FlorisssantLahars

	TOURING COLORADO GEOLOGY
	Florissant Lahars	Back to Roadcuts
1		Along the Teller County 1 road south of Florissant and the Florissant Fossil Beds. Latitude: 38º 49' 7" Longitude: 105º 15' 31" Elevation: 8160'
2		Lahars are dirty, wet and often hot mudflows associated with volcanoes. The word is Indonesian in origin What is a lahar and how we can recognize one? A lahar is a volcanic debris flow or a rapid flowing mixture of mostly volcanic rock and water originating on the slopes of a volcano. Although most vulcanologists would prefer that it be caused by a volcanic eruption, it is not always possible to be sure that it was or whether it was caused by remobilization of volcanic materials long after an eruption. For modern lahars, it is fairly easy to make the distinction; for ancient deposits, it is not. Lahars are not all that different from pyroclastic flows (or nuées ardentes). Both are associated with volcanoes and are composed primarily of volcanic material. The difference is in the delivery method. The material carried with pyroclastic flows is supported by air as opposed to lahars where it is carried by water. So how do you tell the difference? Photo 1 is that of a lahar about 6.5 miles south of the Florissant Fossil Beds Visitor Center. As you can see, it is made up of a variety of different size clasts, most volcanic, some not.
3		Photo 2, 3 and 4 are more detailed pictures of the lahar. Although not definitive, pyroclastic flows tend to show stratification while lahars do not. Although you tend to envision pyroclastic flows as very violent and chaotic flows, they are, with the exception of the very bottom layer, very planar in their flow characteristics, oftentimes preserving stratigraphic relationships as they move hundreds or every thousands of meters. Lahars, however, are like avalanche deposits, unsorted, containing fragments from fine mud to boulders as large as houses. There is usually no trace of bedding except perhaps near the bottom of the flow and more rarely at the very top (Francis, 1998). As you can see in the above photos, the distribution of clasts is chaotic. So, lahars are terrifying, destructive acts of Nature! Something to be feared, avoided, and that we should never want to happen. This is true. Modern lahars have destroyed whole towns and killed thousands of people. They have caused some of the most destructive modern catastrophes. However, we owe a great deal to the lahars around Florissant. Thanks to them we have the Florissant fossil beds.
4		About 35 million years ago, the area around Florissant was covered by a beautiful forest of gigantic sequoias, much like those seen today in California. These large trees grew to tremendous heights towering over everything around them and providing shelter for wide and varied forms of life. Suddenly, to the west, the Guffey volcano erupted ejecting lava and ash onto the surface and more significantly for the Florissant area, initiated a very large lahar that flowed down Four Mile Creek to Evergreen Station then north up Twin Creek burying everything in its path (Simmons, 2001). The sequoias being so much higher than the mudflows only had their lower parts buried, but it was enough to eventually kill them off. The unprotected portions eventually rotted away, but the lower 15 feet or so of the trees buried by the lahar were preserved.
5		Perhaps fairly soon after, perhaps longer, another eruption occurred or perhaps heavy rains caused earlier deposited volcanic materials to again flow. This lahar (see photo above) blocked off the stream flowing through this valley and a lake was formed. For a time, the valley returned to a fairly normal state. It was populated by insects, birds, fish, and all manner of plants and animals. Occasionally the volcanoes to the west erupted, dumping small amounts of ash on the area. Plants and animals were caught up in this and buried on the bottom of the lake. But this wasn’t the last of the lahars. At least once more, the valley would be buried by one of these mudflows covering over the lake deposits sealing off the more fragile fossil-bearing shales helping to preserve them for 35 million years so that we could eventually find them and marvel at the fossils preserved therein.