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-VargasDepartment of Earth SciencesFlorida 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

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958312261

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Western DeepGold Mine,

Witwatersrand,South Africa

Bertha RogersOil Well,

Oklahoma

KolaSuperdeep

Borehole, KolaPeninsula,

Russia

Ocean DrillingProgram Hole504B, Costa

Rica RiftAverage

Oceanic Crust

AverageContinental

Crust

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How deep can we drill into the Earth?

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Western DeepGold Mine,

Witwatersrand,South Africa

Bertha RogersOil Well,

Oklahoma

KolaSuperdeep

Borehole, KolaPeninsula,

Russia

Ocean DrillingProgram Hole504B, Costa

Rica RiftAverage

Oceanic Crust

AverageContinental

CrustCore Mantle

Boundary

Dep

th in

Met

ers

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.

Question: How deep can we drill into the Earth?

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- seawaterYellow - sedimentGreen - basaltPale green - mantle

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Example 1: Understand what is being subducted in subduction zonesIt 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- seawaterYellow - sedimentGreen - basaltPale green - mantle

50

100

150

km

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.

http://ffden-2.phys.uaf.edu/645fall2003_web.dir/elena_suleimani/generation_small.mov

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://www.iodp.org/nantroseize/

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).

http://www.marine-geo.org/margins/

You can find more information about this project by following the links below to the Integrated Ocean Drilling Program and Margins Program

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

• 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.