Interesting geological features on La Gomera      [Home]      Date: 12.02.2015

Interessantes zur Geologie und Verwitterung der Insel


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

dike near La Laja (district of San Sebastin)

basaltic dike in a tuff environment, near Los Almcigos (district of Alajer)

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

volcanic bomb, or not, that is the question - (El Cedro)

a 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)

recrystallization of calcite and zeolites builds such tuff balls

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 the left picture is magnetic

fractured basaltic scoria with bluish spots of maghemite

porphyritic andesite with pyroxene phenocrysts

picritic basalt with augite, oxidized olivin, and calcite

vesicular basalt with plagioclase (feldspar) phenocrysts

xenoliths (foreign rocks) embedded in basaltic lava

phonolith, showing freshly broken face and weathering clefts

same specimen, with manganese 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


Spheroidal Weathering ("Wollsackverwitterung") of mafic and intermediate rock

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 obviously eroded away)

vesicular basalt 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

phonolith, weathered outside and fresh fractures (dike by the road to Erque)

phonolith with layered structure

ferrous hydroxide mud in a brook (near El Guro)

basalt columns

shrinkage during solidification creates hexagonal columns

weathering attacks the edges of cracked columns, thus creating rounded corestones

weathering column corestone, seen from above, with a guess of the former hexagonal shape

weathering of basalt (roadcut exposure; the red coating is oxidized volcanic ash from farther uphill)

pyroclastic material "farther uphill", rich in iron oxides, 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)

more of this kind, somwehat hidden in a vertical cliff (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 at Epina)

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

a neatly spherical corestone (Pedro Cojo, near Arure)

... the same, close up

erosion made these boulders roll down (the rear one is 1.2 metres in diameter)

the front one, 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 Chijer)

mafic pyroclastic agglomerate (near Chijer)

saprolitified corestones, still embedded ...

... and here fallen down (near Chijer)

corestones with 'rindlets' and their weathering grus

what once was an agglomerate (near Chijer) ...

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

grossly reduced volcanic bomb or block (near Chijer)

Fe2O3-biased rindlets (near Chijer)

broken corestone, showing unweathered porphyritic basalt

grus and the core remains of onion skin weathering

gabbro-grus weathered to spheroidal shapes (near Chijer)

corestone, modified to saprolite by acidic water from overlying soil (La Quintana)

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 Lpari, Italy)

vesicular (porous) obsidian, with trapped gas bubbles (Isola di Lpari, Italy)

almost pumice (rapid cooling under pressure relief) (La Gomera)

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)


The Island of La Gomera is made up mostly of basalt, with some trachyte and phonolite. The rocks can come in

various configurations: From obsidian (amorphous glass; rare on this island) to effusive basalt lava (crystalline

matrix), vesicular basalt (showing gas bubbles) to, finally, pumice (when high gas concentration in the melt has

caused the lava to froth). All this, when ejected explosively, can drop out as scoria and other pyroclastic material.

Weathering, quakes etc. can cause the bedrock (and the typical basalt columns) to crack. As edges and corners

are more prone to weathering, the rocks and boulders will become more and more rounded in shape.

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

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

enclose a 'corestone' until, finally, also the latter has turned 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.


Viele der auf der Insel als "Vulkanbomben" bezeichneten gerundeten Steine sind sogenannte "Corestones", d. h. durch unterirdische

"Wollsackverwitterung" des Gesteinskrpers entstandene "Rundsteine", die durch druckentlastungs- und/oder thermisch bedingte,

konzentrische Klfte dann weiter "zwiebelschalenartig" verwittern (NB: die dem englischen Wort "Corestone" entsprechenden Namen

"Kernstein" und "Steinkern" sind im Deutschen schon anderweitig besetzt; daher hier das tentative "Rundstein"). Allerdings verwittern

auch in pyroklastisches Sediment eingebettete Vulkanbomben oder Blcke oft schalig und hinterlassen derartige "Rundsteine".