Stratigraphy of the Hawai‘i Scientific Drilling Project core
(HSDP2): Anatomy of a Hawaiian shield volcano
Michael O. Garcia and Eric H. Haskins
Department of Geology and Geophysics, University of Hawai‘i, 2525 Correa Road, Honolulu, Hawaii 96822, USA
(mogarcia@hawaii.edu)
Edward M. Stolper and Michael Baker
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
[
1
]
The Hawai‘i Scientific Drilling Project (HSDP2) successfully drilled
3.1 km into the island of
Hawai‘i. Drilling started on Mauna Loa volcano, drilling 247 m of subaerial lavas before encountering 832 m
of subaerial Mauna Kea lavas, followed by 2019 m of submarine Mauna Kea volcanic and sedimentary
units. The 2.85 km stratigraphic record of Mauna Kea volcano spans back to
650 ka. Mauna Kea
subaerial lavas have high average olivine contents (13 vol.%) and low average vesicle abundances
(10 vol.%). Most subaerial Mauna Kea flows are ‘a‘
a
(
63%), whereas the Mauna Loa section contains
nearly equal amounts of p
a
hoehoe and ‘a‘
a
(like its current surface). The submarine Mauna Kea section
contains an upper,
900 m thick, hyaloclastite-rich section and a lower,
1100 m thick, pillow-lava-
dominated section. These results support a model that Hawaiian volcanoes are built on a pedestal of pillow
lavas capped by rapidly quenched, fragmented lava debris. The HSDP2 section is compared here to a 1.7 km
deep hole (SOH1) on Kilauea’s lower east rift zone. Differences in the sections reflect the proximity to
source vents and the lower magma supply to Kilauea’s rift zone. Both drill core sections are cut by
intrusions, but the higher abundance of intrusions in SOH1 reflects its location within a rift zone, causing
more extensive alteration in the SOH1 core. The HSDP2 site recovered a relatively unaltered core well
suited for geochemical analyses of the single deepest and most complete borehole ever drilled through a
Hawaiian or any other oceanic island volcano.
Components:
21,448 words, 20 figures, 5 tables
.
Keywords:
stratigraphy; core logging; petrography; scientific drilling; basalt; Hawaii.
Index Terms:
8486 Volcanology: Field relationships (1090, 3690); 3615 Mineralogy and Petrology: Intra-plate processes
(1033, 8415); 3625 Mineralogy and Petrology: Petrography, microstructures, and textures; 3641 Mineralogy and Petrology:
Extrusive structures and rocks.
Received
6 June 2006;
Revised
20 October 2006;
Accepted
6 November 2006;
Published
28 February 2007.
Garcia, M. O., E. H. Haskins, E. M. Stolper, and M. Baker (2007), Stratigraphy of the Hawai‘i Scientific Drilling Project core
(HSDP2): Anatomy of a Hawaiian shield volcano,
Geochem. Geophys. Geosyst.
,
8
, Q02G20, doi:10.1029/2006GC001379.
————————————
Theme:
Hawaii Scientific Drilling Project
Guest Editors:
Don DePaolo, Ed Stolper, and Don Thomas
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3
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Geochemistry
Geophysics
Geosystems
Published by AGU and the Geochemical Society
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Geochemistry
Geophysics
Geosystems
Article
Volume 8
,Number2
28 February 2007
Q02G20, doi:10.1029/2006GC001379
ISSN: 1525-2027
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1. Introduction
[
2
] Although Hawaiian shields are the most studied
volcanoes on earth [e.g.,
Tilling and Dvorak
, 1993],
their interiors are poorly sampled and not well
understood. This is a consequence of their rapid
growth and subsidence [
Lipman
, 1995;
Moore and
Chadwick
, 1995], as well as limited subaerial ero-
sion and faulting, which expose only a relatively
thin veneer of the volcano (<5% [e.g.,
Garcia et al.
,
1995]). The Hawai‘i Scientific Drilling Project
(HSDP) was designed to address this problem.
The goal of this project was to penetrate the deep
interior of a relatively young shield volcano, avoid-
ing rift zones to minimize the effects of alteration
[
DePaolo et al.
, 2001]. Mauna Kea volcano (last
eruption
4ka[
Wolfe et al.
, 1997]) was selected as
the target for this study (Figure 1). The project’s
feasibility was first evaluated on the basis of a ‘‘pilot
hole’’ (HSDP1) drilled in 1993 along the coast of the
island of Hawai‘i in the city of Hilo (Figure 1). The
HSDP1 drilling achieved
90% core recovery with
280 m of Mauna Loa lavas and sediments overly-
ing
776 m of Mauna Kea lavas [
DePaolo et al.
,
1996]. The success of HSDP1 led to the drilling of
HSDP2, which began 15 March 1999 at a site
2km
south of the HSDP1 drill site (Figure 1). When
drilling operations halted on 23 September 1999,
the final penetration depth was 3098 meters below
sea level (mbsl), with
95% core recovery.
[
3
] This paper describes the downhole stratigraphy
for the HSDP2 borehole, based on data compiled
from on-site logging of the core, supplemented
with examination of 265 thin sections. We provide
the primary geological, volcanological, and petro-
graphic context for many geochemical studies of
the HSDP2 core. In addition, we compare the
stratigraphy of the core with new stratigraphic
and petrographic data for the Hawai‘i Scientific
Observation Hole 1 (SOH1) drilled into K
lauea’s
east rift zone (KERZ). This comparison of two
cored sequences of geochemically similar rocks
allows us to evaluate the effects of distance from
vent and emplacement conditions on the proportion
of rock types (e.g., flows versus fragmental rocks)
and their petrographic characteristics (e.g., vesicu-
larity and phenocryst content). Finally, we present
a model for the submarine growth of the flanks of
Hawaiian volcanoes.
