Facts of the Matter
Researchers dig deep to find Earth's makeup
THE EARTH has three major layers: the outer crust, the mantle and the core. We have direct access only to the crust, which is half as thick in proportion to the earth as the skin of an apple. It would really help us to understand many different aspects of Earth's composition and processes if we could get some samples from the mantle.
The thickness of the crust varies. Under the continents, where it is stable and easy to drill, it ranges from 10 to 40 miles. Oceanic crust is much thinner, averaging about three miles, but it is difficult to drill the sea floor from a ship that is being tossed by waves and wind.
The deepest hole drilled on land was more than 40,000 feet, but the 7-mile-deep hole on the Kola peninsula near the Norwegian border with Russia drilled through thick, granitic continental crust.
Although seven miles might seem like a deep hole, it is only a little less than two-tenths of 1 percent of the distance to the center of the earth, the equivalent of about seven inches to the length of a football field.
We know quite a bit about the mantle as it is, but the evidence is indirect. With our understanding of Earth's physical and chemical properties, we can draw conclusions about the composition of the mantle, but a sample would either support or contradict our inferences.
We know that the mantle is different from the crust. There is a distinct boundary between the crust and mantle where the speed of earthquake waves changes.
Our current knowledge of the mantle comes from putting together its seismic properties as compared with fragments of supposed mantle rock that have been brought to the surface in volcanic eruptions. These "xenoliths" have the same seismic properties as the mantle, and their composition is consistent with chemical models that are based on laboratory experiments to determine what kinds of materials could be a source of magma in the mantle.
We know quite a bit more about the composition of the crust from direct observations on land and from drilling on land and at sea.
At various locations around the globe are massive chunks of ancient sea floor that were broken and raised above sea level by the moving sea floor. Similar structures are embedded in mountain ranges worldwide.
The island of Cyprus in the Mediterranean is one example. The island is a 3,571-square-mile piece of oceanic crust and overlying sediment that was tilted and lifted more than 6,500 feet above sea level over a 70 million-year period. Exposed on the island is a complete cross-section, from its sedimentary cover down to the crust-mantle boundary.
Deep-sea drilling gives us other kinds of information about the earth.
Cores taken across the Atlantic Ocean in the 1970s verified sea floor spreading and the process by which new sea floor is created by volcanic eruptions along mid-ocean ridges, which stretch around the earth 50,000 miles like the seams on a baseball. As magma rises along the ridges from the mantle to erupt as lava on the sea floor, it pushes the older sea floor outward.
The verification of sea floor spreading from analysis of the cores led to the theory of plate tectonics, which is a unifying theory of the earth, as significant to Earth sciences as were Newton's revelations on gravity to physics and astronomy.
The oceanic crust and overlying layers of sediment contain the history of the sea floor, which in turn reflects the history of the earth over 200 million years. Sediments contain microfossils of extinct life forms that show evolutionary change and are indicators of past climate change.
From chemical information in the sediment, we can interpret and model the kinds of interactions that have occurred and can occur within Earth's ocean-climate system and use that knowledge as one tool to assess the range of possibilities of future climate change.
Both the sediments and the crustal rock below contain small crystals of natural magnets that reveal the intensity and direction of Earth's magnetic field at the time they were formed. Reversals of the magnetic field occur irregularly and are preserved in rock as distinctive stripes of magnetic polarization on the spreading sea floor like the lines on a bar code that can be correlated with the same reversal events in continental rocks.
The sediments also contain particles of clay from rivers on a distant continent that might now be vastly different from how it was when the particles entered the ocean. There are also remnants of wind-blown dust from the continents in the sedimentary layers. Micrometeorites that bombard the earth accumulate at the rate of 40,000 tons per year, mostly in the oceans. These occur in the sediment as fine metallic or rocky pieces less than 1 millimeter in size, and provide information about the compositions of extraterrestrial matter.
The ability to drill into the sea floor is a recent technology in an ongoing study since man first threw a weighted rope into the water to measure its depth. Early scientific studies were unorganized and not very accurate, consisting of soundings taken by European ships as they sailed for the New World.
HMS Challenger conducted the first worldwide scientific global survey from 1872 to 1876. It crudely measured water depth but was mostly concerned with the biology and chemistry of sea water.
There was no real knowledge of the sea floor until the middle of the 20th century. The sonar that located submarines in World War II also could map the topography of the sea floor, and ships began routinely mapping bottom topography after the war.
The United States began a project to drill through the crust into the mantle in 1961. This project, called "Mohole," was intended to drill through the shallow crust under the Pacific Ocean off Hawaii, but it was abandoned in 1966 because of lack of funding.
The first deep-sea drilling took place in 1968. The Glomar Challenger, a ship fitted with a drilling rig and computer stabilizer, could bore a hole 3,000 feet deep in the sea floor below 2.5 miles of sea water. For 15 years it drilled and brought back core samples for analysis.
The JOIDES Resolution was the next advancement in deep-sea drilling, capable of drilling through nearly 7,000 feet of sediment and rock. Since 1985 it has drilled 2,000 holes in 700 sites and collected 40,000 cores that total 150 miles in length.
The new Japanese drilling ship Chikyu is a significant step in the quest to read the stories of the oceanic crust and its overlying sediments. Able to drill 4.5 miles into the sea floor, it will vastly increase the repertoire of data from which we can learn more about our planet's history, composition and processes.
With any luck, Chikyu will bring back pieces of the mantle for the first time.
Richard Brill, professor of science at Honolulu Community College (home.honolulu.hawaii.edu/~rickb
), teaches Earth and physical science and investigates life and the universe. His column is published on the first and third Sundays of every month. E-mail questions and comments to firstname.lastname@example.org
picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at email@example.com