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.

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.

Sierra Nevada uplift

geologic controversy - Rise of the Sierra Nevada

The Sierra Nevada range of eastern California is one the most famous ranges in the world, but the timing and processes that led to its elevated topography is a major question mark. Christopher Henry (2009) summarizes this controversy and explains that most researchers once considered Sierran uplift to have mostly occurred during the last 10 million years, but more recent studies argue for much earlier uplift in the Mesozoic. The issue is related to the proposed “Nevadoplano” of DeCelles (2004), who argued that the present day Great Basin region (mostly Nevada) was a high plateau during the Mesozoic, much like the modern day Altiplano of the central Andes. The Sierra Nevada region would have formed the flank of this great plateau. Mulch et al. (2006) and Cassel et al. (2009) supports an early rise model with stable isotope evidence that suggest the Sierra Nevada was near its current elevation by at least the Eocene and Oligocene. Major batholithic volcanism also occurred at this time and is consistent with thickened crust. In contrast, Ducea and Saleeby (1996) propose the Sierra Nevada was uplifted in the late Cenozoic following the removal of a dense eclogitic root; citing thermobarometry of mantle xenoliths. Both sides seem to have support look for more work to be done on this issue.

Figure 1 of Henry (2009) showing modern day topography overlain by flow direction of ancient rivers.

Sources

Henry, C. D., 2009, Uplift of the Sierra Nevada, California, Geology,

Cassel, E.J., Graham, S.A., and Chamberlain, C.P., 2009, Cenozoic tectonic and topographic evolution of the northern Sierra Nevada, California, through stable isotope paleoaltimetry in volcanic glass: Geology, v. 37, p. 547–550.   

DeCelles, P.G., 2004, Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S: American Journal of Science, v. 304, p. 105–168, doi: 10.2475/ ajs.304.2.105.

Ducea, M.N., and Saleeby, J.B., 1996, Buoyancy sources for a large, unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry: Journal of Geo- physical Research, v. 101, p. 8229–8244, doi: 10.1029/95JB03452.

Mulch, A., Graham, S.A., and Chamberlain, C.P., 2006, Hydrogen isotopes in Eocene river gravels and paleoelevation of the Sierra Nevada: Science, v. 313, p. 87–89.