2. Geologic Setting and Site Selection
[
4
] The HSDP2 drill site is located in a disused
rock quarry on industrial land adjacent to the Hilo
International Airport. The HSDP1 site was regarded
as unsuitable for the lengthier site occupation
required for the second phase of drilling. The HSDP2
site minimized environmental and community
impacts and provided ready access to the services
of the greater Hilo area. The site is also far from the
summits and rift zones of adjacent volcanoes (e.g.,
14 km south of the east rift zone of Mauna Kea,
28 km north of K
lauea’s east rift zone, and 45 km
ESE of the summit of Mauna Kea; Figure 1). Thus
the chances of encountering intrusions and high-
temperature fluids were minimized, enhancing the
likelihood of obtaining the freshest possible rocks
for geochemical and paleomagnetic studies. The
site was also selected to maximize the time interval
sampled as a function of drilled depth (this was
achieved by selecting a site distant from Mauna
Kea’s summit). A bonus from drilling at the chosen
site was the recovery of a
247 m thick section of
Mauna Loa flows.
3. Methods
3.1. Hybrid Coring System
[
5
] An important innovation of the HSDP2 was the
construction and utilization of a hybrid coring
system, which allowed seamless integration of
wireline coring and rotary drilling capabilities. This
system (owned by the Drilling, Observation and
Sampling of the Earth’s Continental Crust Corpo-
ration, DOSECC) is a top-driven, wireline coring
system that attaches to a host rotary rig for contin-
uous coring to depths of
6000 m. During coring
operations, standard wireline coring rods were used
with a cylindrical, thin kerf diamond bit that drilled
a
10 cm hole and recovered core with a diameter
of
6.4 cm. The core barrel was retrieved by a
wireline winch, allowing the rods and coring
assembly to remain at the bottom of the hole.
When drilling operations required removing pipe,
widening the hole, setting casing, or cementing, the
hybrid the coring system was set aside allowing
use of the rig’s rotary capabilities.
3.2. Drilling
[
6
] The HSDP2 ended at a depth of 3098 meters
below sea level (mbsl), exceeding its depth objec-
tive (2440 mbsl) by more than 25% [
DePaolo et
al.
, 2001]. The average coring penetration rate in
the subaerial section (1.8 m above sea level to
1079 mbsl) was >48 m/day, 60% faster than for
HSDP1. The higher coring rate was probably a
result of using the hybrid coring system for HSDP2
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drilling. The coring penetration rates (
25 m/day),
bit longevity, and core recovery were slower in the
upper part of the submarine section due to the
poorly consolidated character of the volcaniclastic
sediments that dominate this part of the submarine
section (Figure 2). The low penetration rates and
poor recovery in the upper submarine section led to
the decision to drill with a tricone rotary bit
through two intervals of this poorly consolidated
material (i.e., 1140–1223 mbsl and 1243–1260 mbsl;
Figure 3). No core or cuttings were recovered in
these two intervals. Core recovery and bit life
improved in the more consolidated hyaloclastite-
dominated section (>1500 mbsl) to an average of
25 m/day until the first occurrence of pillow lavas
at 1984 mbsl (Figure 3). The highly fractured nature
of most of these pillow lavas, along with the
increasing times required for core recovery and bit
replacement at progressively greater depths, reduced
the average penetration rate to
20 m/day below
1984 mbsl. At several depth intervals, the borehole
was widened and straightened using a tricone rotary
bit, and casing strings of decreasing diameter were
inserted for hole preservation (Figure 2). The lower
section of the borehole was cased to 2998 mbsl in
2003.
[
7
] An unexpected result of the HSDP2 drilling
was the discovery of significant artesian ground-
water. The shallowest artesian flow was from a
fresh water aquifer at 100–600 mbsl (in the
region of the Mauna Loa to Mauna Kea transi-
tion), which produced
7300 L/min. This aquifer
may indicate that there are additional sources of
fresh, artesian groundwater elsewhere in Hawai‘i
near the interfaces between volcanoes. Artesian salt-
water aquifers were discovered below
2000 mbsl
within the sequence dominated by pillow lavas.
These aquifers strongly influence temperatures in
the drill hole. Temperatures in the shallow portion of
the HSDP2 hole decrease with depth to
8
°
Cnear
600 mbsl (Figure 2). A similar decrease in HSDP1
was related to seawater infiltration [
Thomas et al.
,
1996]. In the shallow submarine HSDP2 section,
temperatures are nearly isothermal (Figure 2), prob-
ably due to seawater circulation through the highly
permeable, poorly consolidated hyaloclastite units.
Below
1800 m, the temperature gradient is
20
°
C/km with a maximum temperature of 45
°
C
Figure 1.
Topographic and bathymetric map of the island of Hawai‘i, with elevations and depths in meters. The five
volcanoes that make up the island (Ko, Kohala; MK, Mauna Kea; Hu, Hual
a
lai; ML, Mauna Loa; Ki, K
lauea) are
separated by gray lines; a black arrow points to the offshore Loihi seamount (Lo). The sites where drill cores
discussed in this paper were collected (HSDP2, HSDP1, SOH1, and SOH4) are marked by red dots and labeled
according to the year drilling was completed and total depth. The end of shield-stage Mauna Kea shoreline is defined
by a marked break in slope [
Wolfe et al.
