Show Notes
This is part of the magnetic map of the former Soviet Union. It reveals three continental blocks and their amalgamation to form part of Eurasia. The Siberian Craton (a craton is a strong, usually fairly old block of continental crust) collided with and became attached to the East European Craton beginning about 300 million years ago, about the same time as Europe and Africa began to collide with North America to form the Appalachians. Here, the collision produced the Ural Mountains (orogeny means mountain building). The Ural-building collision went on for close to 100 million years.
Much longer ago, at least 520 million years ago and maybe as long ago as 650 million years, a relatively small continental block, labeled “Timan-Pechora Basin” in the image, was amalgamated to the East European Craton. The Timan Suture marks the position of that collision. Today that’s just a range of low hills, but it would have been high mountains at the time of the collision. The Timan-Pechora block was fully part of the East European Craton when Siberia collided with both of them.
For a sense of scale, the distance along the coastline (edge of data) from the Timan Suture to the thin black line is about 400 miles.
This is a very broad-brush look at these magnetic data (I spent the year 1990 making detailed interpretations of the maps for oil exploration).
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.
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.