Show Notes
In the far northwest corner of the flat, flat Texas panhandle, extending into New Mexico, there’s a narrow, elongate magnetic low. The intensity of the anomaly – 250 nanoTesla or more – says it’s fundamentally the expression of a lithologic change rather than a structure; i.e. it represents something pretty strongly magnetic. Its long narrow geometry is that of a dike. And its negative value suggests that it’s reversely polarized, solidifying from magma during a time when the earth’s magnetic field was in the orientation opposite to that today.
All that is interesting, I guess, but the thing has much broader implications. If it is a dike – which is likely in my opinion – that suggests that it formed at a time when extension, pulling apart, was the dominant stress in this area. Dikes can form under compression, but it’s a lot easier for them to intrude if the rocks are pulling apart, opening up cracks into which magma can force itself.
The northeast-southwest orientation is also intriguing, because it points pretty much dead on at a possible branch of the Mid-Continent Rift, a pull-apart feature that runs from Kansas through southeast Nebraska, northeast across Iowa, up into Minnesota, and into Lake Superior. It’s a 1.1-billion-year-old break in North America – a break that failed to completely dismember the continent, but just formed a long narrow trough filled in many places with dense, magnetic basalt. Kind of like the Red Sea today, but not as linear.
This dike in the Texas panhandle isn’t trivial – it’s at least 45 miles (70 kilometers) long. There are additional similar features on trend with it in Kansas. My interpretation is that it represents a far away expression of the extension and intrusion related to the Mid-Continent Rift System. This possible relationship is shown in a map below.
The depth to these rocks in the panhandle is probably only 3500 or 4000 feet, but to my knowledge there is no drilling to those depths in this area, so we don’t actually have rocks to validate this interpretation. But I’d bet a beer that you’d find a reversely polarized dike of basalt or similar lithology, containing a decent amount of magnetite, that solidified around 1.1 billion years ago.
—Richard I. Gibson
More from History of the Earth
The mineral here is just calcite (even though it’s mostly almost black), but it shows interesting features. Cone-in-cone structures are nested cones, seen here in cross section. The inset shows them a little better – in the main photo, they are represented by very narrow vertical triangles.
Mineral Monday + Tectonic Tuesday. Blue quartz is uncommon and is usually colored by inclusions of unusual minerals like crocidolite, tourmaline, or dumortierite. The purplish-blue quartz here, from north of Llano, Texas, is colored by inclusions of ilmenite (iron-titanium oxide). This rock is called llanite for its occurrence in the Llano Uplift of central Texas, and although similar rocks are found in other parts of the world, the variety name llanite really only applies to this location. On a sunny day, the blue quartz in the rocks has an opalescent sheen that sometimes seems to “wink” at you from the outcrop.
Epitaxy, from Greek words meaning “upon” or “above” and “ordered arrangement,” in minerals means crystals of one (or the same) mineral growing in a particular crystallographic position on another (or the same) mineral. It happens because the molecular spacings and orientations happen to be similar, allowing, even encouraging the crystal structure of the second, later mineral to mesh with that of the first. Some mineralogists might say that epitaxy requires the two minerals to be different minerals, but I do not – just two distinct generations of crystallization.
Thule was the far north in Greek and Roman literature, often identified with Scandinavia. Thursday was named after Thor, the Norse god of thunder. Whether these pink minerals are orthorhombic thulite or monoclinic clinothulite would take analysis that I haven’t done, but either way they contain trivalent manganese to give the pink color.