Plinian Newberry Pumice and Related Eruptions of Newberry

Plinian Newberry Pumice and Related Eruptions of Newberry

Plinian Newberry Pumice and Related Eruptions of Newberry Volcano, Central Oregon, USA
V42B-0354
AGU 2003 Fall Meeting
Abstract
Estimation of Maximum Eruption Column Height
and Tropopause Wind Speed
The Newberry Pumice deposit forms a highly elliptical, eastward-extending lobe, and distal deposits are known
from as much as 550 km downwind in Idaho. The 25 cm isopach has an aspect ratio of 7.5:1. Pumice and lithic
isopleths also have a strongly elliptical pattern for smaller clasts. Larger clasts (e.g. lithic fragments greater
than about 10 cm in longest diameter) have a more nearly circular distribution. The maximum crosswind range
for a 3.2 cm lithic fragment is 7 km, and the maximum downwind range is 27 km. These data indicate a
maximum column height of about 25 km and a tropopause wind velocity far in excess of 30 m/s. The three
largest ballistic lithic fragments at a site on the south rim of Newberry caldera have 1.0, 1.12, and 1.3 m
median diameters. The maximum observed thickness of 7-8 m is known from the same locality. The vent
location is probably about 1.2 km from this location but could be as close as 0.5 km. Initial velocities of about
130 m/s and 95 m/s are needed for the measured blocks to reach distances of 1.2 and 0.5 km respectively.
Particle size and lithic content vary significantly with stratigraphic height. As many as seven ash layers may be
observed in the upper part of the deposit. In general, the upper part of the deposit has smaller clasts and a
greater lithic content. It also has a different distribution direction with an azimuth of 75-78o versus 82o for the
lower portion. This change in trajectory indicates that wind directions shifted during the eruption and/or that
lower altitude winds had a different direction than higher altitude winds.
7
325
Newberry Pumice data point (star) derived from the lithic isopleths is
plotted on a dispersal diagram of Carey and Sparks (1986).
ashes
ash
ash
Wa
xin
g
Wa
xin
g
225
200
Note the abundant large lithic blocks and stratification in
this thick, near-vent deposit at site 99-22
175
g
Wanin
Newberry Pumice on the south caldera rim
Note the irregular lower contact (dashed white line) and
the bedding in the lower half of the deposit. Bedding is
absent at site 99-1.
ning
Wa
250
5
9
ash
275
Location of Newberry Volcano and
Distal Tephra Sites
97-2
ash
4
Paulina Lake ashflow deposit at site 99-22
on the south rim of the caldera
surface ash
300
The Newberry Pumice was preceded by at least one small precursory eruption and followed by the Paulina
Lake ashflow and Big Obsidian flow. On the south rim of Newberry caldera, the Paulina Lake ashflow deposit
includes pyroclastic surge beds overlain by a nonstratified deposit of one or more pyroclastic flows. The basal
contact is irregular and suggests erosion. Eight radiocarbon dates for the ashflow deposit span 1,270+/-60 to
2,054+/-230 14C years BP. The mean limiting age is 1320+/-30 14C years BP (1,260+/-40 2σ calibrated age).
Nineteen dates on wood and charcoal found at or near the base of the Newberry Pumice span 1,510+/-80 to
2,210+/-140 14C years BP. The mean limiting age is 1,525+/-40 14C years BP (1425+/-95 2σ calibrated age). A
sequence of deposits at the south shore of Paulina Lake records at least two eruptions prior to the ashflow with
significant time elapsed between eruptions. Either could be the Newberry Pumice.
2
Variations in particle size and lithic content with
stratigraphic height at site 97-2
Stratigraphic Height (cm)
1
Stephen C. Kuehn
Department of Physics, Physical Sciences, and Geology, California State University, Stanislaus, 801 W. Monte Vista Avenue, Turlock, CA 95382, [email protected]
150
pumice
lithics
all
lithics
125
100
75
Waxin
g
50
25
Deposits beneath the Paulina Lake ashflow at site 99-1
10
0
0
20
40
60
% lithics by mass
in >0.5 mm (1 phi)
fraction
80 0
1
-1
2
-2
4
-3
8
-4
-5 0
16 32
Median Diameter
(phi / mm )
5
10
15
These tephra-bearing layers beneath the Paulina Lake ashflow
contain glass with a Big Obsidian eruptive period composition.
20
Maximum Clast Size
(cm)
Thickness variations in the upper (finer) and lower
(coarser) parts of the Newberry Pumice deposit
along a N-S transect ~10 km east of the vent
Newberry Pumice Thickness and Clast Size Distribution
8
3
Near-vent Thickness Distribution for the Newberry Pumice
6
Base map from MacLeod et. al. (1995). Thicknesses in cm.
site 99-1
x
References
Carey, S. and Sparks, R.J.S., 1986, Quantitative models of the fallout and dispersal of tephra from volcanic
eruption columns: Bulletin of Volcanology, v. 48, pp. 109-125.
x
site 97-2
Gardner, J.E, Carey, S., and Sigurdsson, H., 1998, Plinian eruptions at Glacier Peak and Newberry volcanoes,
United States; implications for volcanic hazards in the Cascade Range: Geological Society of America
Bulletin, v. 110, no. 2, p. 173-187.
Acknowledgments
site 99-22
Many people provided support, assistance, or data for work on the Holocene and Pleistocene tephras of
Newberry Volcano and/or provided useful feedback and suggestions. Included among them are Alan Busacca,
Paul Carrara, Larry Chitwood, Rick Conrey, Scotty Cornelius, Julie Donnelly-Nolan, Nick Foit, Bob Jensen,
Diane Johnson, Marvin Lanphere, Scott Linneman, Rob Negrini, Andrei Sarna-Wojcicki, and John Wolff.
Partial financial support for this project was provided by a Geological Society of America Research Grant, a
Sigma Xi Grant in Aid of Research, Washington State University College of Sciences Minigrants, Washington
State University Department of Geology Minigrants, and a Summer Graduate Research Assistantship from the
Washington State University Graduate School.
MacLeod, N.S. Sherrod, D.R., and Chitwood, L.A., and Jensen, R.A., 1995, Geologic map of Newberry volcano,
Deschutes, Kalamath, and Lake counties, Oregon: U.S. Geological Survey Miscellaneous Geologic
Investigations Map I-2455, scales 1:62,5000 and 1:24,000.
Mastin, L.G., 2001, A Simple Calculator of Ballistic Trajectories for Blocks Ejected during Volcanic Eruptions:
USGS Open-File Report 01-45, Software Version 1.1.
Kuehn, S.C., 2003, Plinian Newberry Pumice and Related Eruptions of Newberry Volcano, Central Oregon,
USA: Eos Transactions of the AGU, 84(46), Fall Meet. Suppl., Abstract V42B-0354.