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.
Examine the natural weathered rocks on the top of the hill. There are numerous examples of xenoliths. Please examine the xenoliths and prepare your observations to the following earthscience tasks. Please provide your answers to the cache owner in a timely manner, i.e. less than a fortnight.
-What differences can you see in crystal size between the xenoliths and the host rock?
-Is there any evidence of differences in composition?
-Do you see any examples of xenoliths within xenoliths?