, 1997] and is shown offshore by a heavy black curve. The dashed line shows
the extrapolated shoreline beneath the younger Mauna Loa surface lavas near the HSDP sites.
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at the bottom of the hole (Figure 2). These results
indicate that site selection successfully avoided
high-temperature, hydrothermal conditions.
3.3. Summary of Core Handling Procedures
[
8
] HSDP2 core drilling was conducted 24 hours a
day. Rock core was removed from the core barrel
by the drilling crew and placed in sequentially
numbered,
1.8 m long, oriented PVC half cylin-
ders (with a series of
5 mm diameter water
drainage holes). After being filled with core, these
cylinders were fitted with foam end blocks and
PVC top covers for transport across the drill site to
the processing area. Handling of the core, includ-
ing washing, marking, boxing, longitudinal slab-
bing (to create a 1/3 archive and 2/3 working split
of the core), drying, scanning, and box photogra-
phy were all done within a few days of recovery
according to procedures developed for this project
[
Hawaii Scientific Drilling Project
, 2000].
[
9
] Core logging was done only on the working
split. Several steps were taken to ensure the quality
and consistency of the logging. First, all data were
entered directly into the Drilling Information System
computer database (maintained by the International
Continental Drilling Project at Potsdam, Germany)
using standardized logging forms. The log report
for each box of core consists of three parts: an
annotated digital photograph of the box, a descrip-
tion of the core prepared using the standardized
logging forms, and point count information on rock
mineralogy and vesicularity. Core logging was
Figure 2.
Downhole summary of the HSDP2: (a) generalized stratigraphy with major rock units, (b) temperature
profile with areas of water intrusion, and (c) hole design with casing sizes and depths. Modified after
DePaolo et al.
[2001].
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done by the core logging staff supplemented by
short-term volunteers. To maintain consistency and
accuracy, their work was reviewed and edited first
by M. Garcia and later by E. Stolper. These logs
and additional news concerning the drilling project
are available online at http://www. icdp-online.de/
contenido/icdp/front_content.php?idcat = 714. The
core is currently stored, and available for inspec-
tion and sampling at the American Museum of
Natural History in New York City.
[
10
] The core was divided into units on the basis of
observed contacts and/or variations in features such
Figure 3.
Summary of core logging observations and volatile glass data from the HSDP2 core hole: (a) relative
proportions of rock types versus depth in 250 m increments; (b) abundances of phenocrysts and vesicles versus depth;
and (c) water (purple symbols) and S contents (yellow symbols) of HSDP2 glasses versus depth; volatile contents are
bimodal, with the lower values indicative of degassing under subaerial conditions and the higher values indicative of
submarine eruption [
Seaman et al.
, 2004]. Note the gaps of 17 and 83 m near the top of the submarine section from
tricone rotary drilling in poorly consolidated hyaloclastite. These are the only sections of the hole that were not
continuously cored. Modified after
DePaolo et al.
[2001].
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as mineralogy, structure, lithology, etc. A conser-
vative approach was taken in delineating new units.
Thus sequences of lithologically similar flows not
clearly separated by distinct contacts were com-
bined into single units. These sequences of flows
are similar to the many overlapping flows being
deposited by the current eruption of K
lauea [e.g.,
Hon et al.
, 1994]. However, internal unit bound-
aries (e.g., lobes of a p
a
hoehoe flow, glassy pillow
margins, and the presence of inter-pillow breccia)
were recorded. Logging information was recorded
for each unit in every core box (
3 m of core),
including identification of contacts and their type
(e.g., intrusive or depositional), groundmass tex-
tures, the presence or absence of volcanic glass,
vesicle abundance, extent of alteration and fractur-
ing, any sedimentary features, and general com-
ments. The abundances (in volume %) of
phenocrysts (i.e., crystals >1 mm), groundmass,
and vesicles were determined for each rock unit in
each core box where possible on the basis of two
100-point counts on representative portions of the
core. For the hyaloclastites, selected large clasts
were pointed counted. The point counts were done
under a binocular microscope (or with a 10
hand
lens) using a 16 cm
2
transparent sheet printed with
100 grid intersections. Modal classification of the
volcanic rocks was based on their vesicle-free
normalized phenocryst abundances using the fol-
lowing ranges: aphyric (<1%), sparsely porphyritic
(1–2%), moderately porphyritic (2–10%), highly
porphyritic (>10%). Contacts, internal boundaries,
point count locations and other notable features
were labeled on digital box photographs (Figure 4).
Rock names for flows, pillows, and intrusives were
based both on the normalized (vesicle-free) abun-
dance and identity of phenocrysts. For example, a
basalt with 2–10% total phenocryst content with
both olivine and plagioclase phenocrysts (but
olivine >plagioclase) would have been called a
‘‘moderately plagioclase-olivine-phyric basalt,’’
following the format of
Streckeisen
[1973]. A more
detailed description of logging and point counting
procedures is given by
Hawaii Scientific Drilling
Project
[2000].
[
11
] Glassy, fragmental materials in the submarine
section of the HSDP2 core were classified as
‘‘hyaloclastites,’’ although these deposits were
formed by a variety of mechanisms (K. P. Bridges
et al., Submarine growth of a Hawaiian shield
volcano based on volcaniclastics in the Hawaii
Scientific Drilling Project 2 core, submitted to
Figure 4.
