Introduction
Oceanic volcanoes formed by mantle plumes, such as
those of Hawaii and Iceland, strongly influence our views
about the deep Earth (Morgan, 1971; Sleep, 2006). These
volcanoes are the principal geochemical probe into the deep
mantle, a testing ground for understanding mantle
convection, plate tectonics and volcanism, and an archive of
information on Earth’s magnetic field and lithosphere
dynamics. Study of the petrology, geochemistry, and
structure of oceanic volcanoes has contributed immensely
to our present understanding of deep Earth processes, but
virtually all of this study has been concentrated on rocks
available at the surface. In favorable circumstances, surface
exposures penetrate to a depth of a few hundred meters,
which is a small fraction of the 10 - to 15 -kilometer height of
Hawaiian volcanoes above the depressed seafloor (Moore,
1987; Watts, 2001).
The shield volcanoes of Hawaii are enormous in
comparison to most other types of volcanoes. The average
Hawaiian volcano has a volume of 30,000 –50,000 km
3
(DePaolo and Stolper, 1996; Robinson and Eakins, 2006).
By comparison, stratovolcanoes like Mt. Shasta in California,
or Mt. Fuji in Japan, have volumes of only 50 –500 km
3
.
Hawaiian volcanoes grow upward from the ocean floor by
systematically covering their roughly conical surfaces with
new lava flows. In their main growth phase, they increase in
height at an average rate of 10 –30 meters per thousand years
(DePaolo and Stolper, 1996), and their surfaces are completely
covered with new lava about every thousand years (Holcomb,
1987). The lava flows of these large volcanoes dip gently
away from the summits at angles of about 5 –15 degrees
relative to horizontal (Mark and Moore, 1987). The subhori
-
zontal orientation of the flows, and the fact that they
accumulate systematically with time just like sediments,
means that the flanks of a volcano contain an ordered history
of the volcanism that can be accessed efficiently by drilling.
The particular interest in drilling Hawaiian volcanoes is
that as they grow, they are slowly carried to the northwest by
the moving Pacific plate. Each individual volcano “sweeps”
across the top of the Hawaiian mantle plume as it forms. The
magma-producing region of the plume is roughly 100 km
wide (Ribe and Christensen, 1999), so with the plate moving
at 9 –10 cm yr
-1
, it takes a little over one million years for a
volcano to cross the magma production region. During this
time the volcano goes through its major growth phases,
starting as a steep-sided cone on the ocean floor, growing
until it breaches the sea surface and becomes a small island,
and then continuing to grow, expand, and subside until it
becomes a massive, 100 -km-wide pancake of lava and
volcanic sediment with intrusive rocks at its core. As a
volcano forms, the magma supply comes first from one side
of the plume, then the middle, and then the other side, so
sampling a stack of Hawaiian lavas provides a cross-section
through the plume. The plume itself brings up rock material
that comes from the deepest layers of the mantle (Farnetani
et al., 2002; Bryce et al., 2005; Sleep, 2006). Thus, by drilling
a few kilometers into a Hawaiian volcano, one can in theory
look 2900 km down into the Earth and (if current models are
correct) gather information about the bottom 100 kilometers
of the mantle. No other place on Earth that we know of affords
the possibly of doing this with quite the regularity that is
inherent to Hawaiian volcanoes.
In recognition of the opportunities afforded by drilling in
Hawaiian volcanoes, the Hawaii Scientific Drilling Project
(HSDP) was conceived in the mid-1980s to core continuously
to a depth of several kilometers in the flank of a Hawaiian
volcano. The Mauna Kea volcano, which makes up the north-
Figure 1.
Map showing the boundaries of the major volcanoes of the
island of Hawaii and the locations of the HSDP pilot hole drilled in
1993, and the deep hole drilled in 1999 and 2004–2007. The red
line shows the approximate location of the shoreline of Mauna Kea
when it reached its maximum extent above sea level, at the end
of the shield-building stage about 150,000 years ago (see Fig. 6).
Subsequently, subsidence has moved the shoreline 10 –20 km
closer to the volcano summit.
Hawaii
Pacific Ocean
Deep Drilling into a Mantle Plume Volcano:
The Hawaii Scientific Drilling Project
by Edward M. Stolper, Donald J. DePaolo, and Donald M. Thomas
doi:10.2204/iodp.sd.7.02.2009
4
Scientific Drilling, No. 7, March 2009
Science Reports
Scientific Drilling, No. 7, March 2009
Kea lavas were entered, the hole would remain in Mauna Kea
to total depth. The drill sites were chosen to be (1) far from
volcanic rift zones to avoid intrusive rocks, alteration, and
high-temperature fluids; (2) close to the coastline to mini
-
mize the thickness of subaerial lavas that would need to be
penetrated to reach the older, submarine parts of the volcano;
and (3) in an industrial area to minimize environmental and
community impacts.
