OBJECTIVES

To explore what Earth's largest surface features reveal about its interior

CONCEPTS/VOCABULARY

BACKGROUND

As recently as the 1960s, nearly 2/3 of the Earth's solid surface was unmapped. Before then, almost nothing was known about the topography or rock types beneath the deep ocean, though there was little reason to suspect that the geology of the ocean floor was much different from that of the continents. But defense-related mapping during the Cold War showed the ocean floor to be a wholly unfamiliar realm. An understanding of the fundamental differences between continents and oceans was critical to the discovery of plate tectonics.

EXERCISES

A. Twin Peaks. Because Venus is the planet most similar to Earth in size and composition, there has been significant interest in understanding the evolution of its crust and mantle. The spacecraft Pioneer (late 1970s) and later Magellan (early 1990s) revealed distinct high-standing areas called terrae (Latin for "land") on Venus' surface. These initially appear to be analogous to Earth's continents, but surface elevation histograms for Venus and reveal that the similarity is only superficial (see figure below).

2. Explain the significance of the surface elevation distribution on Earth.

3. What are the most common ranges of elevations on Earth -- i.e. the elevations associated with the two tallest bars on the plot (the modes)?

4. So what is the typical difference in elevation between Earth's continents and oceans?

B. Vive la difference. The differences in elevations between the continents and ocean basins reflect the fundamentally different origins and compositions of continental and oceanic crust.

1. Examine the hand specimens of rock types representative of the continental and oceanic crust. See if you can identify individual minerals in the rocks. On a separate sheet, make notes and sketches that would enable you to identify these rocks and minerals again.

2. The fact that Earth's low-density granitic continents stand above the denser basaltic ocean basins suggests that both are "floating " on a still denser layer. The continental and oceanic rocks constitute only the outermost skin of the solid Earth, the crust. Continental crust is typically 30-60 km thick, while oceanic crust is only 5-10 km thick. Beneath the crust lies the denser mantle, made up mainly of iron and magnesium-rich silicate minerals. Though solid, the mantle is hot enough to flow over long periods of time, just as glacial ice flows as a solid.

Part of the uppermost mantle, the asthenosphere, is particularly weak and will flow when subjected to uneven crustal loads, much as the water in a waterbed readjusts when you lie on it. Over time, this kind of readjustment in the mantle creates a state of balance, called isostasy, in which the total mass of rocks above the asthenosphere is the same from place to place.

Imagine 2 columns of rock -- one on a continent the other in the ocean -- extending from the Earth's surface to the top of the asthenosphere. At that depth (about 100 km below the seafloor), the total masses of the columns must be equal:

B. 2. Isostasy, continued

a. How much is Dh, the difference in elevation between continents and ocean basins that arises from their differences in density? To find out, we need mathematical expressions for the total mass of the rocks in the two columns. As a first step, write an equation for the mass of an object in terms of its density and volume:

Mass = (1)

b. Now write an expression for the total mass of Column 1. Let V_c = Volume of continental crust and V_mc = volume of mantle between the base of the continental crust and the asthenosphere. Then write a similar expression for Column 2.

Total mass of Column 1 = + (2)

Total mass of Column 2 = + (3)

If we make the columns square in map view, 1 km on a side, we can ignore their width and breadth and substitute their thicknesses (T_c, T_o, T_mc, T_mo) for their volumes. Rewrite your equations for the total masses making these substitutions.

Total mass of Column 1 = + (2a)

Total mass of Column 2 = + (3a)

c. Values for most quantities in these equations are shown in Fig. 2. The values of T_o and T_mc, however, are not known. To constrain the values of these unknown quantities we need additional equations that contain them. The value of T_o can be found by writing an equation that relates the thicknesses of oceanic crust and underlying mantle to the depth of the asthenosphere. Write this equation and determine the value of T_o.

T_o =

The value of T_mc (thickness of mantle between the continental crust and the asthenosphere) can be found by setting equations (2a) and (3a) equal to each other (remember, the masses of the 2 columns must be the same) and solving for T_mc. Show your work.

T_{mc =}

d. Finally, determine the value of Dh ,the difference in elevation between continents and oceans, by writing an equation relating the thicknesses of continental crust and underlying mantle to the depth of the asthenosphere.

D_{h =} ____ _km

Is this comparable to the value you determined previously (page I)?

e. You have shown that the different elevations of continents and oceans arise from their contrasting densities and the existence of a weak layer in Earth's mantle. Explain how the principle of isostasy can also account for the existence of the Lake Superior Basin (which is underlain by basalts of the Keweenaw rift).

