The most recent of these occurred approximately 600,000 years 

 ago. Following a period of substantial uplift of the Yellowstone 

 plateau, more than 900 km 3 of rhyolitic pumice and ash erupted 

 resulting in the collapse of a 75 km by 45 km area and the 

 formation of the Yellowstone Caldera (Eaton et al. 1975). The 

 magnitude of this explosion was immense. By way of 

 comparison, the famous Krakatoa eruption of 1883 created a 

 caldera less than 7 km across, yet sent enough dust into the 

 atmosphere to have a measurable effect on global climate. 

 Immediately following this collapse, resurgence within the magma 

 chamber uplifted the floor of the caldera and formed two 

 resurgent domes within the caldera boundary (Fig. 2). Today, 

 most of the thermal features within the Park are clustered around 

 the rim of this caldera as surface and ground waters circulate 

 along systems of deep fractures at the caldera rim and at the 

 intersection of tectonic faults with the rim fractures (Smith and 

 Christiansen 1980). Yellowstone is one of the most seismically 

 active regions of the world, with micro-earthquakes recorded 

 daily (W. Hamilton, pers. comm.). 



Yellowstone Lake is situated largely within the caldera, 

 although the South and Southeast Arms lie outside it (Fig. 2). 

 The lake itself is of glacial origin. The region was covered by 

 ice during the last glaciation ~ 15,000 years ago. The 

 hydrologic outflow of the lake at Le Hardy Rapids on the 

 Yellowstone river flows directly across one of the two resurgent 

 domes within the caldera. Active doming has been raising this 

 outfall nearly 2.5 cm per year since the 1920 's when the first 

 elevations were taken, and short term doming/subsidence events 

 have raised this region as much as 20 cm in 3 to 4 weeks 

 (Hamilton 1987). 



The limnological history of the lake is not well known. 

 Studies by Brian Shero on the diverse diatom record in sediment 

 cores representing the last 1500 years (Shero and Parker 1976) 

 led him to hypothesize that the productivity of the lake was 

 decreasing with time. Both diatom abundance and sedimentation 

 were lower in recent sediments. More detailed studies of 

 sediments deposited in the last 200 years, however, show a more 

 complicated picture (Kaster et al . 1987, Klump et al . 1987). 

 While sediment accumulation rates determined via Pb-210 dating 

 have remained nearly constant, the deposition of biogenic silica 

 has varied by as much as 30% over the last 100 to 150 years 

 (Fig. 3). A variety of factors have been hypothesized as 

 having a role in controlling the productivity of Yellowstone and 

 other lakes in the region. These include nutrient input from 

 forest fires, climate, ecosystem (food web) dynamics, and 

 anthropogenic influences. Given the geological setting of the 

 lake, however, and the potential impact of ground water inputs, 

 particularly geothermally heated and reacted ground waters, we 

 believe that fluctuations in hydrothermal inputs to the lake may 

 be important to the biology and chemistry of the system. 