Photos of parts of two working boxes showing major stratigraphic contacts. (a) Mauna Loa/Mauna Kea;
contact shown by solid blue line (Box 90). (b) Mauna Kea subaerial/submarine contact (Box 384). The boxes are
organized with top at upper left and bottom at lower right, like this page. The solid blue lines denote the unit contacts;
the dashed blue line in the upper photo indicates an internal unit boundary. The blue box with a ‘‘pc’’ label shows the
location of a point count. The scale bar is in inches and centimeters, and it applies to both boxes, which are
approximately 61 cm long. The color chart gives the true colors.
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Geochemistry, Geophysics, Geosystems
, 2007;
hereinafter referred to as Bridges et al., submitted
manuscript, 2007). For each unit, sedimentary
features (e.g., cross bedding, grading, grain size,
sorting, rounding) and clast lithology were noted
using the classification scheme of
Compton
[1962].
Contacts between hyaloclastite units were delineated
on the basis of changes in clast lithology. For
example, a new unit was designated if the hyalo-
clastite changed from monomict (only one clast
lithology) to polymictic (multiple clast lithologies)
or if there was a significant change in the dominant
clast lithology of a polymictic unit. Finer-grained,
well-sorted sections (sandstones) were designated as
new lithologic units only if they separated two
distinct hyaloclastite units. When a lava interval
within a hyaloclastite sequence was thicker than
61 cm (the length of a single row of a working core
box) and its origin was ambiguous (e.g., a large
clast, a flow, or an intrusive), it was defined as a
separate unit. For each box, the maximum clast size
and the average of the ten largest clasts were
recorded along with whether the clasts are matrix
or clast supported. Each clast lithology was given a
rock name that included a description of vesicular-
ity (sparse <5%, moderate 5–15%, abundant 15–
30%, very abundant >30%, and variable) and
mineralogy (e.g., a ‘‘sparsely vesicular, moderately
olivine-phyric basalt’’) based on two 100-point
modes. However, if a hyaloclastite was polymictic
yet was composed mostly of one clast type, it was
given a modified name such as ‘‘basaltic hyalo-
clastite (polymictic, dominantly highly olivine-
phyric basalt clasts)’’. The following observations
were also made for each clast lithology: (1) relative
abundance (sparse <5%, common 5–20%, or
abundant >20%); (2) common clast size (small,
1–5 cm; medium, 5–10 cm; large, 10–15 cm; or
very large, >15 cm); (3) size range (cm); and
(4) rounding (subangular, angular, subrounded or
rounded). General comments such as vesicle size
and shape, phenocryst size and shape, and the
presence of secondary minerals were also noted.
4. HSDP2 Stratigraphy
[
12
] A total of 345 lithologic units were identified
in the HSDP2 drill core (Table 1). These units were
grouped into four depth zones; subaerial Mauna
Loa (surface to 246 mbsl), subaerial Mauna Kea
(246–1079 mbsl), shallow submarine Mauna Kea
(1079–1984 mbsl), and deep submarine Mauna
Kea (1984–3098 mbsl). Nearly all of the subaerial
section (99.0% of the Mauna Loa section; 99.8% of
the Mauna Kea section) consists of lava (151 units);
the remainder includes 12 thin ash layers, three
soils, and one sand deposit spanning a total depth of
3.9 m (Table 1). In contrast, the 2.2 km thick
submarine section is dominated by sediments
(55.6%), most of which (98.3%) are classified here
as hyaloclastite. Lava comprises 41.7% of the
submarine section, with massive units (see below)
mostly in the shallow portion (<1984 mbsl) and
pillows observed only below this depth. Intrusions
invade only the submarine section, representing
1.5% of the total section. The submarine section
has been subdivided here into an upper sequence
(1079–1984 mbsl), with abundant hyaloclastite
(
82%) and no pillow lavas (Table 1), and a lower
sequence dominated by pillow lavas (
61% below
1984 mbsl; Table 1, Figure 2). An
8 m thick
sandstone, the thickest epiclastic unit in the HSDP2
section, separates these two sequences (see auxiliary
material
1
Table S1, a complete listing of all units).
This thick sediment may record a volcanic hiatus.
These four zones are discussed in detail in the next
section.
4.1. Mauna Loa Section (Surface to
246 mbsl)
[
13
] Drilling started on the
1.34 ka Pana‘ewa
lava flow [
Moore et al.
, 1996]. Whole-rock major,
trace element, and isotopic analyses indicate that
all of the flows from the surface to a depth of
246 mbsl (i.e., to the base of unit 41) are from
Mauna Loa [e.g.,
Blichert-Toft et al.
, 2003;
Rhodes
and Vollinger
, 2004]. The Mauna Loa section
consists of 100 subaerial lava flows divided into
32 units with six ash units, two soils, and a
sandstone (Table 1; Table S1). These rocks and
sediments are thought to have been deposited over a
period of
100 ka [
Sharp and Renne
, 2005]. The
average Mauna Loa flow unit resurfacing interval at
the HSDP2 site is thus
3 ka. This estimate is
consistent with those based on the HSDP1 section
and from the upper slopes of Mauna Loa (3–5 ka
[
Beeson et al.
, 1996]). The
3 ka between succes-
sive units at the HSDP2 site was apparently insuf-
ficient to develop significant soils (i.e., recoverable
by drilling) on the Mauna Loa lava flows except
where ash was present (two of the six ashes in the
section have significant overlying soils, >10 cm
thick; units 33 and 38). However, this interval was
long enough and the environment was wet enough
(rainfall in Hilo averages
330 cm/yr; http://www.
