Table Mountain, South Africa

Watching a film set in Cape Town, South Africa, today, I wondered about the geology of Table Mountain, which looms above the city.  As is often the case, the natural history of that famous massif is complex.

About 500 million years ago, during the Cambrian Period, a rift valley developed in the southern portion of a continental plate that included today's Continents of South America, Antarctica and Africa.  Initially flooded by a shallow sea, the rift accumulated sediments over the next 100 million years.  Throughout the late Paleozoic and early Mesozoic Eras, this region was folded, lifted and eroded by tectonic forces as Earth's Continents merged into Pangea and then rifted apart.

Among the sedimentary rocks that formed within the ancient rift valley was a thick layer of hard quartzite sandstone; resistant to erosion, this Table Mountain Sandstone now outcrops as the steep cliffs of Table Mountain and the Cape Fold Ranges of western South Africa.  Of note, the western end of this sandstone formation is now found in Argentina, separated from its South African segment as the Atlantic Ocean opened, some 150 million years ago.

Evolution of Large Flightless Birds

Looking at the current distribution of ratites (large flightless birds including ostriches, rheas, emus and cassowaries) and understanding the history of Continental Drift, one is inclined to assume that a common ancestor of these birds lived on Gondwanaland (the unified Southern Continents) after the breakup of Pangea and that they diversified as the component Continents split apart.  Indeed, this has been the accepted theory of ratite evolution since the dawn of plate tectonics.

Pangea, formed by the merger of all major land masses during the Permian Period, split into Laurasia (the Northern Continents) and Gondwanaland (the Southern Continents) during the Triassic Period (some 200 million years ago -MYA).  Africa (the current home of ostriches) and South America (now inhabited by rheas), broke from Gondwanaland about 140 MYA and split from one another 100 MYA.  Madagascar (the former home of the elephant bird, an extinct ratite) rifted from Africa 160 MYA, joining India-Antarctica-Australia; India-Madagascar split from Antarctica and Australia about 90 MYA.  Finally, Australia (now inhabited by emus and cassowaries) broke from Antarctica 55 MYA and Madagascar split from India 75 MYA, drifting back toward the southeastern Africa Coast.

Fossil and recent molecular evidence suggest that ratites evolved from flight-capable species about 60 MYA, soon after the demise of the dinosaurs.  This suggests that the various ratites evolved in geographic isolation though, apparently, from a common flying ancestor (or family of ancestors); no doubt, the fossils of early ratites also lie beneath the ice of Antarctica.  If nothing else, the unfolding natural history of these birds illustrates the fact that common sense, devoid of scientific evidence, can be misleading; nevertheless, I suspect that the mystery of ratite evolution has yet to be fully solved.

Zealandia

Zealandia is a long, relatively narrow fragment of continental crust that split from Antarctica early in the Cretaceous Period (about 120 million years ago) and from Australia toward the end of the Cretaceous (about 80 million years ago as the Tasman Sea opened).  This continent, 93% of which is submerged beneath the sea, has since drifted northeastward and now stretches NW to SE, from the tropics, north of New Caledonia, to the sub-Arctic zone, southeast of New Zealand; while most of Zealandia is hidden by ocean waters, it covers an area half the size of Australia.

Northern Zealandia, which is composed of two parallel ridges separated by a long, narrow graben, lies on the Australian Plate while Southern Zealandia (from New Zealand's southern island southward) lies on the Pacific Plate; compression between these plates forced up the Alps on New Zealand's southern island and subduction of the Pacific Plate beneath the Australian Plate produced the volcanoes on New Zealand's northern island.  New Caledonia, New Zealand and numerous small islands are the only segments of Zealandia that currently poke above the southwestern Pacific.

Since rifting from Australia and Antarctica, the portion of Zealandia visible above the ocean has expanded and contracted as sea levels have fallen and risen, respectively.  As one might expect, marine sediments are thus found on New Caledonia and New Zealand while fossils of Mesozoic plants and animals from Australia and Antarctica have been discovered on those land masses, attesting to the origin of Zealandia.

Formation of the Indian Ocean

About 200 million years ago (MYA), the Tethys Sea split Pangea into Laurasia (the northern continents) and Gondwanaland (the southern continents); the latter included Africa, India, Antarctica, South America and Australia.  Since that time, the Indian Ocean has formed as these Southern Continents split from one another.

In the mid Jurassic Period, 150 MYA, the Atlantic Ocean began to form, splitting Europe from North America and Africa from South America.  By the early Cretaceous Period, some 130 MYA, a landmass including Madagascar, the Seychelle Islands and India rifted from the rest of Gondwanaland and, by 100 MYA, Africa split from Antarctica as the Southwest Indian Ridge began to form.  About 85 MYA, India rifted from the Madagascar-Seychelles land mass and Australia began to split from Antarctica along the Southeast Indian Ridge.  Finally, the Mid Indian Ridge (which trends NNW to SSE and connects the Southwest and Southeast Indian Ridges) began spreading as well, accelerating India's movement toward southern Asia; its northwestern extension, known as the Carlsberg Ridge and ending in the Gulf of Aden, began to open about 62 MYA and India finally slammed into Asia 55 MYA, lifting the Himalayas (a process that continues today).

