This document is a descriptive version of the metadata for the digital Geologic Map of the State of Hawai'i. It is intended to be more user friendly than the conventional metadata text document (filename: Hawaii_metadata_20070514.txt). Key data herein are presented in tabular format. Ideally a GISoriented user would glance through this text, print a few tables or open them on an adjacent computer screen, and then begin analyzing the data contained in the GIS databases. A few readers may find this document helpful for its explanation of decisions made while assembling data for the geologic map.

Quaternary (2-0 Ma) extrusion rates change significantly along the Cascade Range volcanic arc. The extrusion rate north of Mount Rainier is about 0.21 km 3 km -1 m.y. -l-the rate in southern Washington and northern Oregon south to Mount Hood is about 1.6 km 3 km-! m.y.-l; in central Oregon the rate is 3-6 km 3 km -1 m.y.-•; and in northern California, the rate is 3.2 km 3 km -• m.y.-•. Eruption style also changes along the arc but at latitudes different from rate changes. At the ends of the arc, volcanism is focused at isolated intermediate to silicic composite volcanoes. The composite volcanoes represent ---30• of the total volume of the arc. MMic volcanic fields partly ring some composite volcanoes, especially in the south. In contrast, volcanism is diffused in the middle of the arc, where numerous overlapping marie shields and a few composite volcanoes have built a broad ridge. Contrasting eruption style may signify diffuse versus focused heat sources or may reflect changes in permeability to ascending magma along the arc. INTRODUCTION GEOLOGIC SETTING We report here estimates of volume, timing, and extrusion rate for the Cascade Range volcanic arc, using new isotopic ages and revised geologic mapping completed since 1980. The rate calculations are based on volumes measured while compiling 1:250,000 and 1:500,000 scale maps of the Cenozoic volcanic rocks of the Cascade Range in Washington, Oregon, and California [Smith, 1987, 1990a, b; Sherrod, 1987; Sherrod and Smith, 1989], whereas previous estimates [e.g., McBirney, 1976; White and McBirney, 1978; McBirney and White, 1982] were based on much less complete data. In comparing different parts of the volcanic arc from southern British Columbia to California, we find that extrusion rates change abruptly along the arc, diminishing by an order of magnitude north of Mount Rainier across a boundary separating extensive seismicity to the north and scant seismicity to the south [Weaver and Michaelson, 1985]. In the following discussion, Quaternary is the time from 2 Ma to the present [Harland et al., 1982]. Extrusion rates are normalized to cubic kilometers per kilometer of arc length per million years (km3 km-l m.y. -l). Except for lowdensity air fall tephra, volumes of different eruptive products are not adjusted to dense-rock-equivalent (DRE) magma.Volumes for some volcanoes are published. For others, volumes were estimated by conventional methods, either by subdividing the volcanoes into regular geometric solids or by measuring areas at successive elevations with a planimeter [Church, 1981]. Thus the calculated extrusion rates are only approximate, and the error probably ranges from -+ 10% at well-studied volcanoes to +-30% at the poorly known centers. However, the rates differ by close to an order of magnitude for different arc segments, so the conclusions are not likely to change significantly as more detailed mapping and geochronology and refined volume calculations become available.

At Newberry Volcano, central Oregon, more than 0.5 m.y. of magmatic activity, including caldera collapse and renewed caldera‐filling volcanism, has created a structural and thermal chimney that channels magma ascent. Holocene rhyolitic eruptions (1) have been confined mainly within the caldera in an area 5 km in diameter, (2) have been very similar in chemical composition, phenocryst mineralogy, and eruptive style, and (3) have occurred as recently as 1300 years ago, with repose periods of 2000–3000 years between eruptions. Holocene basaltic andesite eruptions are widespread on the flanks but are excluded from the area of rhyolitic volcanism. Basaltic andesite in fissures at the edge of the rhyolite area has silicic inclusions and shows mixed basalt‐rhyolite magma relations. These geologic relations and the high geothermal gradient that characterizes the lower part of a drill hole in the caldera (U.S. Geological Survey Newberry 2) indicate that a rhyolitic magma chamber has existed beneath the caldera throughout the Holocene. Its longevity probably is a result of intermittent underplating by basaltic magma.

The central Oregon section of the Cascade Range volcanic arc is characterized by relatively high Quaternary volcanic extrusion rates and hot‐spring discharge rates, and by high conductive heat flow. However, a large area of near‐ zero near‐surface conductive heat flow occurs in the younger volcanic rocks, due to downward and lateral flow of cold groundwater. Alternate models for the high heat flow observed in older rocks on the flanks of the Cascade Range involve (1) a laterally extensive midcrustal heat source or (2) a narrower, spottier deep heat source that is confined to the Quaternary arc and is flanked by relatively shallow conductive heat flow anomalies caused by regional groundwater flow. We simulated groundwater flow and heat transport through two cross sections west of the Cascade Range crest: one in the Breitenbush area, where there is no major arc‐parallel normal faulting, and one in the McKenzie River drainage, where major graben‐bounding faults exist. Measured temperature profiles, hot‐spring discharge rates, and geochemical inferences constrain the results. The numerical simulations provide some estimates of regional‐scale permeabilities; simulated bulk permeabilities of ∼10−14 m2 in the youngest (0–2.3 Ma) rocks and ∼10−17 m2 in the oldest (18–25 Ma) rocks allow the thermal observations to be matched. In general, permeability decreases downsection, but for rocks of any age, permeability at very shallow (<50 m) depths is probably much higher than the bulk permeability values required by the thermal observations: this is indicated by high recharge rates in 0–7 Ma rocks (>1 m yr−1) and well‐test data from domestic wells in rocks older than 7 Ma (which indicate permeability values of about 10−14 to 10−12 m2). In the simulations, the alternate conceptual models for the deep thermal structure were represented as wide or localized deep heat sources. We found that either model can satisfy the observations. Thermal observations in the Breitenbush area seem to require significant advective heat transfer, whereas the sparser observations in the McKenzie River area can be satisfied with either advection‐ or conduction‐dominated simulations. Available regional gravity, magnetic, and electrical geophysical data do not clearly favor either of the two alternate models. Deep drilling in areas of high heat flow in the older rocks would be the most definitive test. The actual thermal structure is probably more complex than either of the models considered here.

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