Tuff Schist

The adventures of an unstable geologist

115 notes

scienthusiasts:

DID THE VIKINGS USE ICELAND SPAR TO NAVIGATE?

Iceland spar is a clear, transparent, colorless crystallized variety of calcite (calcium carbonate, CaCO3). Large pieces are split along natural cleavage planes to form natural rhombs. Iceland spar is probably best known for exhibiting the optical property of double refraction - so, anything viewed through the crystal appears double. It has many uses, in everything from precision optical instruments to LCD screens, and was even used during WWII to make bombsights. The perfect, flawless, colorless Iceland spar that is used in optical instruments sells for more than $1000 per kg. Historically, the first, best quality, and most abundant source of this clear calcite was in Iceland, which is where it got its name. Recently, Iceland spar has been in the news because of new research that shows that this mineral was almost certainly used by the Vikings for navigation.

Over 1,000 years ago, the Vikings sailed from Scandinavia to America without the use of magnetic compasses, astrolabes, maps or any other known device. Yet they still managed to find their way in spite of the clouds, fog, and long summer twilights characteristic of near Polar Regions. Centuries old Viking legends attribute their navigational success to the use of mysterious, glowing “sunstones” to find the position of the sun and set the ship’s course even on cloudy days. Although still controversial, many researchers now believe that these “magical sunstones” were in fact Iceland spar.

When Iceland spar splits light into two polarized rays, the two different images will have different brightness depending on the polarization. Because sunlight is polarized when it enters the Earth’s atmosphere, it is possible to change the orientation of a piece of Iceland spar to determine the direction of the sun. This is done by moving the crystal until the relative brightness of the two images are equal, which only happens when the crystal is aligned to the sun. This is possible even when the Sun is hiding behind clouds or just below the horizon.

Continue reading…

12 notes

brabrabie:

Turbidites

Turbidites are sediments deposited through deep submarine turbidity currents along sloping shelves. They are usually deposited offshore of an active convergent margin wherein tectonic activities could trigger the density-based avalanches. 

14 notes

entertainmentnerdly:

Lake Natron, Tanzania. This shallow salt lake can reach 60°C/140°F temperatures, and depending on rainfall, the alkalinity can reach a pH of 10.5, almost as alkaline as ammonia. Red-pigmented bacteria thrive under the surface, giving it the deep red color. [1000x679][OS] Unknown photographer via /r/EarthPorn http://ift.tt/1rRNkuu

entertainmentnerdly:

Lake Natron, Tanzania. This shallow salt lake can reach 60°C/140°F temperatures, and depending on rainfall, the alkalinity can reach a pH of 10.5, almost as alkaline as ammonia. Red-pigmented bacteria thrive under the surface, giving it the deep red color. [1000x679][OS] Unknown photographer via /r/EarthPorn http://ift.tt/1rRNkuu

21 notes

itssedimentary:

Minnesota Morton Gneiss
Not only is the Morton Gneiss of Minnesota absolutely gorgeous, it’s also really freaking old. Zircon crystals in the gneiss have been dated to ~3.6 Ga [that’s 3.6 billion years old!]
At their age, these rocks are from the Archean Eon, the second eon in Earth’s history (the Hadean is the first), which lasted from 4.0 - 2.5 Ga
This rock is the second oldest terrestrial rock I’ve ever seen, with the Acasta Gneiss being the oldest that I’ve seen (and the very oldest that exists).
To read a little bit about the Acasta Gneiss, the oldest rock on Earth at 4.0 Ga, click HERE
Image by author

itssedimentary:

Minnesota Morton Gneiss

Not only is the Morton Gneiss of Minnesota absolutely gorgeous, it’s also really freaking old. Zircon crystals in the gneiss have been dated to ~3.6 Ga [that’s 3.6 billion years old!]

At their age, these rocks are from the Archean Eon, the second eon in Earth’s history (the Hadean is the first), which lasted from 4.0 - 2.5 Ga

This rock is the second oldest terrestrial rock I’ve ever seen, with the Acasta Gneiss being the oldest that I’ve seen (and the very oldest that exists).

To read a little bit about the Acasta Gneiss, the oldest rock on Earth at 4.0 Ga, click HERE

Image by author

15 notes

Prograding Delta Deposits

itssedimentary:

Typically when you think of a sediment deposit, you think of a fining upwards sequence. That means that the heaviest/coarsest stuff is going to be at the bottom and the lighter stuff is going to be at the top, ‘cause you know, gravity settling. In laterally migrating streams, we see these…

43 notes

earthstory:

Basins and RangesHere’s a cool fact: 20 million years ago, Nevada was half as wide as it is now. Literally.My last post showed a very small normal fault of the Basin and Range province in the western U.S. This satellite photo shows the Basin and Range at its true extreme. The state of Nevada, and several others, is crossed by a series of ranges that trend almost north-south, with valleys/basins in-betweenThe continent being pulled apart created this topography. As the continent is pulled apart, normal faults have grown, with the rocks dropping down to fill in the space required by extension. The ground dropped down by these faults has filled in with sediments shed from the higher-standing basins, creating mountain ranges, fairly flat basins filled with sediment, and sharp, sometimes spectacular boundaries between them.-JBBImage credit: http://en.wikipedia.org/wiki/Basin_and_Range_Province#mediaviewer/File:Basin_range_province.jpg

earthstory:

Basins and Ranges

Here’s a cool fact: 20 million years ago, Nevada was half as wide as it is now. Literally.

My last post showed a very small normal fault of the Basin and Range province in the western U.S. This satellite photo shows the Basin and Range at its true extreme. The state of Nevada, and several others, is crossed by a series of ranges that trend almost north-south, with valleys/basins in-between

The continent being pulled apart created this topography. As the continent is pulled apart, normal faults have grown, with the rocks dropping down to fill in the space required by extension. The ground dropped down by these faults has filled in with sediments shed from the higher-standing basins, creating mountain ranges, fairly flat basins filled with sediment, and sharp, sometimes spectacular boundaries between them.

-JBB

Image credit: http://en.wikipedia.org/wiki/Basin_and_Range_Province#mediaviewer/File:Basin_range_province.jpg