Interesting geological features on La Gomera      [Home]      Date: 05.08.2014

Interessantes zur Geologie und Verwitterung der Insel

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

dyke near La Laja (district of San Sebastin)

dyke near Los Almcigos (district of Alajer)

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

the pyroclastic sediments of Las Pelotillas (district of Valle Gran Rey)

Las Pelotillas, slowly evolving from the lapilli tuff

recrystallization of calcite and zeolites builds such tuff balls

incipient weathering generates patches of maghemite on basalt

fractured basaltic scoria with bluish spots of maghemite

picritic basalt with augite, oxidized olivin, and calcite

gabbro from an intrusion

amphibole and pyroxenes (appr. 1.5 cm long)

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

manganese dendrites on a cleft surface in weathering phonolith

picturesque weathering pattern (near Agulo)

picturesque mosaic pattern (Villa Romana del Casale, Sicilia, Italy)

ignimbrite (welded tuff) of oxidized ash and lapilli

vertical profile (red: iron oxides suggest explosive ejection of volcanic ash)

white: silicates of Al, Mg, Ca ...; yellow: Fe-hydroxides; red: the heavier Fe-oxides

vertical profile

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

Whamm - giant wave hitting a sturdy pillar almost 10 metres tall

nightfall in Valle Gran Rey

nightfall in Valle Gran Rey

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

trachytic basalt with weathering rind

basaltic ash tuff with weathering rind

porphyritic andesite with augite phenocrysts

vesicular basalt with plagioclase (feldspar) phenocrysts

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

a breadcrust-type volcanic bomb (Isola di Vulcano, Italy)

basalt columns

shrinkage during solidification creates hexagonal columns

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

spheroidal weathering of basalt, showing corestone formation (roadcut exposure)

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

pyroclastic material "farther uphill", which is being washed down when it rains

La Fortaleza de Chipude - the surface corestones suggest that this is an eroded trachytic laccolith

concentric 'peeling' of weathering rock (La 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? ... ;-)

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 a boulder corestone had been resting (near Arure)

corestones and the scree slope of grus (near Chijer)

saprolitified corestones, still embedded ...

... and here fallen down (near Chijer)

pyroclastic agglomerate (near Chijer)

the remains of blocks and bombs, weathering away ...

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)

corestone with 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)

saprolitic basalt stump (La Cancela, above Hermigua)

saprolite (possibly a pyroclastic surge deposit, above Epina)

metamorphic aureole adjacent to a 7 metre wide intrusion (above Epina)

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

obsidian (a volcanic glass) indicating: no gas, rapid cooling (Isola di Lpari, Italy)

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

highly vesicular basalt (almost pumice)

vesicular basalt (brownish spots are weathered olivin phenocrysts)

massive, effusive basalt

gabbro: the plutonic equivalent of basalt (here: slightly metamorphic)

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The Island of La Gomera is made up mostly of basalt (with some trachyte and phonolite). This can come in various

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

vesicular basalt (showing fine 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 (and quakes) can cause the bedrock - and the typical 'basalt columns' - to crack. The edges and corners

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

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

concentric cracks in a way that makes it 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 even occur in proper 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".