VUME Upper Mantle of the Earth

Scientific Ocean-floor Dredging

Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. The some of them are the samples recovered from the ocean floor by dredging and drilling.
Underwater excavation is called dredging. Dredging is a process that involves the aquatic excavation of water beds to remove sediments, pollutants, shellfish and other materials.
A dredge is a machine that scoops or suctions rocks from the bottom of ocean. The methods and machinery used in dredging vary widely. Most dredging is done by ships that tow a dredge along the ocean floor. Self standing dredges and dredge pumping stations are used for routine tasks. A dredge, which is the catch all term for the different types of machinery that perform dredging, can cut away sediment, scoop materials out like a back hoe or suction them through a large pipe to be deposited into a ship, barge or other containment system. Dredging can be used to remove desired materials from the waterbed such as minerals.
While the onboard instrumentation of modern dredges is computer assisted, the basic excavation methods of dredges have remained the same since the late 1800s. The three main types of dredges are mechanical dredges, hydraulic dredges, and airlift dredges.
Mechanical Dredges - Mechanical dredges remove material by scooping it from the bottom and then placing it onto a waiting barge or into a disposal area. The two most common types of mechanical dredges are dipper dredges and clamshell dredges. They are names for the type of scooping buckets they employ.
Hydraulic Dredges - Hydraulic dredges work by sucking a mixture of dredged material and water from the channel bottom. The amount of water sucked up with the material is controlled to make the best mixture. Too little water and the dredge will bog down; too much water and the dredge will not be efficient in moving sediment. Pipeline and hopper dredges are the two main types of hydraulic dredges.

Airlift Dredges - Airlift dredges are special use dredges that raise material from the bottom of the waterway by hydrostatic pressure. They have cylinders that operate like pistons. Material is drawn through the bottom of the cylinder. When it is full, the intake valve closes, trapping the material. Then, compressed air forces the material out through a discharge line to a waiting dump scow or directly to a disposal site. Airlift dredges bring dredged material to the surface with a relatively small amount of water, which is good when environmental contamination is an issue.

Ocean Drilling

Tiny fossils in the core sample tell us what the ocean environment was like at different points in time, and at different spots around the globe.
Deep ocean drilling started in April 1957 when self-declared eclectic group of leading scientists gathered one sunny morning in La Jolla, California for breakfast at Dr. Walter Munk’s (Scripps Institution of Oceanography) house. Their adventures—test drilling the AMSOC (American Miscellaneous Society) hole in Puerto Rico and drilling on a modified Navy freight barge, the CUSS 1.
The first attempt to drill through the sediments and basement of the oceanic crust was undertaken as part of Project Mohole. Mohole proved that deep-ocean drilling was a viable means of obtaining geological samples. The project retrieved, for the first time, a sample of the second layer under the softer sediments of the ocean floor. This layer, whose presence had previously been indicated to scientists only by reflections of seismic soundings, was revealed as basalt, a hard rock formed by the solidification of molten material within the Earth. The finding of basalt, which we now know continuously pours from fracture zones on the ocean floor, provided a major piece of evidence supporting the theory of continental drift.
On drill ships, the sediment and rock cores are brought up from the bottom through the inside of the drill pipe sections. Once on the deck of the ship, they are split in half. One half is studied in the ship's laboratories. The other is stored in special repositories, often called core libraries. There are three such libraries in the United States, on the East, West and Gulf coasts, and one in Bremen, Germany. Scientists from all over the world can come to these libraries and examine cores from all over the oceans-much the way you might go to a library to find a book. These core repositories will be a very valuable scientific resource for many years to come.