5.9 – How and why did Pangaea start breaking apart? What forces cause continents to begin rifting?


5.9 – How and why did Pangaea start breaking apart? What forces cause continents to begin rifting?

1) Mantle Plumes and Gravity Sliding
2) Slab Pull and Slab Suction

Why the supercontinent of Pangaea began to split apart nearly 200 million years ago is not known with certainty. Nevertheless, this event serves to illustrate that perhaps most ocean basins get their start when a continent begins to break apart. This clearly is the case for the Atlantic ocean, which formed as the Americas drifted from Europe and Africa. It is also true for the Indian Ocean, which developed as Africa rifted from Antarctica and India.

It seems likely that super continents existed sporadically during the geologic past. Pangea, which was the most recent of these, was assembled into a supercontinent between 450 and 230 million years ago, only to break up again shortly thereafter. Geologists have concluded that the formation of a supercontinent, followed by continental splitting, must be an integral part of plate tectonics. Furthermore, this phenomenon must involve a major change in the direction and nature of the forces that drive plate motion. Stated another way; over long periods of geologic time, the forces that drive plate motions tend to organize crustal fragments into a single supercontinent, only to change directions and disperse them again. Two mechanisms have been proposed for continental rifting: 1) plumes of hot mobile rock rising from deep in the mantle, and 2) forces that arise from plate motions.





1) Mantle Plumes and Gravity Sliding

Recall that a mantle plume consists of hotter than normal mantle rock that has a large mushroom-shaped head, hundreds of kilometers in diameter attached to a long, but narrow, trailing tail. As the plume head nears the base of the cool lithosphere, it spreads laterally. Decompression melting within the plume head generates huge volumes of basaltic magma that rises and triggers volcanism at the surface. The result is a volcanic region, called a hotspot, that can be more than 1,000 miles across.

Research suggests that mantle plumes would tend to concentrate beneath a supercontinent, because once assembled, a large landmass forms an insulating blanket that traps heat in the mantle. The resulting temperature increase would lead to the formation of mantle plumes that serve as heat dissipation mechanisms.

Evidence for the role that mantle plumes play in continental rifting is available from passive continental margins, the former sites of rifting. In several regions on both sides of the Atlantic Ocean, continental rifting was preceded by crustal uplift and massive outpourings of basaltic lava. Examples include the Etendeka flood basalts of Southwest Africa and the Parana Basalt province of South America.

The Etendeka and Parana Basalts point to a corresponding origin of South America and Africa.

About 130 million years ago, when South America and Africa were joined as a single land mass, vast outpourings of lava produced a large continental basalt plateau. Shortly after this event, the South Atlantic began to open, splitting the basalt province into what is now the Etendeka and Parana Basalt plateaus. As the ocean basin grew, the tail of the plume produced a string of seamounts on each side of the newly formed ridge. The modern area of hotspot activity is centered around the volcanic island of Tristan da Cunha, which is located on the Mid-Atlantic Ridge.

Greenland splits from Northern Europe; Iceland is an oceanic ridge Hotspot

About 60 million years ago, yet another mantle plume is thought to have initiated the rifting of Greenland from northern Europe. Volcanic rocks associated with this activity extend from Eastern Greenland to Scotland. The hotspot associated with this event is presently situated beneath Iceland.

From these studies, geologists have concluded that mantle plumes have played a role in the development of at least some continental rifts. In these regions, rifting began when a hot mantle plume reached to the base of the lithosphere and caused the overlying crust to dome and weaken. As the crust was buoyantly uplifted, it stretched and developed rifts similar to those in present-day East Africa. Simultaneously, decompression melting of the plume head led to vast outpourings of basaltic lavas. Following these episodes of igneous activity, an ocean basin began to open. The proposed mechanism for rifting is gravity sliding off the uplift caused by plume buoyancy.

It is important to note that not all hotspot volcanism leads to rifting. For example, vast outpourings of basaltic lava that constitute the Columbia River basalts in the Pacific northwest, as well as Russia’s Siberian traps, are not associated with the fragmentation of a continent.

2) Slab pull and Slab suction

It is generally agreed that tensional forces, which tend to elongate or pull apart a rock unit, are needed in order for a continent to be fragmented. But how do these forces originate?

Recall that old oceanic lithosphere subducts because it is denser than the underlying asthenosphere. In situations where a continent is attached to a subducting slab of oceanic lithosphere it will be pulled toward the trench. However, continents overlie thick sections of lithospheric mantle. As a result, they tend to resist being towed, which creates tensional stresses that stretch and thin the crust. Whether slab pull can tear a continent apart is still being studied. Perhaps other factors, including the presence of hot spots, or an inherent weakness in the crust, such as major fault zones, may contribute to rifting.

Investigators have suggested that during the breakup of Pangea, the Americas were rifted from Europe and Africa as a result of another force — slab suction. Recall that when a cold oceanic slab sinks, it causes the trench to retreat or roll back. This creates flow in the asthenosphere that pulls the overriding plate toward the retreating trench.

During the breakup of Pangaea, a subduction zone extended along the entire western margin of North and South America. As this subduction zone developed, the trench slowly retreated westward toward a spreading center located in the Pacific. Modern remnants of this subduction zone include the Peru-Chile trench, Central American trench, and Cascadia subduction zone. Slab suction along the entire western margin of the Americas may have provided the tensional forces that rifted Pangea.

In summary, continental rifting occurs when a land mass is under tension, which tends to elongate and thin the lithosphere. This mechanism may be aided by a series of hot spots that weaken and elevate the crust.