I noticed this columns at the entrance to the bank walking past. They're worth a look - there's also some fine sandstone carving on the exterior that seems to be in remarkably good condition - make sure you look up to see it.
1) Describe the columns - mention the crystal size, and colours you can see
2) Of the Aptite Veins, Xenoliths and Phenocrysts (see the description below for information on how to recognise them) which features are present in these columns - make sure you look high and low!
3) Is the granite an example of an intrusive or extrusive igneous rock? Use your observations to question 1 to support your answer
4) Take a photo of yourself at GZ, or an identifying item, being careful not to reveal any of the answers to the questions.
Please submit your answers via message though the Geocaching website or via email - there is no need to wait for a response before logging your find.
Granite Features
Aptite Veins
Aptite veins are thin, sheet-like bodies that cut across host granite. They consist of fine-grained, light-colored minerals like quartz, feldspar, and mica. These veins form when late-stage, silica-rich magma intrudes into fractures in the granite and cools rapidly.
Phenocrysts
Phenocrysts in granite are large, visible crystals within a finer-grained groundmass. They form when minerals like feldspar, quartz, or mica crystallize early during the cooling of granite magma. These early crystals grow larger than the surrounding minerals.
Xenoliths
Xenoliths, are foreign rock inclusions found within igneous rock formations, such as granite. Their formation begins deep within the Earth's crust, where molten magma slowly cools and solidifies to form granite. During this process, pockets of magma may intrude into existing rock formations, incorporating fragments of the surrounding rock into the cooling magma. These fragments, known as xenoliths, are often of different composition and texture compared to the surrounding granite. As the magma cools and crystallises over time, the xenoliths become trapped within the granite, preserving a snapshot of the geological history of the area.
Xenoliths are usually visible. They have a different color and density than the surrounding igneous rock. Xenoliths can be as small as a grain of sand or as large as a football, and as long as several meters.
Intrusive vs Extrusive
Igneous rocks are formed by the cooling of molten rock and are classified into two categories: intrusive and extrusive.
-
Intrusive Igneous Rock: This type forms when molten rock cools beneath the Earth's surface. The slow cooling process allows for the development of large crystals. The longer the cooling period, the larger the crystals that form.
-
Extrusive Igneous Rock: This type forms when molten rock cools on the Earth's surface. Exposure to the elements, such as rain, causes the rock to cool quickly. As a result, any crystals that form are typically very small and often not visible to the naked eye.