A s explained in Chapter 7, a thin layer of marine

sedimentary rocks blankets the interior regions of most continents. These rocks tell us that those places must have been below sea level when the sediment accumulated.

Tectonic activity can cause a continent to sink,

A narrow, low-volume

allowing the sea to flood a large area. However, at

mid-oceanic ridge results from slow spreading

particular times in the past (most notably during the

(a)

Cambrian, Carboniferous, and Cretaceous periods), seas flooded low-lying portions of all continents simultaneously. Although our plate tectonics model ex- plains the sinking of individual continents, or parts of continents, it does not explain why all continents should sink at the same time. Therefore, we need to explain how sea level could rise globally by hundreds

A broad, high-volume

of meters to flood all continents simultaneously.

mid-oceanic ridge results from rapid spreading

The alternating growth and melting of glaciers

(b)

during the Pleistocene Epoch caused sea level to fluc- tuate by as much as 200 meters. However, the ages of

Figure 1 (a) Slow sea-floor spreading creates a narrow,

most marine sedimentary rocks on continents do not

low-volume mid-oceanic ridge that displaces less seawater and

coincide with times of glacial melting. Therefore, we lowers sea level. (b) Rapid sea-floor spreading creates a

wide, high-volume ridge that displaces more seawater and raises

must look for a different cause to explain continental

sea level.

flooding. Recall from Section 11.4 that the new, hot lithosphere at a spreading center is buoyant, causing the

siderably more seawater than a low-volume ridge and mid-oceanic ridge to rise above the surrounding sea

would cause a global sea level rise. floor. This submarine mountain chain displaces a huge

Sea-floor age data indicate that the rate of sea- volume of seawater. If the mid-oceanic ridge were

floor spreading has varied from about 2 to 16 centime- smaller, it would displace less seawater and sea level

ters per year since Jurassic time, about 200 million would fall. If it were larger, sea level would rise.

years ago. Sea-floor spreading was unusually rapid The mid-oceanic ridge stands highest at the

during Late Cretaceous time, between 110 and 85 spreading center, where new rock is hottest and has

million years ago. That rapid spreading should have the lowest density. The elevation of the ridge de-

formed an unusually high-volume mid-oceanic ridge creases on both sides of the spreading center because

and resulted in flooding of low-lying portions of con- the lithosphere cools and shrinks as it moves outward.

tinents. Geologists have found Upper Cretaceous ma- Now consider a spreading center where spreading is

rine sedimentary rocks on nearly all continents, indi- very slow (e.g., 1 to 2 centimeters per year). At such

cating that Late Cretaceous time was, in fact, a time

a slow rate of spreading, the newly formed litho- of abnormally high global sea level. Unfortunately, sphere would cool before it migrated far from the

because no oceanic crust is older than about 200 mil- spreading center. This slow rate of spreading would

lion years, the hypothesis cannot be tested for earlier produce a narrow, low-volume ridge, as shown in

times when extensive marine sedimentary rocks accu- Figure 1. In contrast, rapid sea-floor spreading, on the

mulated on continents.

order of 10 to 20 centimeters per year, would create a high-volume ridge because the newly formed, hot

DISCUSSION QUESTION

lithosphere would be carried a considerable distance What factors other than variations in sea-floor away from the spreading center before it cooled and

spreading rates and growth and melting of glaciers shrunk. This high-volume ridge would displace con-

might cause global sea-level fluctuations?

188 CHAPTER 11 OCEAN BASINS

䊳 11.5 SEDIMENT AND ROCKS OF

Layer

A s explained in Chapter 7, a thin layer of marine