Monzanite from Thomasville, NC
This boulder is composed of two rocks: monzanite and basalt. The monzanite has large black and white speckles, making it look almost like chocolate chip ice cream. The basalt is a smooth medium gray, and forms lines that cut through the monzanite.
Figure 1: The monzanite boulder at Northern Wake Campus.
Figure 2: The monzanite boulder at Southern Wake Campus.
In the monzanite, the dark minerals are larger than the light minerals. Also, in the monzanite, if you look at the rock from the correct angle, you may notice that the black minerals are all arranged in a particular orientation (which is a feature called lineation). In the basalt, the mineral grains are too small to be seen with the naked eye.
Figure 3: The monzanite surface. Notice that the black minerals are larger than the white ones. This likely reflects details of the cooling history of the rock. Black ballpoint pen for scale.
Figure 4: Close up of the basaltic dike. Notice that the minerals are all dark gray or green, and that they are very small. Their miniscule size suggests rapid cooling.
How did it form?
Monzanite is a coarse-grained igneous rock, which means that it formed by very slow cooling of magma deep within the Earth. The basalt dikes that cut across the monzanite are fine-grained igneous rocks, meaning that they formed by rapid cooling of lava near the Earth’s surface. The monzanite has also experienced some metamorphism, resulting in the lineation.
How would a geologist figure out how it formed using rock characteristics?
In the monzanite, an interesting feature is that the dark-colored minerals are much larger than the light-colored minerals. We know from laboratory experiments that dark-colored minerals usually have higher melting temperatures than lighter-colored minerals. This means that in a cooling magma, the dark colored minerals crystallize first at high temperatures while the light-colored minerals are still liquid. Other laboratory experiments have shown that when minerals have a longer time to cool, they grow larger, and when they don’t have very long to cool, the minerals only grow to small sizes. The fact that the dark-colored minerals are larger than the light-colored ones suggests that this rock cooled very slowly at first for a long time, which would have allowed the dark colored minerals to crystallize and grow large. Then, once it was cool enough for the light colored minerals to crystallize out of the melt, the rate of cooling sped up, which meant that the light colored minerals did not have as long as the dark colored minerals to grow large. It is fascinating how we can chart the cooling history of an igneous rock like this using grain size.
The basaltic dikes that cut through the monzanite have a completely different chemical composition than the monzanite, which is reflected in their very different color schemes. The monzanite’s composition includes a lot of silica and feldspar, while the basalt’s composition includes much less silica and much more iron and magnesium. These different chemical compositions indicate that the sources for the magmas that formed these rocks came from different places. Likely, the magma for the monzanite came from melting of continental crust, perhaps during a continental collision, while the basalt came from melting part of the Earth’s mantle, perhaps during a continental rifting event.
We can tell that the monzanite is the older of the two rock types because the basalt has cut through it to form thin vertical forms. Logically, the basalt could not have assumed these forms on its own without the monzanite already there to support the cooling lava that would become the basalt.
Figure 5: The surface of the monzanite with a basalt dike running through it. Since the dike cuts through the monzanite, it suggests that after the monzanite cooled, there was an igneous intrusion event that caused magma of this dark color to cut through the monzanite and then cool quickly.
The lineation, or lining up of darker colored minerals, that we see in the monzanite is another interesting feature. It suggests that at some point after the monzanite formed, it was subjected to high heat and pressure, which caused a small amount of metamorphism to occur. High heat and pressure are the only things that can cause minerals to change shape and rotate like we see here to form the lineation.
Figure 6: The surface of the monzanite showing the lineation, or way that the dark minerals have aligned themselves. The lineation in this image runs top to bottom. Black ballpoint pen for scale.