Welcome to Mrs. Sippi's SmarterScience web site

Why does the Mariana Trench exist? That's geology!

What's the explanation of this remarkable crevasse in the earth's skin? Plate tectonics! The skin of the earth consists of separate plates that move relative to each other. Plates are formed at ocean ridges and consumed at trenches. The Mariana Islands owe their origin to subduction, the process of thrusting one plate under another. The Mariana Trench is the location where the Pacific Plate ducks steeply under the Philippine Plate.

Cross section: converging plates on the left and diverging plates on the right.

Plate tectonics is all about plate boundaries. There are three kinds of plate boundaries:

1. spreading centers and ridges, where plates separate (diverging plates);
   
   
2. trenches, where plates collide
   
   and where one ends up ducking under the other (converging plates)
   
   ...and where mountain chains can form (the Himalayas are the result of two plates bumping into each other);
   
   
3. faults such as the San Andreas, where plates slide past each other laterally.

Earthquakes occur where plates slide past each other. The hypocenters of earthquakes reveal the location of the plates, in this case of the subducted Pacific Plate. To a depth of about 100 km the plate is subducted at a gentle angle. From about 100 to 680 km the plate is subducted at a large angle.

A hypocenter is the real location of the earthquake. The epicenter is the spot at the earth's surface right above the hypocenter. Its vertical projection on the earth's surface.

The Mariana Islands are a classic example of an island arc. When a subducting plate gets deep enough, it gets hot enough for the plate to start melting. The relatively high water content of the subducting material (from the sea, after all) speeds up this melting process.

Rising magma derived from the subducting plate is lighter than the surrounding rocks and starts to move to the surface, where it forms volcanoes. Islands. A combination of the rising magma and other motions and tensions can lead to a spreading center: Where plates diverge. That is how so-called back-arc basins are formed.

Geochemists look at the chemical composition of geological materials: rocks (including soft stuff like sand and clay), minerals and melt inclusions. These give them clues about what happened at what temperature and at what pressure. It helps them reconstruct geologic history at a specific location.

The Japanese submersible Kaiko, sampling mud at the bottom of the Mariana Trench.
(Photograph courtesy and copyright Dr. Hideto Takami.)

A few numbers
Age of the Marianas: 155 MY

Velocity: 44 mm/yr

Back arc spreading velocity: 22 mm/yr

Angle: 64 degrees (less to a depth of about 100 km)

Vertical velocity: 5.9 mm/yr

ODP Leg 185
The Ocean Drilling Program (ODP) is the successor of the Deep Sea Drilling Program (DSDP). ODP LEG 185 was undertaken from February 16 - April 12, 1999. Leg 185 officially ended in Tokyo on June 14, 1999. The completion of the first Ocean Drilling Program expedition dedicated to the study of subduction factories. Its focus was the Mariana Trench and the Izu-Bonin Arc. More particularly, recycling (of water, carbon dioxide, uranium and lead). The drilling ship - the JOIDES Resolution - drilled at depths of about 6 km, which was quite a challenge. Co-chiefs were Dr. John Ludden and Dr. Terry Plank. Staff scientist was Dr. Carlota Escutia.

Leg-185 links:

• location map
• Scientific prospectus
• ODP Leg 185: Izu-Mariana Margin Logging Summary (LDEO)
• public information site
• Participants
• Preliminary reports (pdf file)
• Initial reports.