What can (and cannot) be learned from scientific drilling

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What can (and cannot) be learned from scientific
drilling
A mini-lesson for introductory geoscience
students and high school teachers, using examples
from NSF Margins Initiatives
Rosemary Hickey-Vargas Department of Earth
Sciences Florida International University
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How do we know what the Earths interior is like?
From Press Seiver, Understanding Earth, 3rd
Edition, W.H. Freeman Co.
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Answers

• Apply indirect geophysical techniques like the
  interpretation of seismic wave velocities,
  variations in gravity and magnetism
• Look at accidental fragments of mantle and deep
  crust brought to the surface in rising magma
  (xenoliths)
• Drill into the Earth, recover rock and sediment
  cores and make down-hole measurements

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How deep can we drill into the Earth?
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(No Transcript)
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Question How deep can we drill into the Earth?
Answer Not very far! We have only drilled into
the Earths crust, and no more than 0.2 of the
depth to the Earths center. No drill hole on
continental or oceanic crust has reached the
Earths mantle.
Question What, then, can we learn from
scientific drilling?
Answer Many things. Here are two examples from
the National Science Foundations Margins
Initiatives.
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Example 1 Understand what is being subducted in
subduction zones
In a subduction zone, oceanic crust, including
basalt, sediment, and the fluids contained in
these materials, is carried beneath the
lithosphere of the overriding plate. Some of
this material can be accreted onto the overriding
plate, and some fluids migrate upward as the
crust is subducted to increasing depths. The rest
is carried into the mantle. The whole cycle of
inputs and outputs is the Subduction Factory.
Blue- seawater Yellow - sediment Green -
basalt Pale green - mantle
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Example 1 Understand what is being subducted in
subduction zones
It is important to understand what happens to
this material as it is subducted, how much of it
is melted to form the magma found at subduction
zone volcanoes, and how much is ultimately
recycled into the deep mantle. In order to
calculate this, the starting material is
inferred from a reference hole, drilled outboard
of the deep sea trench. The drill core samples
the basalt and sediment that may be wholly or
partially subducted along a specific subduction
zone.
Blue- seawater Yellow - sediment Green -
basalt Pale green - mantle
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Each of the two Margins Subduction Factory Focus
areas has at least one reference hole
1) Izu-Bonin-Mariana Arc, Mariana Arc segment
Reference hole on the Pacific Plate
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ODP Site 801 section
What kinds of sediments are these?

• Pelagic brown clay
• Chert porcellanite
• Volcaniclastic turbidites
• Radiolarite

How old are the sediments?
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2) Central American Arc
Reference holes on the Cocos Plate
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ODP Site 1039 Site 1253 composite section
What kinds of sediments are these?

• Diatom ooze
• Silty clay
• Calcareous clay
• Siliceous ooze
• Chalk

How old are the sediments?

• Questions
• What are the major differences between these
  sediments and those from Site 801?
• Can you think of a way that having different
  sediments on the subducting plate might affect
  the process of subduction?

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You can find detailed information about the
reference holes (and deep ocean drilling in
general) using the links below to the Ocean
Drilling Program and Margins databases

• http//www.odplegacy.org/
• http//www.marine-geo.org/margins/

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Example 2 Understand how subduction zone
earthquakes and tsunamis are generated
The seismogenic zone is the part of the interface
between the subducting and overriding plate where
earthquakes originate. It has both an upper and
lower depth limit. Scientific drilling can help
to understand the behavior of the material in and
near the seismogenic zone.
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Tsunamis can form when earthquakes occur under
the ocean, and ocean water is displaced by motion
of the seafloor. The behavior of the material in
and near the seismogenic zone affects the
generation of a tsunami. Watch the animation and
notice how the motion of the lithospheric plate
is transferred to the ocean water. The yellow
arrows show where the tsunami appears at the
ocean surface.
http//ffden-2.phys.uaf.edu/645fall2003_web.dir/el
ena_suleimani/generation_small.mov
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The Nankai Trough, just south of Japan, is a
Margins Focus area where the seismogenic zone is
being explored using drilling (NanTroSEIZE). This
area has a 1300 year historical record of great
and tsunamigenic earthquakes, including one in
1944 (magnitude 8.1) and one in 1946 (magnitude
8.3).
You can find more information about this project
by following the links below to the Integrated
Ocean Drilling Program and Margins Program
http//www.iodp.org/nantroseize/
http//www.marine-geo.org/margins/
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In NanTroSEIZE, drilling will be used determine
the kinds of rocks and sediments in each plate,
the kinds and amounts of fluids in the rocks and
sediments, the pore pressures in these materials,
and their stress states, all of which are needed
to understand why and how earthquakes occur here.
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The proposed drill sites are superimposed on a
cross-section constructed from a seismic survey.
In the survey, seismic waves are produced
artificially from a ship that moves across the
area to be examined (Line 5). The waves are
reflected from features like sediment layers and
faults which show up as dark lines. The survey
cross-section is used to plan where drill sites
should be located. After drilling is complete,
the seismic cross-section can be used to
correlate specific sediment layers and fault
surfaces, now characterized with actual samples,
from one drill site to another.
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What features of the plate boundary region are
being examined with each of the drill
sites? What feature on the cross-section could
be correlated between several drill sites?
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Summary

• Earths interior is explored using indirect
  geophysical techniques, examination of rare
  accidental deep samples, and scientific drilling
• Scientific drill holes have only reached levels
  within the Earths crust, to a maximum of about
  12 km in continental crust and 5 km in oceanic
  crust. Drilling has not reached the mantle.
• Although drilling samples only the outermost part
  of the Earth, many important Earth processes can
  be investigated, for example, we can find out
• What is delivered to a subduction zone on the
  subducting plate
• The nature of Earth materials that are involved
  in the generation of earthquakes and tsunamis in
  subduction zones

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Some topics for thought investigation

• Look at a chart of the Geologic Time scale and
  compare the ages of the sediments at the two
  Margins SubFac Focus area reference holes.
• Make a list of all the differences between the
  two sediment columns at the two reference holes
  shown in slides 10 12.
• Think of some differences in the inputs of each
  Focus area that might result from the differences
  in outputs, such as the lavas and gases emitted
  by subduction volcanoes.
• Look at a map of the Earths lithospheric plates
  and identify the overriding and subducting plate
  for each of the three Margins Focus areas
  mentioned here.
• Look at a list of historic tsunamis and determine
  in which subduction zones the earthquakes that
  triggered these events occurred.
• Look at the NanTroSEIZE webpages, and find out
  the special characteristics of the riser
  drillship Chikyu, which will be used to drill
  through the Nankai decollement.
• Look at the seismic cross-section shown in slide
  17-18. What features of the plate boundary are
  being examined with each of the drill sites? How
  well could the drilling locations be chosen
  without the seismic survey results?
• Assume that new technology allows you to drill to
  15-20 kilometers. Choose a place among the
  Margins sites that you would drill and explain
  what you might discover there.