Transcript RHV_Margins_Mini_Lesson.v8

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
How do we know what the Earth’s
interior is like?
From Press & Seiver,
Understanding Earth, 3rd Edition,
W.H. Freeman & Co.
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
How deep can we drill into the Earth?
Kola
Western Deep
Superdeep Ocean Drilling
Gold Mine, Bertha Rogers Borehole, Kola Program Hole
Witwatersrand,
Oil Well,
Peninsula,
504B, Costa
Average
South Africa
Oklahoma
Russia
Rica Rift
Oceanic Crust
Average
Continental
Crust
0
Depth in meters
5000
10000
15000
2111
3585
7000
9583
12261
20000
25000
30000
30000
35000
Kola
Western Deep
Superdeep Ocean Drilling
Gold Mine, Bertha Rogers Borehole, Kola Program Hole
Average
Witwatersrand, Oil Well,
Peninsula,
504B, Costa
Average
Continental
South Africa
Oklahoma
Russia
Rica Rift
Oceanic Crust
Crust
Core Mantle
Boundary
0
3585
9583
12261
2111
7000
30000
Depth in Meters
500000
1000000
1500000
2000000
2500000
3000000
2900000
Question: How deep can we drill into the Earth?
Answer: Not very far! We have only drilled into
the Earth’s crust, and no more than 0.2% of the
depth to the Earth’s center. No drill hole on
continental or oceanic crust has reached the
Earth’s mantle.
Question: What, then, can we learn from scientific
drilling?
Answer: Many things. Here are two examples
from the National Science Foundation’s Margins
Initiatives.
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.
km
50
Blue- seawater
Yellow - sediment
Green - basalt
Pale green - mantle
100
150
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.
km
50
Blue- seawater
Yellow - sediment
Green - basalt
Pale green - mantle
100
150
Each of the two Margins
Subduction Factory Focus
areas has at least one
reference hole
1) Izu-Bonin-Mariana Arc,
Mariana Arc segment
Site 801
Reference hole on
the Pacific Plate
ODP Site 801 section
What kinds of sediments are these?
•Pelagic brown clay
•Chert & porcellanite
•Volcaniclastic turbidites
•Radiolarite
How old are the sediments?
2) Central American Arc
Sites
1039 &
1253
Reference holes on
the Cocos Plate
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?
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/
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.
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/elena_suleimani/generation_small.mov
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/
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.
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.
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?
Summary
 Earth’s 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
Earth’s 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
Some topics for thought & investigation
•
•
•
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•
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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 Earth’s 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.