Pyroclastic Flow explained in the Weminuche Wilderness - CATASTROPHIC GEOLOGY LESSON

Описание: The San Juan volcanic field is part of a much larger volcanic region that was active throughout the Southern Rocky Mountains from about 40 million to 18 million years ago. Most eruptive centers lie within the region bounded by the Rocky Mountains on the east, the Colorado lineament on the north, and the Uncompahgre -San Luis Uplift on the south. The volcanic activity within this triangular area produced lavas and pyroclastic debris that covered all of south-central Colorado and the north-central part of New Mexico. The eruptions that formed this middle Tertiary volcanic field began with the formation of stratovolcanoes made of alternating layers of lava and ash of andesitic composition. Geophysical evidence suggests that at this time one or more batholiths intruded to shallow levels in the crust. Early in the history of this intrusive activity, lava and ash were generated at deeper levels from magmas ofthe intrusion. The rocks formed from these first eruptions blanketed a late Eocene erosion surface oflow relief (Epis and Chapin 1975). As eruptions continued, the volcanic aprons enlarged, forming a continuous cover ofvolcanic rocks. Because ofextensive erosion, the exact size ofthis volcanic cover cannot be determined. However, the lavas and ash of these early eruptions still account for about two-thirds ofthe volcanic rocks in the San Juan Mountains (Steven 1975). Distribution of volcanic rocks in the Southem Rocky Mountains. As the magma chamber enlarged and continued its relentless push toward the surface, the volcanic eruptions in the San Juans area became more violent. The near-surface magma supported the overlying roof and volcano. Once the magma breached the surface , the roof rapidly collapsed, forming a cylindrical pit, or caldera, 5 to 15 kilometers in diameter. The vented magma produced ash-flow sheets; fifteen have been identified in the San Juans , according to Steven and Lipman (1976). Each eruption was many times more powerful than the May 18, 1980, eruption of Mount St. Helens, and each caldera produced characteristic ash flows. Fifteen calderas have been identified, and two others are postulated by U.S. Geological Survey geologists.

Pyroclastic rocks or pyroclastics (derived from the Greek: πῦρ, meaning fire; and κλαστός, meaning broken) are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic. Commonly associated with unsieved volcanic activity—such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions—pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock.
Pyroclastic rocks may be a range of clast sizes, from the largest agglomerates, to very fine ashes and tuffs. Pyroclasts of different sizes are classified as volcanic bombs, lapilli, and volcanic ash. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas-and-ash mix of a pyroclastic flow event.

A pyroclastic flow (also known scientifically as a pyroclastic density current is a fast-moving current of hot gas and volcanic matter (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gases can reach temperatures of about 1,000 °C (1,830 °F). Pyroclastic flows normally touch the ground and hurtle downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. They are a common and devastating result of certain explosive eruptions.


http://geology.wikia.com/wiki/Pyrocla...

https://en.wikipedia.org/wiki/Pyrocla...

https://en.wikipedia.org/wiki/Pyrocla...

Stratigraphic Problems of Pyroclastic Rocks:
https://link.springer.com/chapter/10....

La Garita Caldera:
https://en.wikipedia.org/wiki/La_Gari...

Basalt flows and associated tuff, breccia, and conglomerate of late-volcanic bimodal suite:
https://mrdata.usgs.gov/geology/state...