Elemental distribution

Ocean floors are composed of a dark, fine-grained rock called basalt that is more depleted in silicon and potassium and is richer in magnesium and iron than are the abundant light-colored granitic rocks on the continents. Igneous rocks that form where one oceanic plate is being thrust below another are generally intermediate in composition. Certain ore deposits occur only where specific plate tectonic processes take place, thereby enabling a geologist to focus the search for these deposits.

Earth's Oceanic and Continental Crusts

The surface of the earth may be broadly divided into the oceanic crust and the continental crust. The oceanic crust is on the average “heavier,” or denser, than the continental crust. Both the continental and oceanic crusts are less dense than the underlying rocks in the earth's mantle. The continental and oceanic crusts can thus be considered a lower-density “scum” floating on the denser mantle, somewhat analogous to an iceberg floating in water. Because the denser oceanic crust sinks lower into the mantle than the continental crust, much of the oceanic crust is covered by the oceans, but the less dense continental crust is mostly above the level of the oceans. Also, seismic waves from earthquakes indicate that the oceanic crust is much thinner (about 6 to 8 kilometers) than the continental crust (about 35 to 50 kilometers). The density difference between the oceanic and continental crusts is related to the kinds of minerals composing them. The oceanic crust contains more of the denser iron- and magnesium-rich minerals, olivine (iron and magnesium silicate) and pyroxene (calcium, iron, and magnesium silicate), than does the continental crust. The continental crust contains much more of the less dense minerals, quartz (silica) and alkali feldspar (potassium, sodium, and aluminum silicate), than does the oceanic crust. In addition, the oceanic crust contains much of the feldspar called calcium-rich plagioclase (calcium, sodium, and aluminum silicate) than does the continental crust.

This difference in mineralogy between the oceanic and continental crusts is reflected in their average elemental composition. The oceanic crust is enriched in elements concentrated in olivine, pyroxene, and calcium-rich plagioclase, and the continental crust is enriched in those elements concentrated in quartz and alkali feldspar. Thus, the continental crust contains larger concentrations of silicon dioxide (60 weight percent in the continental crust versus 49 weight percent in the oceanic crust) and potassium oxide (2.9 versus 0.4 weight percent) and lower concentrations of titanium dioxide (0.7 versus 1.4 weight percent), iron oxide (6.2 versus 8.5 weight percent), manganese oxide (0.1 versus 0.2 weight percent), magnesium oxide (3 versus 6.8 weight percent), and calcium oxide (5.5 versus 12.3 weight percent) than does the oceanic crust. The other major elements, aluminum and sodium, are fairly similar in concentration in both the oceanic and continental crusts.

The mantle is even denser than the crust, since it contains the dense minerals olivine, pyroxene, and garnet (magnesium and aluminum silicate) in the rock called peridotite. It does not contain the less dense minerals, quartz and feldspar. Thus, the mantle is even more enriched in iron oxide and magnesium oxide and more depleted in potassium oxide, sodium oxide, and silicon dioxide than are the crustal rocks.

Composition of Oceanic Crust

Oceanic and continental crusts also vary substantially in composition. The continental crust is considerably more heterogeneous than is the oceanic crust. The oceanic crust consists of an upper sediment layer (about 0.3 kilometer thick), a middle basaltic layer (about 1.5 kilometers thick), and a lower gabbroic layer (about 4 to 6 kilometers thick). Basalts and gabbros both contain olivine, pyroxene, and calcium-rich plagioclase. They differ only in grain size; the basalts contain considerably finer minerals than do the gabbros. The basaltic and gabbroic layers are thus very similar in composition. They are also of fairly constant thickness across the oceanic floors. The gabbroic layers disappear over oceanic rises, or linear mountain chains on the oceanic floors. The basaltic rocks are believed to form at the rises by about 20 to 30 percent melting of the underlying peridotite in the upper mantle. The newly formed oceanic crust and part of the upper mantle are believed to be slowly transported across oceanic floors, at rates of about 5 to 10 centimeters per year, to where this material is eventually subducted or thrust underneath another plate.

