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Earth of fire

Actualité volcanique, Article de fond sur étude de volcan, tectonique, récits et photos de voyage

Publié le par Bernard Duyck
Publié dans : #Excursions et voyages

To complete the description of this part of the Iberian west, two sites are interesting: Ribeira d'Ilhas and Lomba dos Pianos

Geological Setting:

In the Lusitanian and the Algarve basins, the last phases of rifting occurred in the late Early Cretaceous, while the initiating of the expansion of the seabed of the Iberian Abyssal Plain began around 133-128 Ma.

Tectonic inversion of these basins is a consequence of the simultaneous occurrence of a rapid collision and subduction between the Iberian, European and African plates, which began in the Cretaceous and peaked during the Miocene in the Lusitanian Basin. The Algarve has experienced the compression effects of convergence Africa - Iberian Peninsula (Spain and Portugal today) to the late Cretaceous and early Miocene.

 The Late Cretaceous alkaline magmatism in the onshore sector of the West Iberian Margin. Circles indicate all of the sampled intrusions. Portuguese Geology and batimetry adapted from data available at http://www.iambiente.pt/atlas/est/index.jsp / Doc Rui Miranda - Ref. by sources

The Late Cretaceous alkaline magmatism in the onshore sector of the West Iberian Margin. Circles indicate all of the sampled intrusions. Portuguese Geology and batimetry adapted from data available at http://www.iambiente.pt/atlas/est/index.jsp / Doc Rui Miranda - Ref. by sources

Summary of the most important geodynamic events in Iberia during the Cretaceous. Dates for the limits of the epochs and ages of the Cretaceous period from Gradstein et al. (2004) . Ages from geochronological data in full lines. Geologically interpreted ages in dashed lines / Doc Rui Miranda - Ref. by sources

Summary of the most important geodynamic events in Iberia during the Cretaceous. Dates for the limits of the epochs and ages of the Cretaceous period from Gradstein et al. (2004) . Ages from geochronological data in full lines. Geologically interpreted ages in dashed lines / Doc Rui Miranda - Ref. by sources

Ribeira d'Ilhas - stratified cliffs and laccolith (dark rock) - photo reflexosonline.

Ribeira d'Ilhas - stratified cliffs and laccolith (dark rock) - photo reflexosonline.

Ribeira d'Ilhas:

The beach of Ribeira d'Ilhas, surrounded by the lava of volcanic complex of Lisbon, has its cliffs, cut by the valley of the river Cucos, interesting structures : stratified sediments and  differently eroded from the Cretaceous,  which are traversed by numerous faults fulfilled by dykes.


On the edge of the ocean, one can see traces of a laccolith surrounded by a metamorphic aureole, exposed by erosion.

A laccolith - from the Greek Lakos, cavity and lithos, rock - is an intrusive rocks mass, rounded at the top, which creeps into a sedimentary series. The melting body does not reach the surface or stops in the lithosphere to extend laterally between parallel layers.

Ribeira d'Ilhas - Doc. Carta Geologica de Portugal

Ribeira d'Ilhas - Doc. Carta Geologica de Portugal

Laccolith by the sea - picture J-M. Mestdagh 11.2015

Laccolith by the sea - picture J-M. Mestdagh 11.2015

Lomba dos Pianos:

The site is located between the beaches of Samara / Assafora and the Magoitoe, a beautiful beach range of sizes and a consolidated dune, formed by sand accumulation generated by the combined action of marine and wind erosion, established there about 10,000 years ago, around Sintra.

This magmatic sill, 3 km long and 17-20 meters thick, is dated to about 72 million years. It formed when magma do an intrusion into the sedimentary limestone (Cretaceous in horizontal layers), during the formation of the volcanic complex of Lisbon.

 

Sintra tectonic map - Doc. Kullberg MC & JC

Sintra tectonic map - Doc. Kullberg MC & JC

Lomba dos Pianos - photo geopt.org

Lomba dos Pianos - photo geopt.org

A disused quarry makes a good observation site of the basalt / dolerite (*) sill in the sediment layers of sandstone and marl. In crevices can be observed zeolites (**) white and pink.

The prismatic disjunction, perpendicular to the cooling surface, and like organ pipes, earning him his name.

In addition to the prismatic disjunction, there is a spheroidal disjunction causing desquamation of diabase; the origin of this erosive phenomenon is the presence of almost perpendicular fractures which are preferential sites for the water in motion, with consequent alteration of the rock.

 

(*) The dolerite is a medium grained rock, close of  the gabbro, forming intrusions in small veins. Plagioclase often form small rectangular crystals between larger of pyroxene. Olivine is common, in rounded grains often weathered in brown-orange color.

(**) A zeolite,  from the Greek "Boiling stone" is formed of crystal structures characterized by a microporous aluminosilicate skeleton whose empty spaces are initially occupied by cations and water molecules. Naturally formed in sediments, zeolites have the property of swelling under the effect of heat. They can hydrate and dehydrate reversibly.

 

Lomba dos Pianos - photo Geocaching

Lomba dos Pianos - photo Geocaching

Sources :

- Age constraints on the late Cretaceous alkaline magmatism on the West Iberian margin – Rui Miranda & al. - link

- Geocaching – Piano lessons - link

- Mindat - Ribeira d'Ilhas - link

- Mindat - Zeolites of Lomba dos Pianos, Portugal - link

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