The Breakup: Pangea comes apart

The super-continent, Pangea, lasted 100 million years. but, as we have seen, the Earth's crust is not static. The direction of plate movement shifted over time and the continents began to pull apart rather than converge. Rifts developed in the curst, eventually breaking completely through the crust and leading to the breakup of the supercontinent. Modern day rifting can be observed in the East African Rift Valley. Tension (two forces pulling in opposite directions) slowly began to pull North America away from the other merged continents. As the crust was pulled apart, it stretched, thinned and uplifted to the point of breaking.


Figure 1.19 and 1.20: As rifting occurred, blocks of crust slid down faults to form a basin. The basin was filled with sediments and lava flows. Eventually, the entire basin was tilted and faulted. Figure by J. Houghton.

The rifts occurred along a series of cracks in the Earth's crust roughly parallel to the present coastline. Along a series of faults, blocks of crust slid down the faults to form down-dropped basins bounded by tall cliffs that came to be known as Triassic rift basins (Figures 1.19 and 1.20). The eroding cliffs filled the adjacent basins with poorly sorted, red-colored sandstones and shales. These basin deposits are part of a sequence of rocks know as the Newark Supergroup, with thicknesses reaching up to 6 km in some places. Deposits are found in the Connecticut Valley of Massachusetts and Connecticut, and in the Newark Basin, which stretches from southeastern New York across New Jersey, Pennsylvania and Maryland. There are more Triassic rift basins located off the east coast that are buried by continental shelf sediments (Figure 1.21 ). During the Jurassic, the final break between the plates of North America, Africa and Baltica occurred many kilometers to the east of today's coastline at what is now the Mid-Atlantic Ridge. Other fragments of Pangea gradually broke into the modern continents, slowly moving into their present positions over the next several hundred millions of years.

Figure 1.21: The Triassic rift basins of the Northeast formed as North America broke away from Pangea during the Triassic and Jurassic. Figure by J. Houghton.

Figure 1.22: The softer sediments of the Newark rift basin were quickly worn away, forming valleys between the more resistant ridges of hardened lava flows. Figure by J. Houghton. 

The tension released by the pulling apart of plates, resulted in numerous faults and volcanoes. Alternating with the sandstone and shale being deposited in the basins were deposits of ash and lava flows originating from volcanoes along the rift area. The flay-lying beds of the Newark and Connecticut Valley basins were eventually faulted again and tilted, exposing the edges of the layers of sediment and cooled lava. The hardened lava was more resistant to the erosion that the sediments in the basin, so ridges of cooled lava were left standing as the sediments around them wore away (Figure 1.22).


Figure 1.23: Triassic: 273 million years ago.

Figure 1.24: Cretaceous: 94 million years ago.

As the supercontinent gradually broke apart, the continents moved into the geographic positions we see today (Figures 1.23 and 1.24). The Atlantic Ocean began to widen. The east coast of North America no longer experienced the strong tectonic activity associated with the compression and rifting of a plate margin. Instead, the tectonic activity gradually moved with the Mid-Atlantic Ridge hundreds of kilometers off the coast in the Atlantic Ocean. The Northeast, remaining a “passive margin” through to the present, began a long period of erosion that would continue through the Cretaceous and into the Tertiary. 

Figure 1.25: Pangea Breaks Up:

  • Pangea begins to split.
  • Rifts are created in the crust.
  • Triassic/Jurassic rift basins form.
  • Rift basins filled with sediments and lava flows.
  • Rift basins later tilted, faulted and eroded.
  • Long period of erosion.