This EarthCache takes you to Fitzalan Square in Sheffield. More specifically the stone panelling at either side of the shop front, which we are here to look at. We are here to look at the phenocrysts.
A phenocryst is a crystal in igneous rock, which is larger than the crystals of the rock that it forms part of.
Phenocrysts can have different forms:
Euhedral - these crystals are well defined, sharp and easily recognisable edges.
Anhedral – these crystals do not have well defined edges.
Variations in the size, volume and orientation of phenocrysts can tell us more than just a collection of large crystals in an igneous rock.
The size of phenocrysts change depending on where they were formed in the flow of the igneous rock as it cooled. Concentrations of phenocrysts are found to be nearly zero at the margins of the flow , and their size increases inwards towards the centre of the flow. The increase in size and abundance of phenocrysts toward the center of the dike is interpreted as resulting from flow differentiation.
Flow Differentation
This is the change of the composition of magma as it cools, as this happens different parts of the magma cool at different rates, and this this gives rise to different flows within the magma which have different temperatures, in the this different flow, crystallisation of the rock takes place at different rates. Also contamination the magma by the wall rocks melting into it from the walls of the magma chamber can affect the make up of the magma and its flow rate.
The primary cause of change in the composition of a magma is cooling, which is an inevitable consequence of the magma being created and migrating from the site of partial melting into an area of lower stress - generally a cooler volume of the crust.
The alignment of phenocrysts can help to show the direction of the magma flow, which can be shown by phenocryst alignment parallel to the direction of supposed flow.
This being an EarthCache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1. Please take your time to examine the panelling.
(a) Which direction do these phenocrysts point (i,e, upwards, downwards, horizontally, at an angle)? Is the alignment the same on both side panels of the shop front?
(b) Please measure the length of the longest phenocryst. How long is it?
(c) Please describe the colour, appearance and shape of the phenocrysts.