#pseudotachylite

[geologyjohnson]

The weird black and pink rock I'm stood next to its called Pseudo-tachylite and it formed in a spectacular and cataclysmic event. Around 2 billion years ago, a 15km wide chunk of rock face planted into what would become south africa at a velocity of around 200,000km+ an hour. It created a crater 60 km deep and 300 km wide with a central uplift region around 17 km high. This was one of the biggest impacts in Earth's history, including being larger than the one that caused the K-Pg. Luckily the microbes that inhabited earth at this time were pretty tough critters so life continued in the aftermath. Today most of the structure has been eroded away, a ring hills 90 km across is all that survives on the surface near the towns of Parys and Vredefort South Africa. These hills represent rock that was tilted upwards by the blast during the impact. The underlying rock made of hard granitic gneiss was shattered and blasted outwards as huge blocks carried in a slurry of melted rock. The melted rock quickly cooled to glass as water poured into the new crater. This rock is the pseudo-tachylite I'm stupid next to in the photo. It was injected into the bedrock in these wonderfully messy veins by the force of the impact. I've wanted to see this place in person for a long time and it did not disappoint! If you are in the area, contact Karen or Graeme at Otters Haunt for a tour of the quarry and the geological details! #geologyjohnson #geology #meteor #crater #pseudotachylite #vredefort #parys #archean #precambrian #paleoproterozoic #SouthAfrica #granite #gneiss https://www.instagram.com/p/Ccc_th8twwv/?igshid=NGJjMDIxMWI=

[earthstory]

Glassy This is such a neat rock. Ever seen obsidian? It’s formed when rock is molten and cools fast enough that it can’t form crystals. The dark material in this rock is very similar – a rock type called a pseudotachylite. A tachylyte is the term for basaltic glass, typically formed when basaltic lava enters the ocean and rapidly cools. The dark portion of this rock, a pseudotachylite, forms when the rock is rapidly heated and cooled, leaving behind a dark, glassy rock with either very fine crystals or no crystals. This rock started as a part of the Precambrian crust in Colorado. In this case, the rocks are found in a 1.4 billion year old fault zone. The rocks were fractured and heated by friction caused by the faulting. -JBB Image credit: James St. John https://flic.kr/p/yggCSf

[earthstory]

Pseudotachylite

Rub your hands together. Feel warm? Now imagine that you could squeeze them together harder, and that well, your hands were rocks, so that there was a whole lot more friction. How hot would your imaginary rock hands get if they kept rubbing together. Hot enough to melt maybe?

That’s a pseudotachylite. In active fault zones, rocks are rubbing together rapidly, with huge amounts of friction between the rocks. That friction can be converted to heat energy when the fault moves, so much heat that the rock can melt.

A fault pseudotachylite is created from the friction of rocks rubbing together in a fault zone. The heat melts a small amount of the rock, creating a thin layer of molten rock that rapidly cools, leaving it with an extremely fine-grained or even glassy texture. Often, chunks of the surrounding rock are found chewed up and trapped within the glassy pseudotachylite. Other pseudotachylites can be found in impact zones, produced when rocks are again rubbed up against each other at high speeds during the impact.

This pseudotachylite is found within 1.3 billion year old crystalline rocks in Homestake Creek Valley, in Colorado.

-JBB

Image credit: James St. John https://flic.kr/p/yVWugg

References: http://geology.about.com/od/rocks/a/aa_fractfrict.htm http://www.lpl.arizona.edu/~rlorenz/pseud.html