Interesting geological features on La Gomera      [Home]      Date: 16.05.2013

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)

dyke near Arure (district of Valle Gran Rey)

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

the pyroclastic sediments of Las Pelotillas

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 in a broken rock

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

is this a volcanic bomb that splashed into lava? (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

more of this kind, somwehat hidden in a vertical cliff

...and more yet, closer to the top

is this rock about to lay an egg, and who laid the other one? ;-)

an exfoliated piece ('onion skin' fragment) of a rock

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)

 

On La Gomera, many of the rounded stones and boulders termed "volcanic bombs" are really "corestones" that

have formed by underground spheroidal ('onion skin') weathering of bedrock. Concentric cracking is considered

to be the result of pressure relief (after removal of overburden) or, more frequently, of thermal stressing. The latter

can cause pyroclastic material, volcanic bombs or blocks, and even grus to form spheroidal weathering patterns.

 

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".