Drilling and Downhole Logging
The main phase of HSDP2 drilling in 1999 consisted
primarily of successive periods of coring to predetermined
depths, followed by rotary drilling to open the hole for instal
-
lation of progressively narrower casing strings (Fig. 2). No
commercially available system could satisfy both the coring
and rotary drilling requirements, so a hybrid coring system
(HCS) was designed and fabricated. The HCS employed a
rotating head and feed cylinder to drive the coring string,
and it was attached to the traveling block of a standard rotary
eastern part of the island of
Hawaii, was chosen as the
target (Fig. 1). The drill sites
are located within the city of
Hilo at elevations just a few
meters above sea level. The
project proceeded in three
phases of drilling. What we
refer to as “HSDP1” involved
coring a pilot hole to a depth
of 1052 meters below sea
level (mbsl) in 1993 (Stolper
et al., 1996; DePaolo et al.,
1996). The deep drilling
project,
referred
to
as
HSDP2, took place in two
phases. In the first phase a
hole was core drilled in 1999
to a depth of 3098 mbsl
(3110 m total depth; DePaolo
et al., 2001b). In the second
phase the hole was cased
(2003) and then deepened in
2004–2007 to a final depth of
3508 mbsl (3520 m total
depth). After each phase of
coring, an integrated set of
investigations characterized
the petrology, geochemistry,
geochronology,
and
the
magnetic and hydrological
properties of the cored lavas.
Most of the funding for this
long-term
project
was
provided by the National
Science Foundation (U.S.A.)
through
its
Continental
Dynamics
program,
but
critical support for drilling was received for the 1999
and 2004–2007 phases from the ICDP. We summarize here
the results of the HSDP1 and HSDP2-Phase 1 drilling
and preliminary results of ongoing studies from the
HSDP2-Phase 2 drilling.
Site Location
An abandoned quarry on the grounds of Hilo International
Airport was chosen as the site for HSDP2. The HSDP1 pilot
hole was located 2 km to the NNW, north of the airport,
within fifty meters of the shoreline of Hilo Bay (Fig. 1; Stolper
et al., 1996; DePaolo et al., 1996). Although the Mauna Kea
volcanic section was the primary target, the HSDP sites in
Hilo required drilling through a veneer of Holocene Mauna
Loa flows. The Mauna Kea lavas are encountered at depths of
280 –245 m. Because the volcanoes are younger to the south-
east, and overlap with subsurface boundaries sloping to the
southeast (Moore, 1987), it was expected that once Mauna
Figure 2.
[A] Diagram showing the casing diameter in the HSDP2 hole, and the dates when coring and
hole opening were done. Presently the hole is cased to a depth of 2997 mbsl, and is open below that.
[B] Temperature measured in the hole and the inferred relationships to subsurface hydrological features.
Freshwater is shown as light blue, seawater as light green, and brackish waters as intermediate colors.
Temperature survey (red line), done while the hole was flowing and still uncased below 1820 mbsl, suggests
that water is entering the hole below ~2800 mbsl, and additional entry levels are at 2370 and 2050 mbsl.
Circulation of cold seawater through the section below 600 mbsl is rapid enough to cool the rocks to
temperatures 15
°
C–20
°
C below a normal geothermal gradient.
9.1 m (30 ft.)
3518 m (11,541 ft.)
3009 m
(9872 ft.)
610 m (2000 ft.)
115 m (377 ft.)
3110 m (10,201 ft.)
Core 3/15/99
Core 4/4/99
Core 3/17/99
Core 6/6/99
Core 9/06 - 3/07
Core 9/22/99
Case 3/16/99
Case 7/18/99
Case 4/22/99
Case 3/25/99
Case 8/18/03
503 m (1650 ft.)
1646 m (5400 ft.)
Hawaii Scientific Drilling Project
18 5/8”
13 3/8”
9 5/8”
1831 m (6007 ft.)
7”
5”
Hole Design
Downhole Temperature
Temperature (°C)
Depth
(mbsl)
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
3200
50
40
30
20
10
0
Seawater Saturated Rocks
Freshwater
Saturated Rocks
Static
Temperature
Survey
Temperature
Survey
During Flow
Water Entries/
Permeable
Horizons
Seawater
Saturated Rocks
Mixing Zone
A
B
Scientific Drilling, No. 7, March 2009
Science Reports
rig to allow core and rotary drilling from the same platform.
