#diabase

[schisto-city]

Bit of a headscratcher today. When a pyroxene and a pyroxene love each other very much...?

(These rocks are gay!)

[earthstory]

The Karoo LIP: The Destroyer of Gondwana About 500 million years ago, a huge continent was assembled due to collisions between multiple ancient cratons, or cores of continents. This supercontinent, Gondwana (Gondwanaland) contained what we know today as Africa, South America, The Arabian Peninsula, India, Antarctica, Australia, and New Zealand – give or take that’s something like ½ of the planet’s continental landmass. That large continent held together for nearly 400 million years, and was even joined with a few other large landmasses for a time to create Pangaea. It lasted for about 10% of Earth’s history, and here you’re looking at the leftovers of its killer.

183 million years ago, a blob of hot material moved upwards through Earth’s mantle, silently sneaking up on the base of that continent. When it reached shallow depths, it impinged on the crust itself, pushing it upwards into a large dome. Molten rock poured from giant fractures, creating systems of dikes and lava flows that covered the landscape. The welt in the crust was enough to cause the crust to crack, eventually snapping one piece after another away from the continent. There were a few additional helpers – there are a couple of similar mantle plumes that rose up between South America and Africa to break those continents apart, but the Karoo’s damage created far more continental havoc. Two continents (Antarctica and Australia) as well as the Indian Subcontinent and several other slivers of crust (Madagascar) were separated from Gondwana and pushed across the surface by the intrusion of the Karoo Large Igneous Province (LIP). It has different names across different continents, as you see in the final image, but they all formed from the massive plume of hot mantle that rose up beneath Gondwana. Today, the dikes and lava flows of the Karoo province are partially preserved, some of them still trapped under the ices of Antarctica. Where they intruded softer rocks such as shales, the dolerites form resistant layers that are hard to erode, creating koppies like the ones seen in the large shot. Others form cliffs and fill fractures throughout the bedrock of these continents. -JBB image credits: https://en.wikipedia.org/wiki/Karoo_Supergroup#/media/File:Karoo_Koppies.png https://flic.kr/p/RLe8GH http://www.largeigneousprovinces.org/13apr Reference: http://karoospace.co.za/dolerite-karoos-fracking-game-changer/

[fromthedust]

Piet van Stuivenberg (Dutch, 1901-1988)

Mother and Child - 33cm high - granite

Mother and Daughter - diabase - 1958

Mother and Child - diabase

title unknown - carrara marble - 32cm high

title unknown - travertine - 38cm high - 1955

title unknown - carrara marble - 40cm high (sans base)

Head - granite - 37cm high - 1961

Grey

[spatheandspadix]

Along the banks of French Creek, State Game Land 43, May 2017

[atlanticghostcrab]

Action

[aroguexenolith]

So many thin sections

(They're all dolerite/diabase...) I feel like I used to be able to tell clinopyroxene and olivine apart no problem. But nowadays it's more like--"follow your heart it will guide you to the truth." I guess it doesn't hurt to err on the side of less cpx, since it would be the first to totally be taken out of the melt?

[schisto-city]

A thin section is about as flat as you can get, but put enough feldspar next to each other and it can feel a bit like falling into your microscope. Some lovely diabase today!

[earthstory]

Cradle Mountain

This fog-shrouded peak is Cradle Mountain on the island of Tasmania. Although the peak is not the highest on the island at an elevation of 1545 meters above sea level, it is one of the island’s most popular tourist locations in part from the spectacular columnar features that define the mountain’s shape.

Tasmania contains one of the world’s largest outcrops of dolerite, a type of igneous rock related to basalt. Dolerite, also called diabase, forms when basaltic lava gets close to Earth’s surface but doesn’t quite erupt. Dolerite is defined by an “intermediate” grain size – it doesn’t stay hot long enough to form large crystals as in a gabbro, but it doesn’t erupt on the surface where it would cool extremely fast to make a fine-grained basalt. The spectacular vertical columns form perpendicular to the cooling direction due to thermal contracting; the rocks shrink as they cool down, causing fractures to form.

These basalts were produced during the Jurassic, as the large continent Gondwana broke apart, with Australia rifting away from Africa and the other pieces. Tasmania has a long history of rifting, having both spread away from Gondwana during the Jurassic and from Australia itself more recently, leading to an island with large outcrops of igneous rocks.

-JBB

Image credit: https://flic.kr/p/7ht28V

Read more: http://bit.ly/19hVTIw http://bit.ly/1DRxipE http://www.parks.tas.gov.au/file.aspx?id=6531