Interesting geological features on La Gomera                           Spheroidal Weathering:                                

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(1)

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

(2)

Dike near La Laja (district of San Sebastin)

(3)

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

(4)

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

(5)

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

(6)

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

 

(7)

Volcanic bomb, exfoliating (upper Valle Gran Rey)

(8)

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

(9)

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

(10)

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

(11)

Plagioclase decay leads to calcium carbonate deposits

(12)

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

 

(13)

Amphibole and pyroxenes (appr. 1.5 cm long)

(14)

Unweathered olivin beneath black obsidian

(15)

The bluish sheen of maghemite (incipient weathering of basalt)

(16)

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

(17)

Fractured basaltic scoria with bluish spots of maghemite

(18)

Porphyritic andesite with pyroxene phenocrysts

 

(19)

Picritic basalt with augite, oxidized olivin, and calcite

(20)

Vesicular basalt with plagioclase (feldspar) phenocrysts

(21)

Xenoliths (foreign rocks) embedded in basaltic lava

(22)

Phonolith showing freshly broken face and weathering clefts

(23)

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

(24)

Ignimbrite (welded tuff) of oxidized ash and lapilli

 

(25)

Heat-metamorphed soil, weathered to hematite-rich clay

(26)

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

(27)

Typical repetitive vertical profile

 

(28)

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

 

(29)

Nightfall in Valle Gran Rey

(30)

Nightfall in Valle Gran Rey

 

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

 

(31)

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

(32)

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

(33)

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

(34)

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

(35)

Basaltic ignimbrite (welded tuff) with red weathering rind

(36)

Basaltic ash tuff with weathering rind

 

(37)

Trachyte with white weathering rind (mostly kaolinite & halloysite)

(38)

Porphyritic andesite-trachyte

(39)

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

(40)

Phonolith with layered structure

(41)

Ferrous hydroxide mud in a brook

(42)

Basalt columns

 

(43)

Shrinkage during solidification creates hexagonal columns

(44)

Weathering attacks the edges of cracked columns and creates rounded corestones

(45)

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

(46)

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

(47)

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

(48)

La Fortaleza de Chipude - an eroded trachytic laccolith

 

(49)

Concentric 'peeling' of trachyte (Fortaleza de Chipude)

(50)

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

(51)

More of this kind, somwehat hidden in a steep roadcut (near La Laguna Grande)...

(52)

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

(53)

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

(found near Epina)

(54)

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

(from near Las Hayas)

 

(55)

A neatly spherical corestone (Pedro Cojo, near Arure)

(56)

The same as (55),

close up

(57)

Erosion made these boulders roll down

(58)

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

(59)

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

(60)

Agglomerate and the scree slope of grus (near Chijer)

 

(61)

Mafic pyroclastic agglomerate (near Chijer)

(62)

Saprolitified corestones, still embedded ...

(63)

... and here fallen down (near Chijer)

(64)

Corestones with 'weathering rindlets'

(65)

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

(66)

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

 

(67)

Grossly reduced volcanic bomb or block (near Chijer)

(68)

Fe2O3-biased rindlets (near Chijer)

(69)

Broken corestone, showing unweathered porphyritic basalt

(70)

Grus and the core remains of onion skin weathering

(71)

 Spheroidal shapes in gabbro-grus (near Chijer)

(72)

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

 

(73)

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

(74)

Fossilized plant roots (near Presa de Las Rosas)

(75)

Saprolitic basalt stump (La Cancela, above Hermigua)

(76)

Saprolite (possibly a pyroclastic surge deposit, above Epina)

(77)

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

(78)

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

 

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

 

(79)

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

(80)

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

(81)

Almost pumice (rapid cooling under pressure relief)

(82)

Vesicular basalt (brownish spots are weathered olivin phenocrysts)

(83)

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

(84)

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

 

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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' which, eventually, also turns 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.