Core penetration rates averaged 48 m d
-1
through the
subaerial section, but slowed upon entering the submarine
section, where poorly consolidated hyaloclastites (Fig. 3) led
to short bit life and poor core recovery. A 3.5 -inch tricone bit,
driven by the coring unit, was used to penetrate the most
difficult portion of this interval, the only significant depth
interval where core was not recovered. Progressive indura
-
tion of the hyaloclastites with depth enabled an average
penetration rate of ~25 m d
-1
down to the first occurrence of
pillow basalts (1980 mbsl; Fig. 3). The opening of the hole
and setting of the casing also progressed well; the rotary
drilling penetration rate down to 1820 mbsl averaged
~46 m d
-1
.
After casing was set to 1820 mbsl, coring through the
alternating intervals of pillow and hyaloclastite progressed
at a slower rate than in the upper section of hole. Reduced
rates of penetration were expected due to the longer trip
time, but two additional factors contributed. In the thinly
bedded pillow lavas, the core tended to fragment as it was cut
from the formation, thus blocking the core barrel and
resulting in short core runs. Broken core fragments also
tended to drop into the drill string as the core tube was
brought to the surface. These fragments needed to be cleared
from the drill string before sending a new core tube down,
and this process typically added nearly an hour to the core
retrieval process. Higher rates of bit wear also required more
frequent trips to change the bit. In spite of these challenges,
an average penetration rate of ~21 m d
-1
was maintained down
to 2986 mbsl, where a zone was encountered in which the
basalts were thoroughly broken and unstable. This broken
zone triggered some deviation of the hole from vertical and
presented additional problems with rubble caving into the
hole and threatening to jam the bottom hole assembly (BHA).
The drillers tried various strategies to deal with the caving,
but they achieved only a small amount of additional progress
before the decision was made to terminate coring operations
at a depth of 3098 mbsl and run downhole logs. The hole was
then left filled with heavy mud.
The HSDP Phase 2 drilling commenced in March to
August 2003, by first enlarging the diameter of the hole
below the 7-inch casing from 3.85 -inches to 6.5 -inches , and
then installing a 5 -inch casing to bottom (Fig. 2). Because
the casing weight was beyond the capacity of any Hawaii-
based drill rigs, a rotary rig was acquired for the project.
This “hole-opening” phase proved difficult due largely to
unexpected high formation fluid pressures. Before the start
of hole opening, the well produced artesian water at a modest
rate from depths of 2605 mbsl, 2370 mbsl, and 2059 mbsl.
However, soon after the hole was widened, strong water flow
began. As depth increased, formation pressures increased.
The peak wellhead pressure was measured at ~11 bar, and
water flow rates reached as high as 250 L s
-1
. Initial efforts at
controlling flow with increased mud weight were only
partially successful, as was an alternative cementing strategy.
As a result, progress for most of the hole opening was
difficult, dangerous, and slow. Eventually, after the hole had
been opened down to about 2732 mbsl, a decision was made
to allow the hole to flow freely, with periodic mud “sweeps”
conducted to ensure that cuttings were fully cleared from the
hole. This strategy was successful and the penetration rate
increased from <20 m d
-1
to nearly 100 m d
-1
. Hole opening
then continued down to 2997 mbsl, where caving problems
were again encountered. After several attempts at drilling
through the problematic zone, each resulting in a tempo
-
rarily stuck BHA, the decision was made to terminate hole
opening and to begin casing.
Challenges during the hole-opening phase continued
when improper lifting tools were used, and late in the process
a 2347-m string of 5 -inch casing was dropped into the hole.
After the condition of the dropped casing was checked, it was
left in the hole. The casing was completed by threading an
additional 610 -m string onto the top of the dropped casing;
1052 m
3110 m
HSDP2
Phase 2
(2003-2007)
Depth (mbsl)
245 m
1079 m
Figure 3.
Lithologic column of the HSDP2 drill cores. Boundary
between Mauna Loa (ML) and Mauna Kea (MK) lavas is shown,
as well as the subaerial-submarine transition, and depths at which
the first pillow lava and the first intrusive rocks were encountered.
Patterns represent different lithologies as indicated. The total depth
of the hole is measured from a reference level 11.7 meters above sea
level; the depth scale used here is in meters below sea level (mbsl).
Science Reports
Scientific Drilling, No. 7, March 2009
7
the bottom of the casing was at a depth of 2997 mbsl. As the
follow-on coring work began in late 2004, we discovered that
the bottom joint of the dropped casing string had been
damaged. It was necessary, using special tools, to cut a
window through the side of the bent casing to extend the
hole. After rubble was cleared from the hole down to
3098 mbsl, coring proceeded in two stages (December 2004
to February 2005, and December 2006 to February 2007) to
a total depth of 3508 mbsl. The first coring effort averaged
only 6 m d
-1
and reached 3326 mbsl. At that point the rotary
rig was sold, and a leased coring rig was used. The coring
done in early 2007 achieved about 8 m d
-1
, but problems with
the leased rig and exhaustion of project funds resulted in
only 180 m of additional core.