The thickness of sediment on ocean floors varies considerably. It is nearly absent over the newly formed basalts at oceanic rises. It is thickest in basins adjacent to continents where weathering and transportation processes carry large amounts of weathered sediment into the basins. The composition of oceanic floor sediment varies as well. It contains varied amounts of calcite or aragonite (calcium carbonate minerals), silica (silicon dioxide), clay minerals (fine, aluminum silicate minerals derived from weathering), volcanic ash, volcanic rock fragments, and ferromagnesian nodules.

Finally, a few volcanoes composed of basalt form linear chains on the ocean floor, away from the rises or subduction zones such as the Hawaiian Islands. These ocean-floor basalts are similar in composition to those at oceanic rises, except that they contain greater amounts of potassium. The amount of basaltic rocks produced by these ocean-floor volcanoes is insignificant, however, compared to the vast amounts of basalt produced at oceanic rises.

Composition of Continental Crust

In contrast to the oceanic crust, the continental crust is quite heterogeneous in mineralogy and chemical composition. About 75 percent of the surface of the continents is covered by great piles of layered rocks called sedimentary rocks. The average thickness of these sedimentary rocks on the continental crust is only about 1.8 kilometers, although they may locally range up to 20 kilometers in thickness. The main kinds of sedimentary rocks on the continents are the very fine-grained shales or mudrocks (about 60 percent of the total sedimentary rocks), the coarser-grained sandstones (about 20 percent of the total), and limestones or dolostones (about 20 percent of the total).

The shales or mudrocks are composed of very small grains of mostly clay minerals and quartz. The resultant composition of the shales is often high in the immobile elements, aluminum and potassium, and low in the mobile elements, sodium and calcium.

Sandstones vary in composition depending on which rocks weather to form the sandstone, the distance of the sandstone from the source, and the intensity of weathering. Sandstones formed close to a source of granitic rocks may have a composition similar to that of the granitic rock: high in silicon and potassium and low in magnesium, iron, and calcium compared to basaltic rocks. Sandstones formed a long distance from the source have more time to be weathered. Thus, these sandstones may have most of the unstable minerals weathered away to clays or soluble products in water (for example, sodium), and they may be enriched in silicon because of the abundance of the stable mineral quartz.

Limestones typically form in warm, shallow seas by the action of organisms to produce most of the calcium carbonate in these rocks. Thus, limestones are enriched in calcium and depleted in most other elements. The dolostones are enriched in magnesium as well as calcium.

Some places, such as the Great Plains in the United States, consist mostly of alternating limestones and shales formed in ancient, shallow seas. (Thus, the average composition of the surface rocks in these areas may be considered an average of that of shale and limestone in whatever proportion they occur.) The average composition of sedimentary rocks on the continents is significantly different from that of the granitic rocks that weathered to form them. The average sedimentary rocks on continents are much more enriched in calcium (because of carbonate rocks), carbon dioxide (also because of carbonate rocks), and water (because of incorporation in clay minerals), and they are depleted in sodium (because of its solubility).

The thickness of these sedimentary rocks is still small compared to the 35- to 50-kilometer thickness of most of the continental crust. Only about 5 percent of the continental crust by volume is composed of sedimentary rocks. Most crustal rocks are igneous rocks or their metamorphic equivalents. Metamorphic rocks form in the solid state at high temperatures and pressures because of their deep burial in the earth. A substantial percentage of these igneous rocks of the upper continental crust are either granitic rocks (quartz and alkali feldspar rock) or andesitic rocks (plagioclase-rich rock). Basaltic rocks probably compose only about 15 percent of the upper continental crust.

