Xenolith or Autolith?
Xenolith is Ancient Greek word for "foreign rock". A xenocryst is an individual foreign crystal included within an igneous body.
A xenolith is a rock fragment which becomes enveloped in a larger rock during the larger rock's solidfication and hardening. To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped. An included rock of similar type is called an autolith (same rock) or a cognate inclusion.
Most of the time, a xenolith is a rock embedded in magma while the magma was cooling. Magma is the molten rock beneath the Earths crust that emerges as lava during a volcanic eruption. The rock that forms from cooled magma is called igneous rock. Xenoliths are different types of rock embedded in igneous rock. Xenoliths provide important information about the composition of the otherwise inaccessible mantle. Basalts, kimberlites, lamproites and lamprophyres, which have their source in the upper mantle, often contain fragments and crystals assumed to be a part of the originating mantle mineralogy. Analyzing rock composition of xenolith can reveal the geological information before the host rock is formed. Xenolith could even provide information about the composition of inaccessible mantle.
The base and steps are State War Memorial constructed of grey granodiorite quarried from Harcourt, central Victoria. This stone has been quarried for 150 years and utilised around Australia. Granodiorite is a granite-like rock, both in its crystalline texture and intrusive origin. The feldspar mineral, which differentiates the rock from granite, is a form in which the element sodium is an important constituent. Fragments or xenoliths of preexisting intruded rocks can be seen in various stages of assimilation into the igneous mass of the granodiorite.
To the right of the northern door of the memorial you should notice an impurity with a distinctive shape embedded in the granodiorite rock. Please send your answers for the following earthscience tasks to the cache owner by message or email. You can log your find as soon as you like after your visit. Your find log may be deleted if the answers are not sent within 14 days.
Task 1. Do you think you are looking at a xenolith or an autolith? Please give an reason for your choice.
Task 2. There are other examples of xenoliths visible around the base of the memorial, please compare to the observed sample. Are they the same colour, shape and size as the observed sample? If not, please describe how they differ.
Task 3. How does the basic granodiorite surface compare to the observed xenoliths? Please describe colour, crystal size.
Task 4. Please provide a photograph of yourself or a personal item with the lion head to prove you visited the earthcache site.