Xenolith+Steps

=The Old Post Office=

The point of interest on this building are the steps made of an igneous rock called granodiorite. Molten rocks can occur below ground where they are called magmas, and above ground, where they are called lavas. This one cooled below ground, so was a magma.

The granodiorite is grey, with black and white minerals in it. When magmas are generated by melting rocks in the crust, they are hot and buoyant - so they naturally want to rise to the surface, if they can, before they cool. This is like the hot wax in a lava lamp (though the wax in a lava lamp is more like molten rock //below// ground - so it should really be called a magma lamp! Feel free to tell the manufacturers they clearly don't know their geology terminology! )

As magmas rise they have to move the overlying rocks out of the way. Many of these rocks fall into the magma and ultimately melt, adding to the volume of the magma. You can occasionally see chunks of this foreign rock that fell into the magma, in various states of being melted. These are called xenoliths (this literally means foreign rock - it is foreign to the magma).

Find 3 xenoliths: 1) one which is uniform dark grey, with nice sharp edges. This fell into the magma and remains unchanged - it is a mudstone. 2) Find some with small crystals in it, the edges may be a bit more rounded. This has started to heat up and change. 3) Find one that has rounded edges, and lots of larger crystals in it. This one had been sitting in the hot magma longer and almost melted.
 * EXERCISE:**

This granite makes for good building stone because it is not porous (no holes in it at all) and does not absorb water. It's made of hard minerals which are not damaged by abrasion and it is not soluble like limestone.

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