Interesting geological features on La Gomera
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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
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dyke near La Laja (district of San Sebastián)
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dyke near Los Almácigos (district of Alajeró)
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dyke near Arure (district of Valle Gran Rey)
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this area near Arure is locally known as Las Pelotillas - i.e. The Little Balls
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the pyroclastic sediments of Las Pelotillas
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Las Pelotillas, slowly evolving from the lapilli tuff
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recrystallization of calcite and zeolites builds such tuff balls
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basalt with patches of maghemite (scra- pings are magnetic)
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fractured basaltic scoria with bluish- grey maghemite
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picritic basalt with augite, oxidized olivin, and calcite
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gabbro from an intrusion
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amphibole and pyroxenes (appr. 1.5 cm long)
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druse of calcite / hematite / analcime (?) upon ignimbrite
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ignimbrite (welded tuff) of oxidized ash and lapilli
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picturesque weathering pattern in a broken rock
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vertical profile (red: iron oxides suggest explosive ejection of volcanic ash)
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white: silicates of Al, Mg, Ca ...; yellow: Fe-hydroxides; red: the heavier Fe-oxides
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vertical profile
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pyroclastic sediment, exposed by roadcut
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La Caldera (a flank crater) and Calvario (an eroded laccolith)
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Whamm - giant wave hitting a sturdy pillar almost 10 metres tall
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nightfall in Valle Gran Rey
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nightfall in Valle Gran Rey
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Spheroidal Weathering
("Wollsackverwitterung") of mafic and intermediate rock
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basalt with weathering rind
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basaltic ash tuff with weathering rind
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porphyritic andesite with augite phenocrysts
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vesicular basalt with plagioclase (feldspar) phenocrysts
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basalt columns
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shrinkage during solidification creates hexagonal columns
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weathering attacks the edges of cracked columns, thus creating rounded corestones
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spheroidal weathering of basalt, showing corestone formation (roadcut exposure)
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weathering of basalt (roadcut exposure; the red coating is oxidized volcanic ash from farther uphill)
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pyroclastic material "farther uphill", which is being washed down when it rains
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La Fortaleza de Chipude - the surface corestones suggest that this is an eroded trachytic laccolith
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concentric 'peeling' of weathering rock (La Fortaleza de Chipude)
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a roadcut revealed this typical 'onion skin' disintegration
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more of this kind, somwehat hidden in a vertical cliff
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...and more yet, closer to the top
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an exfoliated piece ('onion skin' fragment) of a rock
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is this a petrified onion ...or is the rock laying an egg?
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corestones and the scree slope of grus (near Chijeré)
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saprolitified corestones, still embedded ...
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... and here fallen down (near Chijeré)
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is this a corestone - or a volcanic bomb that splashed into the lava? (El Cedro)
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a breadcrust-type volcanic bomb (Isola di Vulcano, Italy)
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a neatly spherical corestone (Pedro Cojo, near Arure)
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... the same, close up
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erosion made these boulders roll down (the rear one is 1.2 metres in diameter)
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the front one, close-up (El Barro, near Arure)
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... here's where a corestone had been resting (near Arure)
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pyroclastic agglomerate (near Chijeré)
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the remains of blocks and bombs, weathering away ...
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corestones with 'rindlets' and their weathering grus
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what once was an agglomerate (near Chijeré) ...
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... lies now, exposed by erosion, upon weathering grus
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grossly reduced bomb or block (near Chijeré)
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corestone with Fe2O3-biased rindlets (near Chijeré)
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broken corestone, showing unweathered porphyritic basalt
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grus and the core remains of onion skin weathering
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gabbro-grus weathered to spheroidal shapes (near Chijeré)
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corestone, modified to saprolite by acidic water from overlaying soil (La Quintana)
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phonolitic saprolite strata beneath a stratum of soil (La Quintana / Arure)
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saprolitic basalt stump (La Cancela, above Hermigua)
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saprolite (possibly a pyroclastic surge deposit, above Epina)
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metamorphic aureole adjacent to a 7 metre wide intrusion (above Epina)
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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.
Sehr
viele der auf der Insel als "Vulkanbomben" bezeichneten gerundeten Steine sind
nur sog. "Corestones", d. h. durch unterirdische
"Wollsackverwitterung" des Gesteinskörpers entstandene "Rundsteine",
die durch druckentlastungs- und/oder thermisch
bedingte,
konzentrische Klüfte 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 Bomben oder Blöcke oft schalig
und hinterlassen derartige "Rundsteine".
