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Abstract

Gravity data were investigated to reveal the presence of subvolcanic intrusions. With few exceptions, these intrusions produce a detectable gravity anomaly. In the past, these gravity anomalies have often been overlooked or misinterpreted because the data reduction procedure was inadequate. A pragmatic method for reducing and interpreting reconnaissance gravity data from volcanoes as well as gravity models of a variety of volcanoes is developed.

Large calderas (diameters greater than 15 km) have relatively low-density intrusions beneath them. All other large volcanic systems that would include small calderas (diameters less than 15 km) have relatively high-density intrusions beneath them. The density contrasts that produce the observed anomalies occur between the intrusion, whose density is usually greater than 2.6 g/cm3, and the country rock. Commonly, the shallow country rock is an older volcanic layer with a density less than 2.5 g/cm3. The result of the contrast is a positive anomaly over the intrusion. For larger calderas, the surrounding volcanic layer is usually thin and overlies dense metamorphic and plutonic country rocks. In this case, we find the intrusion commonly less dense than country rock. The result is a negative anomaly.

In modeling volcanoes of the Cascade Range, gravity data and geologic considerations required a bottom on the intrusion. This may be an actual bottom or the depth at which the density contrast between the intrusion and the country rock disappears. The tops of the intrusions are usually shallow and are significantly wider than overlying craters or calderas. Calderas are associated with wider intrusions. Some intrusions are single cooling units, but more commonly they are an accumulation of the unerupted portions of individual magmatic injections. These injections could occur periodically throughout the life of the volcano, and would generally be accompanied by eruption. Comparing the volume of the intrusion and the volume of the volcanic edifice indicates that only a small part of a magma injection erupts, although some of the apparent intrusive material may be reworked older volcan-ics. Exceptions to the general discussion presented tend to be related to the nature of the country rock.

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