[Widdekind]

Steep-sided Strato-Volcanoes are caused by thick, sticky, viscous lavas, of comparatively low temperatures, and being rich in gases and Silica*. These lavas are generally Granitic (Felsic), not Basaltic (Mafic)**.

* History Channel How the Earth was Made -- Krakatoa (TV)
** http://en.wikipedia.org/wiki/Stratovolcano

Conversely, Earth's largest volcano, the Hawaiian Shield-Volcano Mauna Loa, vents out very fluid lavas, which are Silica-poor*. Their gas contents are low, but their vent-temperatures are high**. These lavas are exclusively Basaltic***. They come from deep Mantle Magmas, from "thousands of miles below the surface", close to the Core#.

* http://en.wikipedia.org/wiki/Mauna_Loa
** http://en.wikipedia.org/wiki/Hawaiian_eruption
*** http://en.wikipedia.org/wiki/Hawaii_hotspot
# National Geographic Channel Naked Science -- Birth of the Oceans (TV)

CONCLUSION (?): Billions of years ago, the early Earth's interior was much hotter, than it is today. Thus, it's Magmas were much more like those supplying the Hawaii Hotspot, being high-temperature Basalts. Such fluid lavas formed shallow-sided Shield Volcanoes, not steep-sided Strato-Volcanoes. This suggests that, somehow, sticky, thick, and comparatively cold but gas-rich Grantic lavas came about by the cooling of the planet*. Perhaps this stems from the Chemical Separation of the planet (?).

* Granitic lavas first formed modern-composition Continental Crusts about 2 billion years ago, primarily Basaltic (Oceanic) lavas being produced before that time. See: http://www.geosciences-forum.com/htopic587-.html

Moreover, as the Earth has cooled, its Mantle has congealed, blocking the up-wellings of most deep Mantle Magmas, and restricting those up-wellings to but a handful of Hotspots scattered around the planet. Presumably, as the planet cools down further, even those Hotspots will become blocked.

[Geosciences-Forum.com]

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