( Blog for wesite at http://users.indigo.net.au/don/ )
Orogeny (-ic, -enesis). Pertaining to the building of mountains and the crustal forces responsible for that - these days embodied in the notion of colliding plates, and crustal crumpling according to Plate Tectonics.
Epeirogeny ( -ic, -enesis). Pertaining to Earth processes leading to continent-wide, vertical adjustments of the crust.
The concept of orogeny is easy. All that observable folding. The crust crumples, .. and obviously it crumples up, because there is plenty of space that way. (It's not so easy to crumple things down by the way, where there is none, yet down is the observable corollary of up in the crumpling scheme of things, and a point Plate Tectonics would rather ignore). And the concept was made easier because this crumpling happened in long linear belts. Obviously it was a consequence of convergent movement of crustal masses. No problem.
However, the concept of epeirogensis is not so easy. How does the crust subside on a global scale to accumulate stratigraphic sequence, and then become elevated to an extent (in the case of Tibet) of several thousands of metres? This question is not addressed in Plate Tectonics, which confines itself solely to what happens at continental margins.
The concept of epeirogenesis is virtually ignored in Plate Tectonics because the vertical processes that apply to the elevation of collisional plate margins to form zones of crustal crumpling ('mountains') apparently do not apply to the interiors of continents. Not according to Plate Tectonics anyway. The word enjoys only a single index entry in Seyfert's 1987 authoritative Encyclopedia of Structural Geology and Plate Tectonics, and refers in turn to a text-entry that only cites the word as an annex to orogensis and taphrogenesis, with no clarification as to meaning or origin. First-level entries on the web are likewise superficial. Alternative texts however show that the term was coined by Gilbert (1890) when considering the relationship of the (orogenic) folding of the Appalachians to their later (epeirogenic) block movements :-
" The displacements of the earth's crust which produce mountain ridges are called orogenic. For the broader displacements causing continents and plateaus, ocean beds and continental basins, our language affords no term of equal convenience. Having occasion to contrast the phenomena of the narrower geographic waves with those of the broader swells, I shall take the liberty to apply to the broader movements the adjective epeirogenic, founding the term on the Greek word 'epeiros' a continent. The process of mountain formation is orogeny, the process of continent formation is epeirogeny, and the two collectively are diastrophism. It may be that orogenic and epeirogenic forces and processes are one, but so long at least as both are unknown it is convenient to consider them separately.
A prescient qualification indeed, indicating a certain suspicion that both may be sourced in the same mechanism, but an uncertainty about how exactly to approach it within the limits of understanding of the day.
Ever since the terms were coined, orogenesis and epeirogenesis have been considered separately - and understandably so because researchers of the day had only comparatively limited access to Earth's crustal extents and could not instantly observe the regional continuities that we can today. However while distinction helped to focus on differences in geological expression, that same distinction has also straightjacketed investigation and consequently resulted in something of a schism in understanding the relationships of each to the other..
Today we can (like our space traveller) regard the Earth in its entirety in 'satellite view'. It is no longer necessary or appropriate to regard the two separately. When we look at the surface of the Earth as a whole and consider its relationship to spin, it is quite apparent that the two are indeed related (as Gilbert hinted) as one - in a scale-hierarchy that sees the relentless imperative of *flatness* as pre-eminent.
Gilbert's qualification had it right: the two are indeed linked, and, in the facts of stratigraphic sequence and continental structure that were mandatory learning for students of geology everywhere, those following knew initimately the limitations of the distinction, particularly through observation that the folds exposed in mountains and their topographic elevation were not directly linked, .. that mountains were *erosional* features that had only an *indirect* connection to so-called 'orogeny'.
Stille, writing in 1936,
" As a matter of fact, orogeny in the tectonic sense generally fails as an explanation for the existence of the topographically great mountains of the earth, such as the Alps of Europe or the Cordilleras of North America. These mountains exist - or still exist - as a result of post-orogenic en bloc movements, for the most part still going on, and belonging to the category of epeirogenic processes. Thus arises the terminologic contradiction, that the mountains as we see them today owe their origin not to what is called orogeny, but to an entirely different type of movement that is to be strongly contrasted with the orogenic process. Orogeny and topographic mountains are indirectly connected in the sense that the orogenic units, at least in the beginning, coincide with the units of epeirogeic movement. Thus the Alps are a unit not only morphologically, but also in relation to orogenic history. "
And not only the Alps and the North Ameritcan Cordilleras. Were he writing today he could have included the South American Cordilleras and the Himalyan extents as well.
"..Epeirogenesis, taphrogensis, and orogenesis", .. (the phrase including the word as it appears in the encyclopedia) . ... Epeirogenesis is linked to orogenesis through the dynamics of taphrogenesis - the horst-and-graben patterns of block faulting. The configuration of the faults marginal to the horsts however tell us that these blocks cannot be 'up-faulted' (because the wedge shape in cross-section precludes it), but are simply elevations that remain when the peripheral grabens collapse.
