Friday, December 20, 2013

A Basalt Trek Geo-Haiku

on a high plateau
jagged basalt Cretaceous
curious travelers walk

and a friend sent in this one -

through windswept fields
we tread on dead volcanoes,
A gentle dog follows

- Sushma Date

Monday, December 16, 2013

Porphyry Copper Deposits And Tectonic Plate Thickness

Just a follow up to my earlier post on porphyry copper deposits with an example from the Malanjkhand mines of Central India.

Nature Geoscience has a paper on the relationship between copper deposits and magmatic arc thickness. The full paper is behind a pay wall but the abstract is helpful enough:

Porphyry copper systems supply about 75% of the world’s copper. They form above subduction zones and are preferentially associated with calc-alkaline magmas. Such magmas result from continuous iron depletion during differentiation, in contrast to tholeiitic magmas that show initial iron enrichment during differentiation. The formation of calc-alkaline magmas is favoured by high water content and oxygen fugacity. These characteristics, as well as magmatic metal contents, are thought to be imparted in the mantle source by fluids of the subducted slab. Yet this process does not explain why porphyry copper systems preferentially occur in thicker arcs. Here I present a statistical assessment of more than 40,000 published geochemical analyses of magmatic rocks from 23 Quaternary-aged volcanic arcs worldwide. I find that magmas of thicker arcs are systematically more calc-alkaline and more depleted in copper than magmas of thinner arcs. This implies that the missing copper in the former accumulates as copper sulphides within or at the base of thicker arcs. Such copper accumulations are an essential step in forming porphyry systems. These results suggest that the thickness of the overriding plate provides a more important control on magma differentiation than the composition of the mantle source, and can explain the preferential association of porphyry copper systems with calc-alkaline magmas and thicker arcs.

Malanjkhand ores would have around 2.4 billion years ago been a copper enrichment at the base or at the deep levels of a magmatic arc system in a subduction zone setting as smaller cratonic blocks converged. The roots of this ancient magmatic arc mountain chain has since been exhumed due to subsequent tectonic movements and erosion to reveal its riches.

Sunday, December 8, 2013

Sunday Humor: Rugby And Geology Don't Go Together

I ran into a friend after many years. Her son used to come to our academy for soccer and rugby coaching. She mentioned that a common friend had recently put up a link to my blog on his Facebook wall and after reading a few of my posts had showed my blog to her son, reminding him who I was.

Her son who is now a teenager exclaimed.. "but Mama how is it that a rugby coach can be clever enough to write a geology blog?"

My friend is now trying to persuade her son to meet me again. I on my part have promised to take him and his friends to a trip to the local geology museum.

Maybe I can convince him that sometimes sports, jocks and rocks do go together.

Tuesday, December 3, 2013

A Dispute About Adam And The Origins Of Humans

More than 40% Americans insist that God created humans in their present form less than 10,000 years ago. Probably a majority don't believe evolution at all or accept that it has played a role but one overseen by God in the history of life.

Now genetics is throwing up another problem for creationists to chew on:

Lexington writes in the Economist -

A trickier controversy has been triggered by findings from the genome that modern humans, in their genetic diversity, cannot be descended from a single pair of individuals. Rather, there were at least several thousand “first humans”. That challenges the historical existence of Adam and Eve, and has sparked a crisis of conscience among evangelical Christians persuaded by genetic science. This is not an esoteric point, says Michael Cromartie, an evangelical expert at the Ethics and Public Policy Centre, a Washington think-tank: many conservative theologians hold that without a historical Adam, whose sin descended directly to all humanity, there would be no reason for Jesus to come to Earth to redeem man’s Fall.

Academics have lost jobs over the Adam controversy. Many Christian universities, among them Wheaton (a sort of evangelical Harvard and Yale, rolled into one), oblige faculty members to sign faith statements declaring that God directly created Adam and Eve, the “historical parents of the entire human race”. John Walton, an Old Testament scholar at Wheaton, suggested that Adam and Eve are presented in Genesis as archetypes, though he called them historical individuals too.

Read the rest here.

Sunday, December 1, 2013

Sunday Image: Cavity Minerals In Basalts

In cracks, fractures, geodes, gas vesicles of the Deccan Basalt lava flows occur secondary minerals of the zeolite family along with silica, calcite, apophyllite and iron hydroxides. They are often breathtaking in their fully faceted form and gorgeous color combinations.

Green apophyllite is one of the most common and appreciated minerals. I say mineral but really apophyllite is a general term used for three different minerals, fluor-apophyllite, hydroxy-apophyllite and natro-apophyllite. Most of the Deccan Basalt apophyllite are fluor- and hydroxy apophyllites. These specimens have a peculiar form. Their name is derived from the Greek apophylliso meaning "it flakes off". They have a basal cleavage like the micas which lends it to being easily split along one plane of weakness but an overall crystalline structure conforming to a tetragonal symmetry.

Here it is in the picture below growing like a creeper of other worldly kryptonite from a lustrous crystalline bed. 

The large off white crystals at the base of the geode are also apophyllite.  Zoom in and you will see large fleshy pink colored sheafs of stilbite clinging to the wall of the geode. The other minerals not easily recognizable are calcite, quartz and heulandite.

Source: The Gargoti Museum