May 18, 2011

Geology Word of the Week: X is for Xenolith

Posted by Evelyn Mervine

Mafic xenolith, Ontario, Canada, 2002. Photo Credit: Ron Schott.

Note: Sorry for the re-post. This post was lost and then mangled somewhat in the blogger mishap last week. I managed to correct the post, but I had to re-post it under a new day and time.   

def. Xenolith:
A foreign rock inclusion, usually in an igeneous rock.

Xenolith literally means “foreign rock” coming from “xenos” (foreign) and “lithos” (stone) in Ancient Greek. A xenolith is a fragment of foreign rock within a host rock. To be considered a xenolith, the inclusion must be different in composition from the enveloping rock. Inclusions of similar rocks are called “autoliths” or “cognate inclusions.”  Xenoliths are generally easy to recognize because they are very different in composition (and often in color) from the encompassing rock. For example, in the picture below the bright green olivine crystals and shiny black pyroxene crystals of a mantle peridotite xenolith stand out in contrast to the fine-grained, gray basalt in which they have been encompassed.

Peridotite xenolith in basalt, Hawaii, 2009. Photo Credit: Einat Lev.

Xenoliths most often occur in igneous rocks. For those of you who are a little rusty on Geology 101, igneous rocks are rocks which form by the cooling and solidification of molten material– either magma or lava. As magma or lava migrates and cools to form igneous rock, it may pick up inclusions of foreign rock. Where do these foreign rock inclusions come from? There are several possible sources. Often, molten magma intrudes into preexisting rocks (known as “country rocks”) and may pick up fragments of this country rock. Commonly, xenoliths are fragments of the walls of a magma chamber or conduit. Xenoliths may also be picked up by lava during explosive volcanic eruptions or may be picked up by lava as it flows along Earth’s surface (if a different type of rock is at the surface).

The term xenolith is most commonly applied to foreign rock fragments in igneous rocks. However, a broad definition of the word xenolith might include foreign rock fragments in sedimentary rocks and inclusions found in meteorites.

Xenoliths are generally small in size relative to the overall body of rock. However, xenoliths can range in size from single crystals (called “xenocrysts”) to rock fragments of several meters.

A small peridotite xenolith in basalt, Hawaii, 2009. Photo credit: Einat Lev.
“Teboho-size xenolith.” Cape Columbine, South Africa, 2009.
Photo credit: Christie Rowe.

Xenoliths are important because by studying xenoliths geologists can learn about the origin and evolution of the host rock. For example, when an igneous rock contains a xenolith, geologists know that at some point the magma or lava that cooled to form the igneous rock was in contact with that foreign rock. Xenoliths are also important because they often allow geologists to sample and study rocks which are difficult to access. For example, mantle rocks are not generally exposed at Earth’s surface (except at ophiolites), so xenoliths of mantle rocks are important for learning about the composition of Earth’s mantle. Some xenoliths come from very deep within the Earth. For example, diamonds are famous and economically valuable xenocrysts that formed at high pressures and temperatures very deep within the Earth, ~140 km deep or deeper. Diamond are brought to Earth’s surface as xenocrysts in kimberlite rock.

Here are a few more pictures of xenoliths:

Peridotite xenolith in basalt, Hawaii, 2009. Photo credit: Einat Lev.

Many peridotite xenoliths in basalt, Hawaii, 2009. Photo credit: Einat Lev.

Xenolith in lamprophyre, Ontario, Canada, 2002. Photo credit: Ron Schott.

Peridotite xenolith collected at Dish Hill Cinder Cone, Mojave Desert. Photo credit: Ron Schott.

Oxidized mantle xenolith collected at Dish Hill Cinder Cone, Mojave Desert. Photo credit: Ron Schott.

Finally, Callan Bentley of the Mountain Beltway blog has a zillion majillion photos of xenoliths on his blogs (Mountain Beltway used to be the NOVA Geoblog):

Xenolith Label on Mountain Beltway

Xenolith Label on NOVA Geoblog

Here is one of Callan’s xenolith pictures that I found particularly striking:

Mafic xenolith in a statue carved from porphyritic andesite, Ankara, Turkey, 2010.
Photo Credit: Callan Bentley. Read more about Callan’s trip to Turkey here.

Thanks to all of my geologist friends and fellow geobloggers who sent me pictures of xenoliths. If you have any good xenolith pictures, post a link below in the comments.