Squashed coffee cups

The tiny cup on the right used to be identical to the standard 8 ounce Styrofoam coffee cup shown on the left for comparsion. The cup was taken to the seafloor at a depth of 4,000 meters (2.5 miles) in a mesh bag attached to Alvin. The crushing pressure (greater than 400 bars) collapased the pore space in the Styrofoam, resulting in a dense, leathery texture. The squashed cup has the same shape as the original because the pressure exerted on it was equal in all directions, causing it to shrink the same amount in all directions.

Geocompass

A typical geologist's Brunton compass (shown in Alvin's port side manipulator) cannot be used on the seafloor because of the extreme pressures, small size and required positioning against rock outcrops. The Geocompass (shown in Alvin's sample basket) allows geologists to measure the orientations of geological structures on the seafloor.

ROVs and manned submersibles make it possible to use many of the techniques of land geology on the seafloor. For example, making geological maps of seafloor exposures along mid-ocean spreading centers as well as other settings is now feasible.

A fundamental aspect of geological maps is the documentation of the orientation of various lines and planes in space. Planes are typically characterized by their strike and dip and lines by their trend and plunge. Sedimentary bedding, lava flow tops, dike margins, igneous layering, metamorphic foliations, joints and faults are just a few examples of planar structures observed along spreading centers.

Land geologists determine the orientation of outcrop-scale features with various types of hand-held compasses and inclinometers, like the Brunton Compass used widely in North America. Obviously, this type of instrument is not appropriate for use on the seafloor and different approaches are required to obtain orientation data.

A Geocompass being used on the seafloor on a previous trip to Hess Deep. The edge of the frame is held against a dike rock - a tabular, frozen, magma conduit.

For the past seven years we have been using a "Geocompass" built by engineers at Woods Hole Oceanographic Institution for use from manned submersibles. It consists of a magnetic compass and two inclinometers in a pressure housing set in an aluminum frame. Data are stored in an internal computer and also routed into the submarine's data-logging systems. In practice, data are recorded as the frame is held against an outcrop surface.

This very simple device has provided the first direct measurements of strike and dip on the seafloor and made it possible to collect fully oriented samples for paleomagnetic and microstructural studies. However, this approach has its drawbacks. The magnetic compass requires calibration for the magnetic field of the submersible. The effect of strongly magnetized rocks cannot be easily determined. The device must be placed against the rock surface requiring significant pilot skills as well as consuming valuable bottom time. It is rather cumbersome, takes up space in the sample basket, and requires an attached cable. This first-generation tool has demonstrated the feasibility and applications of collecting structural data from seafloor outcrops at spreading centers, transform faults and subduction zones, but new technology offers the possibility of greatly improved efficiency and accuracy.

New Laser Surface Mapping instrument designed and built by Harbor Branch Oceanographic Institution engineers.

Laser line scanners, developed by engineers at Harbor Branch Oceanographic Institution, provide one possible means of collecting orientation data and much more. These instruments rapidly scan surfaces to produce a digital map that can have 5 millimeter resolution at 2 meter range – without touching the seafloor. This type of instrument would make it possible to quickly and accurately image and determine the orientation of geologically relevant planes and lines on seafloor outcrops. Thousands of measurements could be made during a single seafloor traverse. It would greatly reduce uncertainties arising from various magnetic fields and provide a means of accounting for local effects of rough surfaces. It would also be capable of making quantitative measurements relevant to studies of slope morphology, sedimentology, hydrothermal vents, biology, structural geology and rock magnetism.

With funding from the National Science Foundation, engineers at Harbor Branch Oceanographic Institution are building a Laser Surface Mapping System that will be the next generation of seafloor geological compass. It will permit observers in manned submersibles or surface controlers of ROV's to essential "point and shoot" to acquire images from which orientation data can be obtained. We hope to use this system in upcoming research cruises as soon as it is tested during the summer of 1999.

Who was Harry Hess?

A longtime Princeton University geology professor, Harry Hess lived from 1906 to 1969. Iin 1960 Hess developed the hypothesis that new seafloor crust is created by the upwelling and spreading of lava along midocean ridges. During World War II, he rose to the rank of rear admiral and also discovered a class of flat-topped submarine seamounts, which he named "guyots." In 1945 he plumbed the greatest ocean depth, nearly seven miles.

How fast does the East Pacific Rise create new crust?

It is spreading out at a rate of 132 millimeters – or about 5.2 inches – a year, which is considered rapid. By comparison, the Mid-Atlantic Ridge spreads at a comparatively languid 10 to 20 millimeters (about 0.4 to 0.8 inches).