Saturday, December 11, 2010
What a Geologist Sees - Part 15 [Original Post Date 2/11/08]
Inclusions and xenoliths. [OK, ya'll. Next time you play Scrabble, if you wind up with an "x", if you can cobble together "xenolith", that should score a lot of points.]
When there are individual components of an igneous or sedimentary rock that markedly-contrast with the matrix, the inquiring mind wants to know - where did that (or those) come from? When the contrasting piece is within an igneous rock, it is a xenolith, as highlighted in previous What a Geologist Sees posts 3 and 14. When the contrasting piece occurs within a sedimentary rock, it is an inclusion.
With the upper slide, I neglected to mark the locality, but I think it is the Eagle Mts. in West Texas. The matrix is on the right, while the xenolith (composed of inclusions) is on the left. The lens cap shows the scale of the inclusions within the breccia boulder that was incorporated into pyroclastic flow. The breccia boulder is older than the pyroclastic matrix and the breccia clasts (pieces) are older than the breccia boulder itself. The pyroclastic ash flows produce the welded ash flow tuffs when the ash flow movement ends. [That is for a separate post.]
The lower photo shows conglomeratic sediments. Each pebble came from a rock older than the conglomerate to which it now belongs. In this case, all of the quartzite pebbles in this conglomerate seem to be from a similar source, but sometimes some conglomerates (with different source rocks) can be quite colorful.
[Before going further, a breccia is composed of angular gravel-sized pieces of rock. A conglomerate is composed of rounded gravel-sized pieces of rock.]
Most conglomerates are associated with old river-channel deposits, the conglomerate in the lower photo represents old Chattahoochee River deposits, out of the current river channel.
Because of their angular corners and edges, breccias are generally found near where they are formed. The rock that produced the angular pieces can be broken by a number of different processes including landslides, faulting, meteor impact, and volcanic explosions (the probable source of the breccia in the upper photo, as this photo was taken inside of an old caldera-type volcano, where explosions would be the rule, rather than the exception).
As stated before, both examples illustrate the Concept of Inclusions that originated with James Hutton and Charles Lyell as one method of determining a relative timeline, without knowing the absolute age of any of the events. With the upper photo, there are at least four events recorded; 1) The brecciation (breakage) of the rock fragments within the boulder; 2) The cementation of the angular breccia fragments; 3) The separation of the boulder from its original locality; and 4) Inclusion of the boulder in the pyroclastic ash flow.
As part of another timeline, the stretched, brown pumice fragments existed before the pyroclastic flow, so they are also xenoliths, of a sort.