M

82$ Spreading Planet

Second Minute Day Year Century Ten One M

Thousand Years Years

Years

Characteristic Temporal SExapenential Seconds)

FIGURE 6.1 The envelope of Earth system phenomena represented across exponential scales of time and space (Fig. 2.3). Geological processes transform oceanic and terrestrial environments over millions and billions of years (Fig. 1.6). Ecological processes operate, on the opposite end of the spectrum, in relation to the persistence of populations and their habitats over periods that may be shorter than seasons. Modified from Earth System Science Committee (1988).

Second Minute Day Year Century Ten One M

Thousand Years Years

Years

Characteristic Temporal SExapenential Seconds)

FIGURE 6.1 The envelope of Earth system phenomena represented across exponential scales of time and space (Fig. 2.3). Geological processes transform oceanic and terrestrial environments over millions and billions of years (Fig. 1.6). Ecological processes operate, on the opposite end of the spectrum, in relation to the persistence of populations and their habitats over periods that may be shorter than seasons. Modified from Earth System Science Committee (1988).

the spectrum of dynamic processes that activate the Earth system (Fig. 6.1). Just as the Earth can be portrayed over space in relation to density layers through the atmosphere and planetary interior (Figs. 1.1 and 1.2), time intervals are represented by depositional layers in rocks, sediments, ice, and various biota.

How can sediment cores and other stratigraphic records be used for interpreting environmental variability?

Over geological periods, from top to bottom, the Transantarctic Mountains contain strata that increase in age from several million to greater than 1 billion years (Fig. 3.1). In the East Antarctic Ice Sheet, there are superimposed layers of snow that have accumulated over several glacial-interglacial cycles during the past half-million years. Over ecological time scales, growth rings in trees and other living organisms reveal seasonal to millennial periods. In general, such strati-graphic sequences are progressively formed by relatively recent deposits overlaying older materials.

Beyond their chronological features, stratigraphic sequences also contain compositional information about the environmental conditions that existed when and where the strata were deposited. For example, extensive coal seams along the Transantarctic Mountains indicate that the environment was wet and warm enough to support the luxuriant growth of temperate forests more than 200 million years ago. Compiled in a global context—from ocean and land areas, lakes and ice, trees and other organisms—such proxy records provide clues for reconstructing the history of the Earth system and its varying environmental conditions over geological and ecological periods (Fig. 6.1).

Why would the Antarctic environment have been wet and warm in the geologic past?

continental separation

Viewed from outer space, the Earth is a blue planet covered mostly by oceans with a scattering of various land masses (Plate 1), which primarily occur in the northern hemisphere (Fig. 1.4). The largest land masses are continents, of which there are seven today: Africa, Antarctica, Asia, Australia, Europe, North America, and South America. Development of these continents and the oceans that separate them is a fundamental feature of the Earth system.

Two hundred million years ago, lush floras with large glossopterid ferns covered the Earth while dinosaurs roamed the land. By 65 million year ago, Glossop-teris and the dinosaurs had gone extinct. Nonetheless, fossils of these species can be found on all continents, including Antarctica. The vast seams of coal running through the Transantarctic Mountains (Fig. 3.1, top) also overlay the Beacon sandstone formation, which exists in Australia, Africa, and South America.

What does the distribution of extinct species and geological formations—across land masses that are now separated by oceans—suggest about past continental configurations and the dynamics of the Earth system?

To interpret the past configuration of the Earth's land masses, take a look at a world map that outlines the opposing continents across the Atlantic Ocean in the northern and southern hemispheres. This view of the planet, along with fossils and distinctive geological features that crossed continental boundaries, motivated Alfred Wegener (1880-1930) to speculate in 1912 that the continents had actually drifted apart from a supercontinent which he called Pangaea (all land).

Wegener argued that if the Earth could move vertically in relation to vertical forces, as with mountains, then it also could move laterally. However, largely because there was no known geophysical process that could laterally transport continents of rock through solid sea floor in a rigid Earth, Wegener's hypothesis of ''continental drift'' was rejected for the next half century.

In 1937, Alexander Du Toit (1878-1948) began to revive the ''continental drift'' hypothesis with geological and paleontological data from the Southern Hemisphere. However, it was not until 1962, when Harry Hess (1906-1969) developed arguments about sea-floor spreading, that the underlying mechanisms for ''continental drift'' began emerging. Rather than having the continents move actively through the ocean, as suggested by Wegener, Hess reasoned that the continents are being transported by giant convection cells within the Earth's interior that create and destroy crustal material (Fig. 6.2).