1
Auxiliary materials are available at ftp://ftp.agu.org/apend/gb/
2006gc001379.
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Table 1.
A Summary of Lithologic Unit Characteristics Within the HSDP2
a
Unit Type
# Occurrence % of Section
Thickness, m
Basalt
(0–5% Ol)
Ol Basalt
(5–12% Ol)
Picrite
(>12% Ol)
Ol Avg%
Vesicularity
Total Avg Max Min #
%
#
%
#
%
Avg
Max
Min
Mauna Loa Subaerial (
1.8–246 mbsl)
P
a
hoehoe
13
31.7%
44.8%
110.9 8.5 23.7 1.0 11 84.6%
1
7.7%
1
7.7%
4.3%
13.7% 33.0% 3.5%
Transitional
4
9.8%
9.5%
23.4
5.9 10.0 1.5
1
25.0%
2 50.0% 1
25.0%
10.1%
10.2% 12.6% 7.3%
‘A‘
a
12
29.3%
37.4%
92.4
7.7 19.4 0.2
3
25.0%
2 16.7% 7
58.3%
12.0%
6.7% 10.3% 1.8%
Massive
3
7.3%
7.3%
18.1
6.0
9.5
3.4
1
33.3%
0
0.0%
2
66.7%
18.5%
5.0%
8.5%
0.5%
Ash
6
14.6%
0.5%
1.3
0.2
0.6
0.1
-
-
-
-
-
-
-
-
-
-
Soil
2
4.9%
0.2%
0.6
0.3
0.4
0.2
-
-
-
-
-
-
-
-
-
-
Sandstone
1
2.4%
0.2%
0.6
0.6
0.6
0.6
-
-
-
-
-
-
-
-
-
-
S
flow
32
78.0%
99.0%
244.8 7.7 23.7 0.2 16 50.0%
5 15.6% 11 34.4%
10.2%
9.2% 33.0% 0.5%
S
nonflow
9
22.0%
1.0%
2.4
0.3
0.6
0.1
-
-
-
-
-
-
-
-
-
-
Mauna Kea Subaerial (246–1,079 mbsl)
P
a
hoehoe
26
20.6%
22.7%
189.5 7.3 34.3 1.3 22 84.6%
2
7.7%
2
7.7%
3.1%
20.8% 35.5% 13.0%
Transitional
16
12.7%
12.0%
100.3 6.3 13.1 1.9
8
50.0%
3 18.8% 5
31.2%
9.3%
12.3% 22.7% 0.0%
‘A‘
a
76
60.3%
62.9%
523.8 6.9 19.1 0.9 26 34.2% 32 42.1% 18 23.7%
8.7%
8.2% 24.0% 0.2%
Massive
1
0.8%
2.2%
18.2 18.2 18.2 18.2 0
0.0%
0
0.0%
1 100.0%
24.8%
6.9%
6.9%
6.9%
Ash
6
4.8%
0.1%
0.4
0.1
0.1 <0.1 -
-
-
-
-
-
-
-
-
-
Soil
1
0.8%
0.1%
1.0
1.0
1.0
1.0
-
-
-
-
-
-
-
-
-
-
S
flow
119
94.4%
99.8%
831.8 7.0 34.3 0.9 56 47.1% 37 31.1% 26 21.8%
7.5%
11.4% 35.5% 0.0%
S
nonflow
8
5.6%
0.2%
1.8
0.2
1.0 <0.1 -
-
-
-
-
-
-
-
-
-
Mauna Kea Shallow Submarine (1,079–1,984 mbsl)
Massive
36
37.5%
15.0%
120.4 3.3 23.0 0.6
8
22.2% 12 33.3% 16 44.4%
12.2%
2.5% 19.0% 0.0%
Intrusion
3
3.1%
1.3%
10.1
3.4
5.8
1.8
1
33.3%
1 33.3% 1
33.3%
8.0%
0.1%
0.3%
0.0%
Hyaloclastite
48
50.0%
81.9%
658.2 11.2 70.9 0.5
-
-
-
-
-
-
-
-
-
-
ss/cong
9
9.4%
1.9%
15.0
2.3
8.2
0.1
-
-
-
-
-
-
-
-
-
-
S
igneous
39
40.6%
16.2%
130.5 3.3 23.0 0.6
9
23.1% 13 33.3% 17 43.6%
12.0%
2.4% 19.0% 0.0%
S
sedimentary 57
59.4%
83.8%
673.2 9.9 70.9 0.1
-
-
-
-
-
-
-
-
-
-
Mauna Kea Deep Submarine (1,984–3,098 mbsl)
Massive
3
3.7%
0.3%
3.5
1.2
1.7
0.7
3 100.0% 0
0.0%
0
0.0%
1.4%
1.0%
3.0%
0.0%
Pillow
26
32.1%
60.7%
675.8 26.0 98.4 1.2 12 46.2% 10 38.5% 4
15.4%
7.0%
1.4%
9.7%
0.0%
Intrusion
10
12.3%
3.8%
42.5
4.3 11.8 0.2
8
80.0%
0
0.0%
2
20.0%
3.8%
0.3%
0.8%
0.0%
Hyaloclastite
38
46.9%
34.8%
387.9 10.2 55.7 1.2
-
-
-
-
-
-
-
-
-
-
ss/cong
3
3.7%
0.3%
3.6
1.2
2.5
0.1
-
-
-
-
-
-
-
-
-
-
S
igneous
39
48.1%
64.8%
721.8 8.8 98.4 0.2 23 59.0% 10 25.6% 6
15.4%
5.5%
1.1%
9.7%
0.0%
S
sedimentary 41
50.6%
35.2%
391.5 1.2 55.7 0.1
-
-
-
-
-
-
-
-
-
-
a
Notes: ss, sandstone; cong, conglomerate; #, number.