Continental Margins and Rift Zones

Most of the granitic rocks and andesites originally formed along subduction zones, where oceanic crust is being thrust or subducted below either oceanic or continental crust. There also may be some basalts formed along these subducted plates. These basalts, andesites, and granitic rocks that formed along continental margins may eventually be plastered along the edges of the continents, resulting in the gradual growth of the continents. Other basalts are formed in portions of continents, called continental rifts, that are being stretched apart much like taffy. These basalts are considerably more enriched in potassium than basalts formed on ocean floors. For example, a large fraction of the states of Washington, Oregon, and Idaho is covered with these rift basalts extruded as lavas since about 20 million years ago. The total volume of about 180,000 cubic kilometers for these basalts is still comparatively insignificant; therefore, basalts make only a small contribution to the composition of the average upper continental crust.

Composition of the Middle and Lower Crust

The composition of the lower continental crust is much more difficult to determine than that of the upper continental crust because the rocks forming the lower crust are not exposed at the surface. Estimates of about 50 percent granitic and 50 percent gabbroic rocks in the lower crust have been reached. Thus, the lower continental crust is more enriched in the basaltic components, calcium, magnesium, iron, and titanium, and depleted in the granitic components, potassium and silicon, than is the upper continental crust.

The average compositions of the middle and lower oceanic and continental crusts are difficult to determine because they cannot be directly sampled. Much of the information about the nature of the crust below the surface comes from the behavior of seismic waves given off by earthquakes, from heat-flow measurements, and from the composition of rock fragments brought up by magma passing through much of the crust. In addition, there are places in the crust where rocks from the lower crust have been uplifted to the surface, so their composition can be examined in detail.

The speed of the earthquake waves through the oceanic crust is consistent with the crust being composed of a thin upper layer of sediment (indicated by P-wave velocities of 2 kilometers per second), a thicker middle layer of basalt (P-wave velocities of 5 kilometers per second), and a thick lower layer of mostly gabbro (P-wave velocities of 6.7 kilometers per second). The thicker continental crust, however, has P-wave velocities (6.1 kilometers per second) consistent with mostly granitic rocks below the overlying sedimentary rock veneer (2 to 4 kilometers per second). The lower continental crust has P-wave velocities (6.7 kilometers per second) similar to those expected for lower-silica rocks like gabbro, so there is probably more gabbro mixed with granitic rocks in the lower crust.

Heat-Flow Measurements

How fast heat flows out of the earth may also be used to estimate the composition of crustal rocks. Variation in heat flow at the surface depends on how much heat is flowing out of the earth below the crust; the distribution of radioactive elements in the crust, such as uranium, thorium, and potassium, that give off heat; and how close magmas are to the surface. Oceanic ridges and continental rift zones, for example, have high heat flow, suggesting that magmas are close to the surface. In contrast, the heat loss from much of the ocean floor and over much of the continents with old Precambrian rocks (older than about 600 million years) is considerably lower because of the lack of magma close to the surface. It is surprising, however, that the oceanic floor and continents with old Precambrian rocks have similar low heat flow, as the abundant granitic rocks in the continents ought to be more enriched in the heat-producing radioactive elements than is the oceanic crust. That suggests that many of the granitic rocks at depth in these parts of the continental crust are depleted in radioactive elements, perhaps because of melting processes carrying away the radioactive elements in the magmas during the Precambrian. Also, this finding is consistent with the presence of abundant basaltic rocks depleted in radioactive elements in the lower crust.

Glimpses into Earth's Interior

There are places on the earth, such as the island of Cyprus in the Mediterranean Sea, that appear to be ruptured portions of the entire oceanic crust and part of the upper mantle. In Cyprus, the lower zone is composed of peridotite, olivine-rich rocks, or pyroxene-rich rocks, as are predicted to occur in the mantle. These rocks correspond to the P-wave seismic velocities of 8 kilometers per second. There is a rather abrupt change to the next overlying layer of mostly gabbros that correspond to the sharp decrease in P-wave velocities to about 6.7 kilometers per second. These rocks grade upward into basalt corresponding to the upper igneous rock layers of the oceanic crust with P-wave velocities of about 5 kilometers per second. The basalt and gabbros are also penetrated by a multitude of tabular igneous dikes that were feeders of magma to the overlying basalt at the surface. Finally, there are overlying sedimentary rocks corresponding to the upper oceanic layers with P-wave velocities of about 2 kilometers per second.