At the conclusion of HSDP2-Phase 2 drilling, the 5 -inch
casing was perforated, cement was pumped into the annulus
at depth, and at 2031 mbsl, the casing was cut at 1635 mbsl
and the top section removed from the hole. The final depth of
the HSDP core hole is about 914 m less than was originally
planned in 1996, but it is still nearly twice as deep as the next
deepest core hole drilled in Hawaii (SOH-2 to 2073 m on the
Kilauea East Rift Zone; Novak and Evans, 1991).
Hydrology
Although the primary purpose of the borehole was to
document the geochemical evolution of an oceanic volcano, a
significant finding was the unexpected hydrology. The tradi
-
tional view of ocean island subsurface hydrology is one of a
freshwater lens (fed by rainfall recharge) “floating” atop
saltwater-saturated rocks that extend to the island’s base.
Circulation of seawater within the basement rocks is
presumed to occur to the extent made possible by permea
-
bility and thermal conditions. The HSDP boreholes showed
that the hydrology of the island of Hawaii is considerably
more complicated and interesting. Whereas it has been
assumed that the youth of the island of Hawaii meant that
artesian aquifers, such as those arising from the buried cap
rocks on Oahu, would be absent, the borehole encountered
multiple artesian aquifers (Fig. 2). Estimated groundwater
flow through the first of these, at a depth of only 300 m, may
represent as much as a third of the rainfall recharge to the
windward mid-level slopes of Mauna Kea. The deeper
artesian aquifers have equally unexpected implications.
Some of the groundwater produced by the deep aquifers was
hypersaline, with chloride concentrations about 20% higher
than seawater. These aquifers must be isolated from ocean
water, and they may have lost 25% of their water to hydration
reactions with basalt glass. Other fluids produced by the
borehole had salinities less than half those of seawater, indi
-
cating that a connection exists between these deep confined
pillow aquifers and the basal fresh groundwater system.
Evidence for freshwater in the formation fluids was found in
the borehole to as deep as 3000 mbsl, implying that the
volume of freshwater within Mauna Kea may be ten times
greater than previously estimated.
Thermal Profile
The downhole temperature profile for the HSDP2 core
-
hole (Fig. 2) yields additional information about the subsur
-
face hydrology of Hawaii. Within the first 200 m of the bore
-
hole, the thermal conditions were consistent with the
expected basal freshwater lens underlain by rocks saturated
with freely circulating saline water. However, at ~300 m a
temperature reversal occurs that was later demonstrated to
be the result of a ~150 -m-thick freshwater aquifer confined
by multiple soil and ash layers present at the interface
between Mauna Loa lavas and late-stage Mauna Kea flows
(Thomas et al., 1996). Below the artesian fresh aquifer, the
temperature falls rapidly to ~9
°
C, reflecting the presence of
an actively circulating saltwater system that draws deep,
cold sea water in through the submerged slopes of Mauna
Kea. Circulation within this system is rapid enough to
maintain a very weak temperature gradient (~7
°
C km
-1
) down
to a depth of ~1600 mbsl where the gradient begins a progres
-
sive rise to ~19
°
C km
-1
at 2000 mbsl. This value is to be
expected for a conductive thermal gradient (Büttner and
Huenges, 2002). Temperature measurements made below
2000 mbsl under static conditions (no internal well flow)
show a nearly constant 19
°
C km
-1
gradient to total depth.
Downhole temperature measurements made during and
soon after well flow show sharper temperature gradients that
are interpreted to reflect flow into or out of the formation
during drilling or production, respectively. The positive
temperature steps at permeable formations indicate entry of
warm fluids from deep within Mauna Kea’s core.
Lithologic Column
A major effort was made to characterize and catalogue the
rock core on-site. This nearly-real-time logging allowed us to
monitor the volcano structure, which helped with drilling
and allowed us to immediately start systematic sampling.
On-site activities included hand-specimen petrographic
description and photographic documentation of the re-
covered core. There were 389 distinguishable lithological
units identified (e.g., separate flow units, sediments, soils).
A simplified version of the lithological column is shown in
Fig. 3. A diagrammatic representation of the internal
structure of the Mauna Kea volcano in the vicinity of the drill
site (Fig. 4) helps explain the significance of the volcanic
stratigraphy.
The core was split longitudinally into a working portion
(two-thirds) to be used for analysis and an archival portion
(one-third) to be reserved for future study. A reference suite
of samples for geochemical analyses, chosen to be
representative and to cover the depth of the core at specified
intervals, was taken on-site and sent to participating
scientists. A key feature of the sampling is that a complete
suite of petrological and geochemical analyses was conducted
on these reference samples, allowing for a high level of
comparability among complementary textural, chemical,