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prh.noaa.gov/hnl/climate/phto_clim.php) to allow
new lavas to bake (i.e., oxidize and redden) the
underlying flows in most cases. In contrast, baked
contacts are absent between successive flows from
the current eruption of K
lauea, even in wet areas.
Therefore unbaked contacts between flows with
similar mineralogy were interpreted as internal flow
contacts (i.e., multiple flows from the same erup-
tion); internal contacts are numerous within some
flow units (e.g., unit 15 with 13 internal flow
contacts; Table S1). Baked zones allowed easy
identification of subaerial flow contacts, which
was important in cases of adjacent flows with
identical flow type and mineralogy (e.g., units 11,
12, and 13).
[
14
] The sand deposit, unit 10, is thin (0.6 m thick),
well sorted, and contains subrounded to subangular,
rock, mineral (mostly fresh olivine with some cli-
nopyroxene and plagioclase) and variably altered
glass fragments in a matrix mainly composed of clay
minerals. In contrast, HSDP1, located only 2 km to
the north and adjacent to the ocean (Figure 1),
recovered three beach sand units, four hydroclastite
deposits, and a
25 m thick carbonate unit [
Beeson
et al.
, 1996]. The rarity of such deposits in the
shallow HDSP2 core suggests that the HSDP2
drilling site was above sea level for almost all of
the last 100,000 years.
4.2. Mauna Loa – Mauna Kea Contact
[
15
] The first major chemostratographic boundary
in the HSDP2 drill core is between Mauna Loa and
Mauna Kea lavas at 246 mbsl (Figure 4). This
boundary was found at a slightly deeper depth
(
280 m) in the HSDP1 core [
Rhodes
, 1996].
The greater depth may reflect the slightly greater
distance (
1 km) of the HSDP1 drill site from the
summit of Mauna Kea. Chemical analyses indicate
that the transition from Mauna Loa to Mauna Kea in
the HSDP2 section is abrupt, with no interfingering
of lavas from the two volcanoes [
Rhodes and
Vollinger
, 2004]. Ar-Ar ages for lavas from both
sides of the boundary indicate that it represents a
major time gap (at least 100 ka [
Sharp and Renne
,
2005]). However, there is no significant lithologic
indication of this time break between units 41 and
42 (Table S1). It is marked by a baked rubble zone
(Figure 4) similar to the many other baked zones in
the Mauna Loa and Mauna Kea sections. Thus this
zone was not recognized as a major time break
during core logging; instead, a soil (unit 38) was
originally picked by the logging team as the Mauna
Loa-Mauna Kea contact [
Hawaii Scientific Drilling
Project
, 2000]. However, rocks above and just
below this soil have compositions typical of Mauna
Loa [
Rhodes and Vollinger
, 2004]. Thus unit 38
represents a significant time break.
4.3. Subaerial Mauna Kea (246 – 1079 mbsl)
[
16
] This depth interval contains 289 flows divided
into 119 flow units (with a total thickness of 832 m)
and 7 sedimentary units (six ashes and one soil,
totaling 1.4 m in thickness; Table 1). The upper
96 m, with 20 interbedded weakly alkalic and
tholeiitic lavas, is part of Mauna Kea’s postshield
sequence [
Rhodes and Vollinger
, 2004]. Only one
flow in this sequence (unit 48 at
277 mbsl) was
dated, yielding a preferred Ar-Ar age of 236 ± 16 ka
[
Sharp and Renne
, 2005]. Combining ages for lavas
from the HSDP1 and HSDP2 cores,
Sharp and
Renne
[2005] inferred an accumulation rate of
0.9 ± 0.4 m/kyr for the postshield section. On the
basis of this estimate, the HSDP2 postshield
sequence accumulated over
100 kyr and had a
flow recurrence interval of
5 kyr, somewhat
longer than for the overlying Mauna Loa section.
However, only one soil was recovered (unit 51,
1.0 m thick) on an ‘a‘
a
flow. The absence of ash
units in this sequence probably inhibited soil for-
mation during the
5 kyr between deposition of
successive flows.
[
17
] A thin ash (unit 64, <0.1 m thick) separates
postshield and shield lavas (Table S1). The 100
subaerial lavas beneath this soil are all tholeiitic
[
Rhodes and Vollinger
, 2004]. Only one of these
tholeiitic lavas (unit 155) was successfully dated; it
yielded an Ar-Ar age of 370 ±180 kyr [
Sharp and
Renne
, 2005]. The estimated accumulation rate for
this section, based on combining ages from the
HSDP1 and HSDP2 sections, is 8.6 ± 3.1 m/kyr
[
Sharp and Renne
, 2005]. Thus this 738 m thick
section formed over
86 kyr and its flow recurrence
interval was
860 years, comparable to that of the
recent rate for the K
lauea shield (
1000 years
[
Holcomb
, 1987]). Although four ashes are present
in this sequence (Table S1), no soils were recovered.
This probably reflects the relatively short time inter-
val between flows during Mauna Kea’s subaerial
shield-building stage.