Support for the sea-floor spreading mechanism also came in the early 1960s with the discovery of mirror-image patterns of magnetic stripes on opposite sides of the 40,000 kilometer volcanic ridge that runs through the middle of the entire ocean. Locked into molten magma as it cooled and spread away from this mid-ocean ridge, these magnetic stripes represent reversals in the Earth's magnetic field as it switched between the north and south polar regions. Today, for example, the north geomagnetic pole is located around the latitude of 79° north (in the vicinity of northwest Greenland), which is more than 1000 kilometers away from the north geographic pole at 90° north. Additional evidence about sea-floor spreading was revealed by the increasing thickness and age of sedimentary deposits from the mid-ocean ridge toward the continental margins.

Subduction Zone

Oceanic

Mid-Ocean BasalticContinental Ridge Crust Granitic

Crust

Sea level

Subduction Zone

Oceanic

Mid-Ocean BasalticContinental Ridge Crust Granitic

Crust

Sea level

FIGURE 6.2 The plate tectonic mechanisms of sea-floor spreading and mantle convection that have been used to explain how oceans are created as continents drift apart. Crust and mantle of the Earth form the lithosphere, which varies in density and thickness between the oceans and continents (Fig. 1.2). Rigid lithospheric plates float on the molten mantle, which extends through the core of the Earth. Convection cells in the mantle, associated with upwelling magma at mid-ocean ridges and downwelling crust at subduction zones, push and pull the continents apart to account for their changing geometry through geologic time (Fig. 6.3). Modified from Wylie (1976).

FIGURE 6.2 The plate tectonic mechanisms of sea-floor spreading and mantle convection that have been used to explain how oceans are created as continents drift apart. Crust and mantle of the Earth form the lithosphere, which varies in density and thickness between the oceans and continents (Fig. 1.2). Rigid lithospheric plates float on the molten mantle, which extends through the core of the Earth. Convection cells in the mantle, associated with upwelling magma at mid-ocean ridges and downwelling crust at subduction zones, push and pull the continents apart to account for their changing geometry through geologic time (Fig. 6.3). Modified from Wylie (1976).

During the 1960s, seismic measurements through the Earth further revealed that the crusts of the continents and oceans were different. Continents have a relatively light granitic crust that is 30 to 40 kilometers thick. In contrast, the ocean basins have a relatively dense basaltic crust that is only 6 to 10 kilometers thick— which is why the continents are above sea level and the sea-floor is an average of 4 kilometers below (Fig. 1.3). Together, these oceanic and continental crusts compose the upper part of the lithosphere, which has a total thickness of 100 to 150 kilometers (Fig. 1.2).

In turn, the lithosphere is broken into seven major ''plates'' (along with several minor plates) around the Earth that are separated by ridges and fracture zones. These lithospheric plates are internally rigid and are floating on the underlying molten asthenosphere (Fig. 1.2) in continuous motion relative to each other and the Earth's rotational axis. In 1965, T. J. Wilson unified this global framework of lithospheric plate motion with observations of continental drift and sea-floor spreading into the theory of plate tectonics.

As hypothesized by Hess, plate tectonics works like giant convection cells: cool, rigid slabs of lithosphere flow downward through the asthenosphere, where they are melted and recirculated in the mantle up through the crust at the mid-ocean ridge (Fig. 6.2). Regions on the Earth where the lithosphere is subducting into the mantle occur at plate boundaries and are characterized by earthquakes and volcanism, such as along the ''ring of fire'' surrounding the Pacific Ocean. Subduction zones also create the deepest trenches in the ocean, such as the Marianas Trench, which exceeds 11,000 meters below the sea surface and is deeper than the height of Mt. Everest (Fig. 1.3).

To illustrate plate tectonics, consider a simple experiment the next time you take a bath. Place several pieces of paper (which correspond to rigid lithospheric plates) on the water surface. Then circle your left hand counterclockwise and your right hand clockwise (top to bottom) underneath the pieces of paper to mimic mantle convection cells upwelling toward the surface of the Earth at the mid-ocean ridge. Subsequent motion of the overlying paper pieces is analogous to the global geometry of lithospheric plates over geologic time.

Oceans with subduction zones spread faster than those without because they are being pulled apart as well as being pushed apart at the mid-ocean ridge. For example, the north Pacific Ocean is spreading 6 centimeters per year, in contrast to the Atlantic Ocean, which is spreading less than 2 centimeters per year in the absence of substantial subduction. Considering that the Pacific Ocean has an average width of 13,000 kilometers, at its current spreading rate it would take around 200 million years for this ocean basin to achieve its modern dimensions. In contrast, formation of the 5000-kilometer-wide Atlantic Ocean of today would have taken around 250 million years. These back-of-the-envelope estimates (Chapter 1: Global Dimensions) indicate that oceans are created over hundreds of millions of years.

Based on the various datasets associated with plate movements, it is now recognized that around 180 million years ago Pangaea fractured into two smaller

Early 0 Jurassic ~ 200 M.YA.

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