What is a Xenolith?
A xenolith (from Greek 'xenos' for foreign, and 'lithos' for stone) is a fragment of rock that becomes encased in an igneous rock during magma ascent and emplacement, but is not derived from the magma itself. It is essentially a piece of the surrounding 'country rock' that gets caught up in the molten material and is preserved as an inclusion when the magma solidifies.
How Xenoliths Form
Xenoliths typically form when rising magma or lava forcefully breaks off pieces of the host rock it intrudes through. These broken fragments are then incorporated into the magma. If these fragments do not completely melt or chemically react with the surrounding molten rock before it crystallizes, they remain as distinct foreign inclusions within the new igneous body.
Characteristics and Examples
Xenoliths can vary greatly in size, from microscopic grains to massive blocks, and their appearance often contrasts sharply with the host igneous rock due to differences in mineralogy, texture, and color. For instance, a basaltic magma might pick up fragments of granite from the continental crust, or mantle-derived peridotite xenoliths can be found in volcanic rocks like kimberlites, providing direct samples of Earth's deep interior.
Importance in Geological Research
Studying xenoliths is crucial for geologists as they act as natural probes, offering direct insights into the composition, temperature, and pressure conditions of the Earth's crust and mantle layers that are otherwise inaccessible. Analyzing these inclusions helps scientists understand crustal evolution, magma generation processes, and the structure of the subsurface.