Igneous rocks (rocks that crystallize out of hot molten material), can be classified into two main categories: Intrusive and Extrusive. Intrusive rocks crystallize from magma beneath the earth's surface. These cool and solidify at a slow rate, resulting in the formation of large crystals in the rock. Extrusive crystallize from lava at the earth's surface. The exposure of lava to air and/or water cause cooling at a rate too fast for large crystals to form.
A secondary clasification system is based on the amount of silica (or silicon dioxide) in the rock. Felsic rocks are high in silica (65%+) and are usually light-colored. As the silica content drops, through Intermediate (55-65%), Mafic (45-55%) and Ultramafic (>45%) rocks, the colour of the rock gets darker, although high iron or magnesium content of Ultramafic rocks can lead to other colors.
The boulders at this location were extracted from behind the Royal Hotel (the building immediately to the South). To log this cache you need to send me the following answers:
1) How many boulders are there?
2) How would you classify the volcanic rock type represented by these boulders (two words needed: intrusive/extrusive and felsic/intermediate/mafic/ultramafic)?
3) What time period were these rocks formed in?
You can log the cache as soon as you send me the answers. I will let you know if there is a problem with the answers