#porphyritic

[earthstory]

Crystal Mush

This is a common texture in igneous rocks – a porphyritic texture. This term refers to igneous rocks that have a population of large crystals surrounded by a fine-grained groundmass or matrix.

A porphyritic texture is produced naturally as magmas cool. A magma is a complicated, multi-component solution – a single molten rock can be composed of 10 different abundant elements. When magma cools, some of these elements start forming minerals, crystallizing out of the melt while other elements remain left behind.

A porphyritic texture therefore is created when some minerals crystallize at higher temperatures than the rest of the magma, forming large crystals surrounded by a groundmass. These larger crystals are plagioclase floating in a groundmass that contains other elements – the photographer labels the rock as an andesite. This type of rock is one way that molten rock can look when it sits inside a “magma chamber”.

The term “magma chamber” probably makes most people picture a big hole in the ground filled with molten rock, but that’s not how the world works. The Earth’s crust is pretty cold, so when molten rock gets into the crust it starts to cool off and crystallize. Most magma chambers aren’t big pockets of molten rock, they’re a mush, filled with a mixture of crystals and molten rock in-between. They’re much more like this rock, a porphyry.

Although we can’t see individual crystals until they reach the surface, scientists have tools that can characterize these mixtures; geophysics. Seismic waves, for example, will travel through just about anything. If a melt is present, even a tiny bit of melt in-between larger grains, seismic waves will travel through that rock and interact with the melt. The presence of a tiny bit of melt therefore can be detected using seismic waves, and that technique has been used to measure the volume and crystallinity of the magma plumbing system beneath many volcanoes around the world. 

-JBB

Image credit: Richard Droker (Creative Commons Licensed)

https://flic.kr/p/6Litaw

[earthstory]

Egyptian diorite vase.

Archaeologists frequently comment that the best workmanship of Ancient Egypt occurs in the earliest eras of that long lived civilisation, and in no area is it truer than the lovely hard stone vases and bowls made in predynastic times. Thousands were found in the earliest step pyramid of Djoser and they are some of the finest stonework ever produced by humans.

The technology used to work such hard stone is a mystery, though lathes and bronze cutting tools tipped with hard materials such as corundum or diamond must have been involved. One example has clearly been centred twice during manufacture on a lathe, exhibiting two surfaces. No evidence such as diamond tipped tools has so far been found from that era, so the puzzle remains. Some of the vases have also been incised with hieroglyphs with a sharp point, such as a diamond octahedron. Saws covered in abrasive powder such as emery (powdered corundum) were also used.

This example from the Field Museum dates from the latest pre dynastic period known as Naqada 2. It was carved out of porphyritic diorite, in which the large white crystals of feldspar grew slowly in a cooling magma before it was quenched, resulting in the finely crystalline matrix. Diorite (diabase to some) is an intrusive equivalent of basalt, that cooled below the surface rather than being erupted, in much the same way that granite is the intrusive analogue of rhyolite.

Loz

Image credit: Madman2001/Wikimedia Commons.

http://www.theglobaleducationproject.org/egypt/articles/petrie.php http://www.theglobaleducationproject.org/egypt/articles/hrdfact3.php

[match-your-steps]

(studying for my geology exam) porphyritic because por because- because it has big parts, which is like pores- this doesn't make any sense but I think it's gonna work

[relatetorocks]

[louie-999]

Yes, even Porphyritic Andesite can be a name for a hero's attack... #geology #anime #porphyritic #andesite #Otters11 #ラッコ11号

[earthstory]

Porphyritic Texture. Porphyritic is a term used in igneous petrology to describe texture in relation to crystal size. Porphryitic refers to one crystal size being much larger than the other, such as in the image below. The k-feldspar crystals are obviously much larger than the rest of the crystals making up the granite.  There are two main types of porphyritic texture, the first, large crystals"floating" in a fine grained crystal groundmass, where it is not possible to differentiate the individual crystals of the ground mass with the naked eye, and the second, where it is possible to differentiate the differing crystals, but one group of crystals are obviously larger than the other. Porphyritic texture forms when magma cools in two stages. The first stage is slow, and elements that form at high temperatures are the first to crystallise, forming large individual crystals "floating" in a magma. The second stage is sudden rapid cooling, forming the smaller sized crystals.  This rapid cooling can occur during an eruption from a volcano, or when a rising column of magma suddenly reaches shallow depths in the crust. -LL Links; http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures/4PorphyriticFineGrained.html http://geology.about.com/od/more_igrocks/ig/igroxtextures/igtexporphyritic.htm http://www.tulane.edu/~sanelson/eens212/textures_igneous_rocks.htm Image; Leah Lynham