epina)


Volcanic ashes and a 7 metre wide dike (above Epina)


Metamorphic aureole adjacent to the dike in the picture on the left



Solidified basaltic lava from magmas with different gas concentrations and cooling rates



Obsidian - no gas, rapid cooling under pressure (Isola di Lípari, Italy)


Vesicular (porous) obsidian, with trapped gas bubbles (Isola di Lípari, Italy)


Almost pumice (rapid cooling under pressure relief)


Vesicular basalt (brownish spots are weathered olivin phenocrysts)


Porphyritic basalt - no gas, slow cooling led to phenocryst growth


Massive, effusive basalt (broken; note the weathering rind on its outer surfaces)



Most of the Island of La Gomera is basalt, with some local trachyte and phonolite intrusions. Depending on the

amount of gas in the magma, the lava rock can come in different configurations: From obsidian (amorphous glass)

to effusive basalt lava (which has a crystalline matrix), vesicular basalt (showing gas bubbles) to even pumice

(when high gas concentration in the melt has caused the lava to froth). All this, when ejected explosively, can drop

out as to what is termed pyroclastic material, such as scoria or volcanic ashes (coarse or fine-grained material).

Weathering, quakes or varying temperatures can cause the bedrock (particularly those typical basalt columns) to

develop cracks and clefts. As edges and corners are more easily weathered away, rocks and boulders become

more and more rounded in shape over the ages.

Pressure relief after removal of an overburden and, more frequently, thermal stress can cause rocks to develop

concentric cracks, which makes them decay layer by layer. What remains is a cluster of peeling-off 'rindlets' that

enclose a 'corestone' which, eventually, will also turn into grus. This process is called 'spheroidal weathering' or

'onion skin weathering'. It is not confined to massive rocks, but can also occur in grus and mudstone [Fig. (71)].

This seems to be a collective phenomenon and as such a manifestation of  the emergent behaviour of nature.