#dolerite

[davidstanleytravel]

Faults in the greenstone dolerite cliffs are seen at several locations on Isle of May, Scotland.

[geologyjohnson]

These rocks have all experienced hydrothermal alteration and contact metamorphism. The 1.4 billion year old Velkerri Formation from Australia is an organic rich mudstone. Some time after burial and lithification, the rock was intruded by hydrothermal fluids that caused metallic sulphide, and ferrous carbonate + quartz mineralisation. I think some maybe related to the intrusion of dolerite around 1.2 billon years ago, and some related to basin inversion around 800 million years ago. The mineralisation and contact metamorphism is widespread but in some areas is subtle and only visible on the microscopic scale. The velkerri formation is the basis for much of what we know about marine chemistry during the early evolution of eukaryotic life. Several important geochemical studies sampled the velkerri formation and used the pyrite in particular to show that the oceans 1.4 billion years ago were low in oxygen and rich in sulphur. This environmental state is thought to have been a major barrier to the development of more complex eukarotes. However, the hydrothermal and contact metamorphic alteration, in particular the micro scale alteration were unknown when these studies were conducted. As these previous studies used bulk sampling methods, it's likely that they unknowingly sampled the later sulphides which will have influenced the results and the interpretations of the previous researchers. This does not mean the previous work is bad and should be disregarded. Simply that the results should be reinterpreted inlight of this new information. That is how science is supposed to work. It is self revising and correcting. That is how science advances and improves. However, getting paper reviewers to understand and accept this is another thing entirely. I'm currently revising this work after a rejection. So let's see if the next version is enough to get the paper out. 😉 #geologyjohnson #geology #geologyrocks #pyrite #blackshale #shale #ore #hydrothermal #igneous #dolerite #metamorphism #science #sciencerocks #earthscience #naturalhistory https://www.instagram.com/p/CdaMy8-sFoZ/?igshid=NGJjMDIxMWI=

[happy-geology]

A Miocene dolerite sill sitting over volcanic sandstone....assigned to a divine place at the shore.

[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/

[geologicaltravels]

2018: Jurassic dolerite is seen intruding sedimentary rocks of the Parmeener Supergroup (mainly Permian and Triassic) along the Three Capes track on the way to Cape Hauy.

[c6h14nightmares]

I like this unusual rainbow lens flare.

Dolerite columns and cliffs, Bruny Island, Tasmania. 

[remash]

apollo bay house ~ room 11 | photos © ben hosking

[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?