Chapter 1: The Earth in Context
Feature Articles
Drilling the Crust
by Stephen Marshak
Detail of drill stem and mud circulation
Credit:W. W. Norton
To drill a well in solid rock, workers generally use a rotary drill, which consists of a drill stem (a long steel tube, turned by a machine) connected to a diamond-studded drill bit, which grinds the rock at the base of the hole and transforms it into powder. Drill bits can grind rock because the diamonds in the bit are harder than rock. Because of the friction between rock and bit, the bit becomes very hot. To keep the bit from melting, and to flush the pulverized rock out of the hole they’re making, drillers constantly pump drilling mud, a mixture of water and clay, into the hole. Drilling mud flows down through the drill stem, out through holes in the bit, and up through the space between the stem and the hole wall.
Typical commercial water-well drillers penetrate less than 100 meters (m) into the ground in their quest for underground water. Oil-well drillers use much larger drills and commonly go to depths of 1,000 m, on occasion to depths of 4–8 km. When drilling into the Earth from a land-based site, workers simply set up a tower on the ground to hold their machinery. In the past few decades, however, the search for oil has led drillers to drill into the sea floor just off a coast. In shallow, near-shore waters, offshore drillers set up an anchored platform, whereas in deeper water, they work from gigantic drilling platforms that float. Some platforms are huge, housing crews of several dozen people at a time.
Geoscientists drill the crust simply to find out what’s below the surface, to learn from older rocks about changes that have taken place through Earth’s history. In the open ocean, scientists must set up their drilling rig on a ship. In the early 1960s, a group of scientists attempted to drill through the oceanic crust to reach the mantle. Their project came to be known as Project Mohole because they hoped to drill a "hole to the Moho." Unfortunately, the attempt failed. A successful program of ocean drilling, the Deep Sea Drilling Project (DSDP), began in the late 1960s. Year after year, the drilling ship Glomar Challenger crisscrossed the sea, ultimately drilling several hundred holes, from 300 m to 2 km deep, in water depths of up to 5 km. From these drill cores of ocean sediments, marine geologists were able to make many discoveries, such as the nature of climate changes over the last 150 million years, the causes of ice ages, the age of the ocean floor, and many other amazing revelations. In more recent years, scientists have used a larger ship, the Joides Resolution, for such open-ocean drilling; this ship provides the platform for the Ocean Drilling Project (ODP), the successor to DSDP.
Perhaps the most impressive on-land scientific drilling has been in operation for many years on the Kola Peninsula, north of the Arctic Circle in Russia. This superdeep hole reaches a depth of about 12 km and may ultimately reach 15 km. Unfortunately, drilling deep holes is prohibitively expensive (costing tens of millions of dollars per deep hole), and drilling holes much deeper than about 15 km simply can’t be done using available technology, because the drill bit melts and the cuttings in the hole can’t be flushed out.
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