URBAN EARTH - Aliens EarthCache

Introduction

This EarthCache isn't about aliens from another world - it's about aliens from this world.

There are two aliens here. One is a large boulder of a rock that has travelled here from afar and differs from the rock which naturally occurs here. The second alien is actually embedded in the first - what you might call a hitchhiker.

You won't need a magnifying glass or anything like that but anyone who relies on spectacles for reading will likely enjoy the experience more by wearing them.

Logging Tasks

IN ORDER TO COMPLETE THESE LOGGING TASKS PLEASE SEND US YOUR ANSWERS USING THE Message this owner LINK AT THE TOP OF THIS PAGE OR USING THE MESSAGE CENTRE OR EMAIL VIA OUR GEOCACHING PROFILE BEFORE SUBMITTING YOUR LOG. PLEASE DO NOT INCLUDE ANSWERS OR SPOILERS IN YOUR ONLINE LOG. YOU CAN GO AHEAD AND LOG YOUR FIND AS SOON AS YOU HAVE SENT YOUR ANSWERS IN ACCORDANCE WITH GROUNDSPEAK GUIDELINES. LOGS WITHOUT ADEQUATE LOGGING TASK EVIDENCE MAY SUBSEQUENTLY BE DELETED.

Based on your study of the Shap granite fragment and the embedded xenolith and using the information on the cache page please tell me:

1. What is the overall height of the large fragment of Shap granite?
   
   
2. Do you think this large fragment of Shap granite is a glacial erratic?
   
   
3. What leads you to your conclusion?
   
   
4. What is the maximum height and width of the xenolith?
   
   
5. Examine the xenolith and describe in some detail how it differs from the granite e.g. does it look different? Does it feel different?
   
   
6. Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
   
   

Background

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  Shap granite - on the right is a typical example of Shap granite - so you'll know what you're looking for at GZ (although the example at GZ is a little bit bigger ).
  
  Shap granite is a coarse grained igneous rock with porphyritic texture. That is to say that some of the crystals in the rock are conspicuously larger than the surrounding crystals. These larger crystals are called phenocrysts. The phenocrysts in the Shap granite are crystals of pink orthoclase feldspar. These formed as the molten rock cooled very slowly, deep underground and tend to be well defined crystal shapes (euhedral). The crystalline structure of granite tends to produce fragments which are rough to the touch and angular in nature when the granite is freshly quarried or plucked from the rock face.
  
  At some point the still molten rock migrated upwards towards the surface and started to cool much more quickly - resulting in the formation of the much smaller groundmass crystals which surround the phenocrysts. The smaller groundmass crystals are made up mainly of quartz, plagioclase and biotite. Small amounts of ilmenite and titanite may also be present. The groundmass crystals tend to be more cramped together, resulting in less well defined crystal shapes (subhedral or anhedral).
  
  This boulder of Shap granite is our first alien - because the underlying bedrock in these parts comprises mudstones and sandstones rather than granite - so the granite can't have formed around here and must have been transported from somewhere else. The clue of course is in the name - Shap granite is from Shap in Cumbria - almost 100 miles from GZ.
  
  
  
• 
  Glacial erratics - an erratic is a rock or boulder that differs from the surrounding rock and is believed to have been brought from a distance by glacial action.
  
  The last glacial period in the UK, called the Devensian encompassed a period from around 115,000 years ago to 11,700 years ago. During that time ice moved from North to South almost like a giant, very heavy frozen conveyor belt, picking up and dragging along rocks as it went - including chunks of Shap granite plucked from the rock face by the ice as it passed. During transportation the rock was abraded against other rocks and worn progressively smoother and rounder until eventually the ice melted and dropped its payload of rock passengers, typically much more rounded than when they were picked up - much like pebbles on a beach that have been tumbled by the action of the sea.
  
  
  
• 
  Xenoliths - xeno-lith literally foreign stone from the greek xenos and lithos.
  
  A xenolith is a piece of rock trapped in another type of rock. Most of the time, a xenolith is a rock embedded in magma while the magma was cooling. Xenoliths tend to be different in obvious ways from the rock they are trapped in - they aren't terribly difficult to spot. The xenolith in the Shap granite at GZ is quite obvious and is on the face of the granite which is facing the pavement.
  
  Magma is the molten rock beneath the Earth's crust that emerges as lava during a volcanic eruption. During the eruption the molten magma, under pressure, forces its way up through the layers of crust to the surface, picking up fragments of foreign rock as it rises which remain trapped as xenoliths when the magma cools and solidifies.
  
  
  

So at GZ we have an alien embedded in an alien - a xenolith of an unidentified rock type embedded in the Shap granite which itself is alien to its location with its underlying bedrock of mudstones and sandstones.

If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there .

Please submit your logging task responses before posting your log.