Sunday, 23 February 2014

nuée ardentes

Nuée ardentes (glowing clouds) at Mt. Pelée. 
photographed (and named) by Alfred Lacroix. 
December 16. 1902. 

Friday, 24 January 2014

crustal growth

here is a neat image showing crustal thicknesses across the planet. The two thickest regions are in Tibet and the central Andes. The colours represent surface elevations.

This image is sources from the USGS, but I got it from Cawood et al., 2012. This paper discusses the current state of affairs of their understanding of the evolution of the continental crust. A major question is whether crustal growth was accelerated early in Earth's history, or if it has been more or less steady going. Here are some of the options:
I like the style of Condie and Aster, 2010. Buenas hondas. 

Below they show the idea that prior to 3 billion years ago, a different set of processes dominated the formation of the continents compared to the typical plate tectonics that we see today. In these early days, the mantle was much hotter and convection likely occurred at fairly shallow levels. Melting of dense, thickened/dripping, basaltic crust is thought to be the dominating mechanism of continental crustal formation at this time. After the earth cooled down a bit, subduction-as-we-know it took over, and continental crust began to form by andesitic volcanism at subduction zones, with the occasional delamination of the mafic roots.

 Cawood, P.A., Hawkesworth, C.J., Dhuime, B., 2012, The continental record and the generation of continental crust, GSA Bulletin.

accretion vs. erosion at subductio zones

Here is a figure from Clift and Vannucchi (2004) showing which subduction zones are adding material to the continental crust (accretion), and which are actively scraping away the continental crust (erosion) and dragging it into the depths.
Accretion is occurring in the Cascadia (forming the coast range), and eroding in the central Andes (exposing magmatic rocks from Jurassic subduction).

Clift and Vannuchi explain that accretion occurs where rivers introduce large volumes of sediments into the subduction trench, such as the Columbia river in Cascadia. In arid regions such as the central Andean coast, sediments are few, and continental erosion dominates. 

Clift, P., Vannucchi, P., 2004, Controls on tectonic accretion versus erosion in subduction zones: Implications for the origin and recycling of the continental crust, Review of Geophysics, 42.  

Thursday, 2 January 2014

Tulul Al-Safa - ghuluww

there is this volcano in syria.

in the desert. far from a plate boundary. no good excuse to be there.

the druze have been hiding out there for years.

there were named after a heretic
who exaggerated a bit too much.
but never got around to changing the name.

nobody knows the chemistry of the lavas. as far as i can tell

Sunday, 3 November 2013

Project Mohole

 Unlocking the secret history of the earth
      - the story of an early attempt at offshore drilling of the ocean crust.
      - and a really cool project name.


Willard Bascom. Director of the Moho project. 1959.

Friday, 13 September 2013

Starbursts and snowballs

when stars explode, earth gets cold

Many folks agree that the planet suffered a small number of drastic cooling events (snowball earth); at least one between 2.2-2.4 billion years ago and another 770 to 550 million years ago, but their triggers are a matter of debate. Katoaka et al. (2013) suggest that the events could be driven by encounters between Earth and cosmic dust and rays originating from stellar explosions (nebulae). They use the ages of stars, and star clusters, to deduce a causal effect between the death (and subsequent growth) of nearby stars in the Milky Way galaxy and periods snowball Earth. They also suggest that some of the major mass extinctions in the Phanerozoic could also be related to encounters with remnants from supernovae.

Image from Katoaka et al., showing interaction of stellar particles with two of the Earth’s protective shields (geomagnetic field, and ozone layer). The heliosphere provides further protection.

Reference, Kataoka, R., Ebisuzaki, T., Miyahara, H., Maruyama, S., 2013, Snowball Earth events driven by starbursts of the Milky Way Galaxy, New Astronomy, 21, 50-62.

Wednesday, 4 September 2013

Archean crustal growth

Has continental crust always formed the same way?

In an introduction to new research published in Geology, Ali Polat (2013) summarizes some of the controversies related to the timing and processes of the formation of Earth’s continental crust. An important questions is whether crustal growth process were different (non-uniformitarian) during the Archean. Many folk suggest that about half of the Earth’s continental crust was emplaced by the end of the Archean (2.5 billion years ago), citing isotopic values that imply the mantle was strongly depleted in incompatible elements at this time. High Nb/Th and Nb/U ratios of Archean komatiites further supports this hypothesis (Th and U are incompatible and concentrate in oceanic crust during partial melting of the mantle). Another question of consequence of this manner concerns the origin of tonalite-trondhjemite-granodiorite (TTG) intrusive suites, which may comprise up to 80% of remaining Archean crust. TTGs are characterized by strongly fractionated REE patterns, typically attributed to residual garnet and are often thought to be derived by melting oceanic crust as it subducts into the mantle. The Earth was much hotter in the Archean, and capable of this process, unlike the planet's later years. TTGs may not be exclusive to slab melting, however, as recent research of Greenland rocks by Nagel et al., 2013 supports an origin by melting thickened oceanic crust, on the upper plate of an island arc. Adam et al. (2013) reach a similar conclusion for Archean rocks in Canada. Key to the geochemical distinction between the two models is the evidence of water (bound in amphibole) in many TTGs, which would be present beneath an island arc crust, but absent in the dehydrated subducting slab. Upper plate melting is a process that continues today, and is consistent with uniformitarian models of crustal formation.

Figure 1 of Polat (2013) showing hydrous melting of mafic lower crust as the key producer of Archean TTGs, as opposed to dry melting of subducted oceanic crust. The image was modified after Davidson and Arculus, 2006)

Source: Polat, A., 2013, Growth of Archean continental crust in oceanic island arcs, Geology, v. 40, p. 383-384.

Nagel, T.J., Hoffmann, J.E., and Münker, C., 2012, Generation of Eoarchean tonalitic-trondhjemitic-granodioritics from thickened mafic arc crust: Geology, v. 40, p. 375–378, doi:10.1130/G32729.1.

Adam, J., Rushmer, T., O’Neil, J., and Francis, D., 2012, Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth’s early continental crust: Geology, v. 40, p. 363–366, doi:10.1130/G32623.1.

Davidson, J.P., and Arculus, R.J., 2006, The significance of Phanerozoic arc magmatism in generating continental crust, in Brown, M., and Rushmer, T.,eds., Evolution and Differentiation of the Continental Crust: New York, Cam- bridge University Press, p. 135–172.