4.4. Upper Submarine Mauna Kea
(1079 – 1984 mbsl)
[
18
] The subaerial to submarine transition in the
HSDP2 section is marked by the first occurrence of
basaltic hyaloclastites (Table S1; Figure 4). This
transition zone includes a 12 m thick subaerial flow
(unit 168) overlying a 1.7 m thick, poorly indurated
Geochemistry
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hyaloclastite (unit 169) and a thin (2.2 m), massive
flow (unit 170). All three units have the same sparsely
olivine-phyric to aphyric mineralogy (Table S1), and
they may thus be part of one eruptive sequence. The
subaerial flow is capped by a reddish, baked ash. No
reddish zones were found below 1079 mbsl, so the
switch from submarine to subaerial volcanism was
abrupt at the HSDP2 site. This depth is also marked
by a significant decrease in vesicularity (Figure 3)
and increase in lava density [
DePaolo et al.
, 2001;
Moore
, 2001]. None of the lavas from this sequence
were successfully dated. However, extrapolation of
the ages for lavas from the subaerial sections of the
HSDP1 and HSDP2 cores lead to an estimated age of
400 kyr for the HSDP2 subaerial-submarine tran-
sition [
Sharp and Renne
, 2005]. This age and its
depth below sea level yields an average subsidence
rate for the subaerial section of 2.7 mm/yr [
Sharp and
Renne
, 2005].
[
19
] Within the uppermost 61 m of the submarine
section, relatively thin massive basalt (2.4 m aver-
age thickness) and clastic sedimentary units (3.1 m
average thickness; dominantly basaltic hyaloclas-
tite with one unit with rounded basalt cobbles)
alternate and occur in roughly equal amounts
(Table S1). Poor drilling conditions in the weakly
consolidated hyaloclastite led to a decision to
switch to tricone drilling in two intervals (1140–
1223 mbsl and 1243–1260 mbsl). No core or
cuttings was recovered in these intervals. Below
1260 mbsl, hyaloclastites are the dominant rock type
in the upper submarine section (
88%; Table S1).
Clasts within these units are generally monomict
and petrographically similar to the interlayered
massive basalts, although they are more vesicular
than the flows (Table S1). Glasses from both rock
types are nearly all degassed [
Seaman et al.
, 2004].
Thus both rock types were probably erupted sub-
aerially and may have been part of the same eruptive
sequences. The depth of emplacement of the rocks at
the base of the upper submarine sequence (now at
1984 mbsl) can be estimated using an average
subsidence rate of 2.7 mm/yr and the inferred age
of the section [
Sharp and Renne
, 2005]. Applying
the emplacement depth correction factor
[0.67*(depth-1079)] yields an emplacement depth
of
600 mbsl for the base of this zone (Bridges et
al., submitted manuscript, 2007).
4.5. Lower Submarine Mauna Kea
(1984 – 3098 mbsl)
[
20
] The start of this sequence is defined by the
first appearance of pillow lava at 1984 mbsl. Pillow
lava is the dominant rock type in this zone (26 units
representing
61% of this zone; Table 1), although
massive lavas are also present (3 units forming
0.3%). Both lava types are interspersed with hya-
loclastites (38 units comprising 35% of this zone)
and three thin sandstone units with silty intervals
(Table S1). This sequence is cut by 10 intrusive units
(some of which have multiple splays; (Table S1).
Sharp and Renne
[2005] successfully dated four
lavas from the sequence yielding Ar-Ar ages of
482 ± 67, 560 ± 150, 683 ± 82 and 760 ± 380 ka.
A linear fit to these ages gives an age of
647 ±
50 ka for the base of the drill hole [
Sharp and
Renne
, 2005]. On the basis of an assumed subsi-
dence rate of 2.7 mm/yr and the age versus depth
relationship from
Sharp and Renne
[2005], this
zone was deposited at depths of
600 to 1350 ±
135 mbsl.
[
21
] Sulfur contents of the glassy pillow margins
from 1984–2140 mbsl are low (
0.02 wt.%),
indicative of subaerial eruption. In contrast, low-
silica (<50 wt.% SiO
2
) pillow lava glasses from
2233–2488 mbsl have higher S (0.058–0.127 wt.%
[
Stolper et al.
, 2004]). These higher S contents
indicate eruption in a submarine environment of
sufficient depth to inhibit S degassing (probably
>500 mbsl [
Seaman et al.
, 2004]), which is consis-
tent with the depths of deposition inferred from
subsidence (773–944 mbsl). The marked change in
glass S content may suggest a change in vent
location (e.g., the deeper lavas came from a sub-
marine rift zone and the shallower units from the
summit or other subaerial vents).
[
22
] Hyaloclastites are less abundant in this zone
than in the upper submarine sequence, and they are
mostly polymictic with many containing silty or
sandy beds (Table 2). Monomictic hyaloclastite
units do occur in this zone, but they are rare,
relatively thin (
2 m; Table 2), and similar in
mineralogy and glass chemistry to underlying or
overlying flows (e.g., units 300 and 301; Table S1
[
Stolper et al.
, 2004; Bridges et al., submitted
manuscript, 2007]) suggesting a possibly close
relationship between the hyaloclastites and adja-
cent flows. All of the analyzed glasses from
hyaloclastite units from shallower than 2460 mbsl
are degassed [
Seaman et al.
, 2004], despite their
deposition at depths up to
925 mbsl (based on
their ages and assuming a subsidence rate of
2.7 mm/yr). At depths greater than 2460 mbsl, the
hyaloclastites contain a mixture of degassed and
undegassed glasses, commonly within the same
unit (e.g., unit 332 [
Seaman et al.
