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Cumulate Rocks - DP/EC161 EarthCache

Hidden : 05/25/2022
Difficulty:
1.5 out of 5
Terrain:
1 out of 5

Size: Size:   other (other)

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Geocache Description:


Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proportions of mafic components and smaller amounts of felsic material than most granites; those are classed as being of intermediate composition.

Syenites are products of alkaline igneous activity, generally formed in thick continental crustal areas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a granitic or igneous protolith to a fairly low degree of partial melting. This is required because potassium is an incompatible element and tends to enter a melt first, whereas higher degrees of partial melting will liberate more calcium and sodium, which produce plagioclase, and hence a granite, adamellite or tonalite.

At very low degrees of partial melting a silica undersaturated melt is produced, forming a nepheline syenite, where orthoclase is replaced by a feldspathoid such as leucitenepheline or analcime.

Conversely in certain conditions, large volumes of anorthite crystals may precipitate from thoroughly molten magma in a cumulate process as it cools. This leaves a drastically reduced concentration of silica in the remainder of the melt. The segregation of the silica from the melt leaves it in a state that may favour syenite formation.

A syenite is an excellent example of a cumulate rock type.

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks.

At the cache coordinates, you have syenite. This syenite shows clear evidence of being a cumulate rock. To validate your FIND, you need to answer the following questions and send them to me.

Questions:

  1. What two colours predominate in this rock?
  2. What is the average crystal size of the two types of crystals in this rock?
  3. What shape are the crystals?

Sienito é uma rocha ígnea plutónica, de textura holocristalina grosseira, composta principalmente por feldspatos (microclinaortoclase e alguma ou nenhuma plagioclase), anfibolas (geralmente hornblenda), clinopiroxena (augiteegirine), pouca ou nenhuma biotite. 

Uma rocha cumulada é formada pela acumulação de cristais. Normalamente é criada por assentamento gravitacional ou por diferenciação de fluxo tal como na figura infra.

No Local das coordenadas vais ver um sienito (com a provável origem da Serra de Monchique no Algarve) que exemplifica muito bem este conceito das rochas cumuladas. Para validares o teu registo, tens de me enviar as respostas às seguintes peguntas:

  1. Quais a s duas cores predominantes nesta rocha?
  2. Qual o tamanho médio dos dois tipos de cristais que consegues observar?
  3. Qual a forma desses cristais?

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