So I promised myself not to do anymore Earthcaches on walls, but these particular piece caught my eye, and it is certainly beautiful even if not in it's natural context. Here we go learning geology on a wall again.

Logging Tasks

Earthcaches can be boring to some - my personal goal is to show you this really beautiful rocktwhat I found and it really doesn't matter whether or not you learn something from this. If you try your best to answer these questions and attach an image as evidence of visitation it will suffice to claim the find. With that in mind, this is the exact rock you are looking for:

1. Look at the part of the wall that is shown in the image (where the green arrow points). You will notice an oddity. Describe this oddity - colour, shape, size and its texture.
2. Is this oddity an amygdule or phenocryst? Why so?
   • Would you also call this oddity a geode?
3. Is this amygdaloidal basalt or porphyritic basalt?
   • Do you think it existed as another category of basalt before this? Use the textures described.

Basalt

Well okay, I've done enough(-ish) on basalt to condense this into as short as I can. Here we go.

1. Igneous rock - formed from molten rock
   • Extrusive igneous rock - cooled rapidly therefore not allowing crystals to grow.
   • These small crystals can be described as aphanitic texture
2. Mafic rock - more magenesium and iron (ferric). These rocks are usually darker in colour. This is opposed to felsic rocks which have more feldspar and quartz and are usually lighter in colour.

Basalt is also a very broad term. It has many subcategories based on it's composition and texture. Let's have a look at basalt textures:

Ropy Basalt

Pillow Basalt - They occur wherever lava is extruded underwater

Scoriaceous Basalt/Scoria - Smaller holes than vesicles

Vesicular Basalt - the holes are called vesicles and are formed from gas bubbles. They are essentially gas bubble fossils, if you think about it.

Tachylite/Glassy Basalt - This glass is formed naturally by the rapid cooling of molten basalt

No idea what to title here - crystals?

Let's talk about crystals.

Phenocryst - remember the aphanitic texture? Well phenocrysts stand out as they are much larger than the other crystals. Phenocrysts form alongside the cooling rock. Rocks that have phenocrysts are described as porphyritic. 

Amygdule - These are secondary minerals that form after the rock has formed, or at least it doesn't form with the rock.

Xenolith - Opposite to amygdules in some way, these are existing rocks that were enveloped and incorporated into the igneous rock. In other words, it existed before the rock.

Geode - Those beautiful rocks that you see being sold. A picture speaks a thousand words. Some amygdules can be described as geodes.