, 2004]).
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Table 2. (Representative Sample).
A Summary of Hyaloclastite Clast Type 1 Characteristics From the HSDP2
a
[The full Table 2 is available in the HTML version
of this article at http://www.g-cubed.org.]
Litho Unit
Depth, mbsl
Thick-ness,
m
Max Clast
Size, cm
Avg. of
10 Largest
Min Clast
Size, cm
Avg Clast
Size,
b
cm
Texture
(Support) Sorting Diversity
c
Rounding
Rock Name
d
Top
Bottom
169
1079.0 1080.7
1.7
11.0
6.0
3.0
large
matrix
poor
mono
subangular
sopb
175
1088.8 1089.8
1.0
12.0
5.0
0.1
large
clast
poor
mono
angular
hv,hopb
178
1092.8 1095.6
2.8
43.0
12.0
0.1
mixed
mixed
mixed
mono
angular
mixed
180
1099.4 1107.1
7.7
18.0
7.0
0.1
mixed
matrix
poor
mono
subangular
mixed
182
1110.2 1110.9
0.7
9.0
4.0
0.1
continuous
matrix
poor
mono
angular
hv,sopb
184
1111.9 1
122.6
10.7
7.0
5.0
0.1
medium
matrix
poor
mono
angular
mixed
186
1126.4 1128.2
1.7
8.0
4.0
0.1
medium
matrix
poor
mono
angular
mixed
188
1223.1 1227.0
3.9
7.0
6.0
0.2
continuous
matrix
poor
mono
subangular
sv,mopb
190
1227.2 1260.5
33.4
15.0
8.0
0.2
continuous
matrix
poor
poly
mixed
mixed
192
1283.6 1285.3
1.8
20.0
7.0
0.1
large
matrix
poor
mono
angular
hv,mopb
194
1287.0 1310.9
23.8
38.0
12.0
0.1
mixed
matrix
poor
mono
mixed
sv,hopb
196
1313.1 1334.2
21.1
20.0
9.0
0.1
mixed
matrix
poor
poly
mixed
sv,hopb
197
1334.2 1335.7
1.5
9.0
4.0
0.2
continuous
matrix
poor
poly
mixed
mixed
198
1335.7 1403.9
68.2
38.0
11.0
0.1
mixed
mixed
poor
mono
mixed
mixed
200
1408.6 1409.8
1.2
10.0
6.0
0.1
small
matrix
poor
poly
blank
sv,m-t-hopb
202
1414.0 1484.9
70.9
30.0
11.0
0.1
mixed
matrix
poor
poly
subangular
sv,hopb
204
1486.2 1494.9
8.6
30.0
15.0
0.2
continuous
clast
poor
poly
subangular
sv,hopb
206
1496.7 1497.2
0.5
8.5
5.8
0.2
continuous
matrix
poor
mono
subangular
sv,hopb
208
1498.7 1505.7
7.0
15.0
8.9
0.2
continuous
clast
poor
poly
subangular
sv,hopb
210
1508.9 1510.2
1.3
17.0
6.0
0.2
continuous
mixed
poor
poly
subangular
sv,hopb
212
1512.8 1520.3
7.5
37.0
9.4
0.2
continuous
mixed
poor
poly
subangular
sv,hopb
214
1521.7 1529.3
7.6
15.0
7.4
0.2
continuous
matrix
mixed
poly
subangular
sv,hopb
216
1531.0 1548.7
17.6
28.0
10.4
0.2
continuous
mixed
poor
poly
subangular
sv,hopb
218
1551.8 1584.5
32.8
24.0
7.7
0.1
continuous
matrix
poor
poly
subangular
mixed
220
1585.6 1591.9
6.3
19.0
8.0
0.2
mixed
mixed
mixed
poly
mixed
mixed
222
1598.2 1599.7
1.5
10.0
7.0
1.0
continuous
clast
poor
poly
subrounded to
subangular
sv,hopb
223
1599.7 1601.8
2.1
8.5
4.8
1.0
mixed
matrix
poor
poly
mixed
mixed
225
1607.0 1635.5
28.5
48.0
11.9
0.1
continuous
mixed
poor
poly
mixed
sv,mopb
227
1636.2 1643.7
7.5
21.0
10.0
0.1
continuous
clast
poor
mono
angular
sv,mopb
229
1644.4 1652.3
7.8
29.0
9.0
0.1
continuous
mixed
poor
mono
mixed
mixed
231
1653.3 1657.0
3.7
21.0
10.0
0.2
continuous
mixed
poor
poly
subangular
mixed
233
1657.6 1660.4
2.9
9.0
4.0
0.1
continuous
mixed
poor
poly
mixed
sv,mopb
a
Hyaloclastite clast type 1 are the most abundant. Units split by intrusion are not included. The term mixed indicates that the multiple clast characte
ristics described for a given unit are in <75% agreement
(see text for details).
b
Avg clast size (cm): small, 1–5 cm; medium, 5–10 cm; large, 10–15 cm; very large, 15 or more cm; continuous, no predominant size of clasts.
c
Diversity: mono, monolitholigic; poly, polymictic.
d
Rock names are abbreviated forms of the classification described in the text. For example, sv,mopb, sparsely vesicular, moderately olivine-phyric
basalt; nv,hopb, nonvesicular, highly olivine-phyric basalt;
and ab, aphyric basalt.
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