Deep drill holes provide scant information about the composition of the crust at depth. Drill cores provide mostly information about sedimentary rocks; they also give some information about the first igneous rocks just below the sedimentary cover. Unfortunately, deep drilling is costly and limited in depth and distribution. Generally, wells are never drilled deep enough to obtain samples from the intermediate and lower crust, and none reach the mantle.

Some volcanoes derive their magma from the upper part of the mantle. These volcanoes often bring up fragments of mantle material and random samples of crustal rocks thrown from the volcanic conduits' walls. Although volcanic vents of this kind are not common and sample an extremely small distribution of lower continent and upper mantle material, they are extremely important.

Finally, meteorites are used as analogies of the interior of the earth. Iron meteorites, rich in iron and nickel, are thought to approximate the composition of the core of the earth. Stony meteorites with compositions close to that of peridotite are thought to match the composition of the mantle of the earth.

Guide to Ore Deposits

A knowledge of the overall distribution of rock types and the corresponding elemental compositions of these rocks over the earth can give geologists a guide to where to look for certain kinds of ore deposits, as certain ores occur in certain kinds of rocks. The most generalized pattern is the association of certain types of ores with certain tectonic environments. Both oceanic rises and subduction zones tend to heat waters and drive the resultant metal-rich waters toward the surface. Oceanic rises often contain sulfide-rich, copper and zinc hot-water deposits. These hot-water deposits at subduction zones are often enriched in copper, gold, silver, tin, lead, mercury, or molybdenum.

One example of subduction zone deposits is the copper porphyry deposits. These important ore deposits are formed in granitic rocks that crystallized at shallow depths below the surface in areas where an oceanic plate is being subducted, or thrust below a second plate. They are especially abundant around the rim of the Pacific Ocean. The copper ores contain low copper concentrations (0.25 to 2 percent) and have some associated molybdenum and gold. These low-grade ores are often profitable to mine because of the large volume of ore (over a billion tons in some places) that can be rapidly extracted from the rock. A geologist looking for such ores designs an exploration campaign to search out only areas with active or inactive subduction zones. Also, the geologist looks for certain compositions of granitic rocks intruded at fairly shallow depths below the surface that have been exposed to erosion near the top of the intrusion, as these are the places where the copper porphyries form. Hundreds of these copper porphyry deposits have been discovered, accounting for about half the copper ores of the world. Copper is used in wires to transmit electricity and in bronze and brass.

Principal Terms

andesite: a volcanic rock that is lighter in color than basalt, containing plagioclase feldspar and often hornblende or biotite

basalt: a dark-colored, volcanic rock containing the minerals plagioclase feldspar, pyroxene, and olivine

granitic rock: a light-colored, intrusive rock containing large grains of quartz, plagioclase feldspar, and alkali feldspar

limestone: a sedimentary rock composed mostly of calcium carbonate formed from organisms or by chemical precipitation in oceans

P waves: the first waves from earthquakes to arrive at a seismic station; because they travel at different speeds through different types of rock, they may be used to deduce the rock types below the surface

peridotite: a dark-colored rock composing much of the earth below the crust; it usually contains olivine, pyroxene, and garnet

plate tectonics: the theory that assumes that the earth's crust is divided into large, moving plates that are formed and shifted by volcanic activity

sandstone: a sedimentary rock composed of larger mineral grains than those forming shales, such that they are deposited from faster-moving waters

sedimentary rock: a flat-lying, layered rock formed by the accumulation of minerals from air or water

shale: the most abundant sedimentary rock, composed of very tiny minerals that settled out of slowly moving water to form a mud

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