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Rock composition - DP/EC162 EarthCache

Hidden : 05/26/2022
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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.

Additionally, based on composition, cumulate rocks can be divided into four classes:

FELSIC- The light-coloured minerals predominate and are composed mostly of feldspar (especially K-feldspar), at least 10% quartz, and less than 15% mafic minerals (biotitehornblende).

INTERMEDIATE- It usually contains roughly-equal amounts of light and dark minerals, including light grains of plagioclase feldspar  (mainly plagioclase) and dark minerals like amphibole (mainly hornblendepyroxene, and/or biotite).  There is little or no quartz.

MAFIC- are dominated by plagioclase and pyroxene and smaller amounts of olivine.

ULTRAMAFIC - Are rocks dominated by dark-coloured minerals mainly by olivine and/or pyroxene.

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.


  1. What two colours predominate in this rock?
  2. Is this rock, based on the above compositional differences, a felsic, intermdiate, mafic or ultramafic type of rock?
  3. What is the average crystal size of the two types of crystals in this rock?
  4. 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.

Além disso, com base na composição, as rochas cumuladas podem ser divididas em quatro classes:

FÉLSICA- Os minerais de cor clara predominam e este tipo de rochas são compostos principalmente por feldspato potássico, pelo menos 10% de quartzo e menos de 15% de minerais máficos (biotite, hornblende).

INTERMÉDIA- Geralmente contém quantidades aproximadamente iguais de minerais claros e escuros, incluindo grãos de feldspato (principalmente plagiocláse) e minerais escuros como anfibola (principalmente hornblende, piroxena e/ou biotite). Contém pouco ou nenhum quartzo.

MÁFICA- São riochas dominadas por plagiocláse e piroxena e quantidades menores de olivina.

ULTRAMÁFICA - São rochas dominadas por minerais de cor escura, principalmente por olivina e/ou piroxena.

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. Com base na classificação por composição, como classificarias esta rocha?
  3. Qual o tamanho médio dos dois tipos de cristais que consegues observar?
  4. Qual a forma desses cristais?

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