Similarly, epeirogenic movements evidence an overall *downwards* gravitational correction globally. Uplift is relative only, giving an appearance of uplift as adjacent blocks subside at different rates; sea level is the final arbiter of erosional control.
Although some sunken continental crust may make up some small parts of today's ocean floors the evidence is thus far otherwise:. So far as is known the ocean floors are everywhere young and comprised of basalt representing relict ridge-spreading and the top therefore of mantle breakthrough. Or to put it another way, the entirety of stratigraphic sequence of geological history that remains on continental crust implies a global run-off of those pre-Mesozoic ('epeiric') seas amounting to thousands of metres in the Tibetan region, dwarfing any calculated eustatic rise and fall of seas from melting or growing ice caps. The Tibetan plateau is precisely that, .. a once-flatness from which the mountains of the present day are being carved with a rapidity that precludes any drift of India from the spreading ridge being responsible for its 'uplift'.
Fig.2. Section across the Himalayas showing the essential flatness from which Tibet is carved and the +5,000m differential between the 'uplift' and Plate Tectonics' explanation of the reason for it, which is that India is thrust under Tibet.
The 'Roof of the World' is capped by the sediments of Cretaceous seas that were never pushed up by any colliding continents in any "dramatic, thrusting and contorting collision", .. not unless India was a wimp and flat on his back from the start, in which case there would have been no resistance to cause any contorting; India should be crumpled too, which it isn't.
Plate Tectonics has a *lot* of back-peddling to do. Even if theories must have, as Feynman says, holes (quote here), and all you need is a way to better define them instead of using "moogleflannel" for patches, they are still holes!
As with many words, 'epeirogeny' was not coined to describe a known geological process, it was a word explicitly coined to draw attention to a *lack* of knowledge, although usage since appears to have transmogrified into concealment rather than display. [A bit like the doctor informing you that your ailment is 'eye-itis'.]
And that is precisely why there is virtually no index entry for it in the enclopedia of PlateTectonics, and why the question is not addressed in the current geological milieu.
Yet prior to Plate Tectonics suchlike questions occupied the finest geological minds of the day. But no more. Just like mountains are taken for granted as being "tossed on high by the crumpling of crust", and transform faults are assumed to be somehow 'tramlines' of mantle flow as continents separate, stratigraphic sequence on the continental crust is simply and unquestioningly taken for granted. Plate Tectonics has no explanation for the continent-wide, vertical fluctuations of crust that led to the accumulation of tens of thousands of metres of stratigraphic sequence other than recognising that somehow it reflects inexplicable vertical crustal movement of continental interiors, which is precisely the criticism it levels at Earth expansion :: "No Mechanism". Isostacy and eustacy don't cut it.
So there we have it. Both Plate Tectonics and Earth expansion recognise this "lack of mechanism" for epeirogensis but only expansion sees it as a question central to geological understanding and worthy of an explanation. And a first-order one too. Plate Tectonics sees it all right but ignores it, attributing all crustal dynamics to horizontal movements stemming from convection. Earth expansion on the other hand sees this 'epeirogeny' as pre-eminent, and lateral dynamics as a necessary consequence, and seeks an explanation for it, recognising it as central to realising that the Earth could never have *had* a mantle of any consequence in Archaean times, and that the relationship of the core to the crust must have been substantially different then from what it is now.
One for the meme-machine? - the gossip rumour-mill, .. that the Earth's magnetic field is actively implicated in the *creation* of the mantle, not just some signal-noise written on some incidental passive receptor like the rocks in my back yard. Surely one indeed to capture the 'everybody-knows-that already' imagination (like 'soup-in-a-pot' for convection and 'crumpling tablecloths' for mountains) - that light has an electromagnetic expression, that energy can be converted into mass, and that we are everywhere surrounded by vacuum energy.
(But through the intensifying lens of what, exactly?). (Moogles? )
Not saying in a direct *causative* way of course, but *related*. Like spin and gravity seem to be related in this embodiment we call the Earth. The gravity Field? The electro-magnetic field? The Higgs field? The vacuum energy field? How many moogle-fields do we need to make this science work? And what sort of field should we be looking at to understand the creation of this Earth and its position in the universe? .. The flotation, subduction, and Archimedes-wet-leg Field ?
The Creator and Knowledge, ..
and the Big Bird born in the Fire.
Phoenix, rising from the ashes of
"Winds of change" indeed. We stand on the threshold, ..etc etc.. If a whole lot of people would just pull the finger out and *observe-and-think* (in that monitoring-ish sort of way) instead of *theorise-and-believe* (in that ringmasterish sort of way), we might get some help in this business of a *theory* for Earth expansion. As a fact it's a lay-down misere.
[Anyway, "This show is not over till the Fat Lady sings." ]
[ See also - Debunking Plate Tectonics - at :-