C. Exceptions and extremes

1. Some continental crust (crust that is granitic in composition) lies below sea level.

a. Using the surface elevation data for Earth in Figure 1, estimate what percent of Earth's surface area lies between 0 and 1000 m below sea level:

b. Study the seafloor map in the lab (or the similar figure in your text), and describe the settings in which this shallowly submerged continental crust occurs.

c. The shallow continental shelves are the repository for most of the sediments that are shed from the continents. They are also areas of tremendous biological productivity, and as a result, they host some of the world's largest petroleum and natural gas fields. Examine the specimens of sedimentary rocks typical of the continental shelf. Make notes and sketches that would enable you to identify these rocks and minerals again.

d. On the seafloor map, identify 2 places where large amounts of sediment have been delivered to the deep sea floor. Why do you think this has occurred?

e. The extent of the shelves depends on sea level. How would sea level be affected if the average density of oceanic crust were slightly lower or higher than the value in Figure 2? How would this affect the extent of the continental shelves? Explain your reasoning.

2. Some oceanic crust (basaltic in composition) stands above the mean depth of the ocean.

a. Using the surface elevation data for Earth in Figure 1, estimate the percent of Earth's surface area that lies between 1000 and 3000 m below sea level:

b. Consult the ocean floor map and describe where this high-standing oceanic crust occurs.

c. What does the height of these parts of the ocean floor suggest about the density of the oceanic crust there? Explain.

d. Name 2 places along the mid-ocean ridge system where oceanic crust stands above sea level.

3. A small percentage of the Earth's surface area lies well outside the averages. Using the surface elevation plot (Fig. 1), the seafloor map, and the global physiographic map in your text, identify the highest continental area and deepest oceanic area.

Highest area on Earth's surface: Elevation:

Lowest area on Earth's surface: Depth:

R1. Granite: Igneous rock representative of the average composition of Earth's continental crust. Rich in the silicate minerals feldspar (especially potassium and sodium feldspars), quartz, hornblende and mica.

R2. Gabbro and Basalt: Igneous rocks representative of Earth's oceanic crust (and lunar lowlands). Gabbro and basalt differ only in crystal size. Both are composed mainly of the silicate minerals feldspar (especially calcium or calcium/sodium feldspar) and pyroxene (especially augite).

Almost all of the major minerals in igneous rocks at Earth's surface are silicates -- that is, minerals composed largely of silica (SiO₄) -- silicon and oxygen arranged in a tetrahedral pyramid, with oxygen atoms at the 4 corners and a silicon atom nestled in the middle. The arrangement of silica tetrahedra determine the physical properties of the minerals.

M1. Feldspar: A large group of tectosilicate minerals (silica tetrahedra arranged in 3-D frameworks), with a range of compositions, including:

a. Potassium feldspar (Orthoclase): KAlSi₃O₈

b. Sodium feldspar (Albite): NaAlSi₃O₈

c. Calcium feldspar (Anorthite): CaAl2Si₂O₈

_{M2. Quartz (SiO2): Another tectosilicate, and one of the most common minerals inrocks at Earth's surface. Gemstones amethyst, aquamarine, and tiger eye are all forms of quartz.}

M3. Mica: Hydrous silicate minerals in which silica tetrahedra are joined into sheets with weak bonds between them. Common varieties are:

a. Biotite (dark-colored): K(Mg, Fe)₃(Al,Fe) Si₃O₁₀(OH,F)₂

b. Muscovite (light-colored: KAl₂(AlSi₃O₁₀)(OH)₂

M4. Amphibole, variety hornblende Ca₂(Mg, Fe, Al)₅ [(Si, Al₄)O₁₁]₂(OH)_2:

A mafic mineral (rich in iron and magnesium) in which silica tetrahedra are arranged in double chains. Note that cleavage traces on crystal cross sections are at angles of about 56° and 124° (compare with pyroxene).

M5. Pyroxene, variety augite Ca(Mg, Fe)Si0₃: A mafic mineral, in which silica tetrahedra are arranged in single chains. Note that cleavage traces on crystal cross sections are at nearly right angles

Rivers carry sediment from the continents to the coasts, where they accumulate on the continental shelf. The coarsest particles settle close to shore; finer ones can be carried further offshore. The following sediment types are listed according to increasing distance of deposition from shore.

A. Sandstone: Rock composed of sand-sized grains (1/16-2 mm), representing high-energy near-shore environments (surf zone)

B. Siltstone/mudstone: Rock composed of clay-sized grains (<1/16 mm), deposited in quiet water and/or deep water.

C. Limestone: Rock composed of chemically precipitated calcite (CaCO3). Calcite precipitation can occur only where there is little or no suspended sediment in the water column. Limestone can be readily identified by its tendency to effervesce in dilute hydrochloric acid. When acid is applied to the rock, bubbles of ancient atmospheric carbon dioxide (CO2) are released.