Page 72 - 4567

The most recent period of rifting began about 20 million years ago as upwelling in the
mantle intruded the base of the lithosphere. Buoyant uplifting of the heated lithosphere led to
doming and stretching of the crust. Consequently, the upper crust was broken along high-angle
normal faults, producing downfaulted blocks, or grabens, while the lower crust deformed by
ductile stretching.
In the early stages of rifting, magma generated by decompression melting of the rising
mantle rocks intruded the crust. Occasionally, some of the magma migrated upward along
fractures and erupted at the surface. This activity produced extensive basaltic flows within the
rift as well as volcanic cones— some forming more than 100 km from the rift axis. Examples
include Mount Kenya and Mount Kilimanjaro, the highest point in Africa, rising almost 6000 m
above the Serengeti Plain.
RED SEA. Research suggests that if spreading continues, a rift valley will lengthen and
deepen, eventually extending to the margin of the continent. At this point, the continental rift
becomes a narrow linear sea with an outlet to the ocean, similar to the Red Sea. The Red Sea
formed when the Arabian Peninsula rifted from Africa beginning about 30 mln years ago. Steep
fault scarps that rise as much as 3 km above sea level flank the margins of this water body. Thus,
the escarpments surrounding the Red Sea are similar to the steep cliffs that border the East
African Rift. Although the Red Sea reaches oceanic depths (up to 5 km) in only a few locations,
symmetrical magnetic stripes indicate that typical seafloor spreading has been occurring for at
least the past 5 mln. years.
ATLANTIC OCEAN. If spreading continues, the Red Sea will grow wider and develop
an elevated oceanic ridge similar to the Mid-Atlantic ridge. As new oceanic crust is added to the
diverging plates, the rifted continental margins gradually recede from the region of upwelling.
As a result, they cool, contract and sink.
Over time, continental margins subside below sea level and material eroded from the
adjacent highlands blanket this once-rugged topography. The result is a passive continental
margin consisting of a rifted continental crust that has been covered by a thick wedge of
relatively undisturbed sediment and sedimentary rock. Not all continental rift valleys develop
into full-fledged spreading centers.

Task 2. Build up a glossary to the most important terms used in the text.

Task 3. Name a modern example of a continental rift.

Task 4. Look at Figure 12.1. Briefly describe each of the four stages in the evolution
of an ocean basin.

TEST YOURSELF

Task 1. Tell whether the sentences are true or false. Correct the false ones.
1. A plan of the site is prepared and registered with the appropriate government agency.
2. A reserve pit usually holds unneeded drilling mud, cuttings, and other materials from the
well.
3. The start of drilling a well is called spudding in.
4. If the oil-water or gas-water contact can be located at least on two sides of the discovery
well, the area of the field can be determined.
5. Cable tool drilling is very fast.
6. Cable tool drilling effectively controls subsurface pressures that is why blowouts are not
common during cable tool operations at all.
7. Today, almost all wells are drilled with rotary drilling rigs.
8. Where the lithosphere is thick, cool, and strong, rifts tend to be broad.