As these four spreading zones continued to open, Africa drifted to the NNW, Antarctica moved southward, India plowed northeastward into Asia and Australia drifted eastward and then northeastward, all surrounding the vast Indian Ocean which has an average depth of 12,900 feet; the portion of the Indian Ocean south of the Southwest and Southeast Indian Ridges is often referred to as the Southern Ocean.  While the Carlsberg Ridge is no longer active, the Southwest, Mid and Southeast Indian Ridges continue to produce oceanic crust; in concert, the African Plate continues to move NNW (lifting the Alps and igniting volcanism in southern Europe) and Australia is drifting NNE.  Most evident are the earthquakes and subduction volcanoes along the western and southern rims of Indonesia, where the Australia Plate dips below the Eurasian Plate.

The Tasmanian Bridge

During the Triassic Period, about 200 million years ago (MYA), the supercontinent of Pangea split into Laurasia (the northern Continents) and Gondwana (the southern continents).  Throughout the Jurassic and Cretaceous Periods (190-65 MYA), as dinosaurs roamed the globe, Laurasia and Gondwana rifted into the continents that we recognize today.

Late in the Cretaceous, about 80 MYA, Australia and Antarctica remained connected but a rift began to form from west to east, opening a long, narrow bay between the continents; nevertheless, they retained a connection via the Tasmanian Bridge.  Throughout most of the early Cenozoic Era (65-40 MYA), Earth's climate was very warm and, despite its gradual movement to the south, the Australian-Antarctic land mass harbored diverse, temperate ecosystems.  Then, near the end of the Eocene (about 38 MYA), the southern end of the Tasmanian Bridge split from Antarctica, the Tasmanian Strait opened and Australia drifted toward the north (a process that continues today).  This produced a circumpolar ocean current around Antarctica, cooling that continent and Earth's climate as well; indeed, the Antarctic ice sheet began to form at that time.

The connection between Tasmania and the Australian mainland has since opened and closed as sea levels have fallen (during glacial epochs) and risen (during warm interglacial periods), respectively.  Humans reached Australia about 60,000 years ago, crossing (perhaps accidentally) from Indonesia; by 30,000 years ago, some of these native Australians had colonized Tasmania, then a peninsula of southeastern Australia.  About 12,000 years ago, as the last Pleistocene glaciers receded, sea levels rose and Bass Strait reformed, isolating the Tasmanian residents from mainland aborigines.

The Antarctic Plate

When the Tethys Sea opened, 200 million years ago (MYA), Pangea was split into Laurasia (the northern Continents) and Gondwanaland (the southern Continents); future Antarctica, as one might expect, was a component of Gondwanaland. Africa split from its neighbors as the South Atlantic opened, India broke free 80 MYA and Australia drifted away about 55 MYA. Finally, losing its connection with South America, Antarctica settled over the South Pole some 20 MYA.

All of this rifting has resulted from the opening of oceans and seaways (a process that continues today). The rift zones, known as oceanic ridges are not continuously active and, today, the Antarctic Plate is only rifting from the Pacific Plate (along the Pacific-Antarctic Ridge) and from the Australian Plate (along the Southeast Indian Ridge); its borders with the Nazca, South American and African Plates are characterized by a complex of compression faults, transverse faults and subduction zones. Since these margins lie deep beneath the Southern Ocean, they are not yet fully mapped and understood.

For much of the post-Pangea period, Antarctica remained in the Subtropical and Temperate Zones; it was thus home to a wide variety of plants and animals that inhabited these climatic regions. Then, as it drifted to the South Pole, Antarctica's fauna changed and, today, fossils of its warmer past lie entombed beneath its thick coat of ice and snow.

A Weddell's Winter

Should the cold, gray Midwestern winter begin to depress you, think of the Weddell seal. This large pinniped, the southernmost mammal on the planet, spends its winter in the dark, frigid waters beneath the Antarctic ice. In order to keep from drowning, these seals must keep their breathing holes open by gnawing away the sea ice that threatens to close off their air supply; over time, this activity wears away their teeth and, though most face little threat of predation, many end up starving to death.

When not tending to its vital portholes, the Weddell seal dives for fish, squid, octopi and small invertebrates, including krill. Protected from the cold by a thick layer of blubber, this true seal is able to dive 2000 feet below the surface and remain submerged for more than an hour; a rich supply of myoglobin in its body stores and releases oxygen to its muscles and vital organs, permitting these prolonged dives. Averaging ten feet in length, Weddell seals may weigh up to 1200 pounds; females are generally larger than the males.

Now, as we enter our winter season, Weddell seals are hauling out on the ice to deliver and raise their pups. Enjoying the Antarctic summer, they will bask in temperatures comparable to our Midwestern winter!

From Flight to Flippers

Fossil evidence suggests that ancestral penguins first appeared along the coasts of Antarctica and New Zealand about 65 million years ago. It is thought that they evolved from large sea birds, similar to the modern albatross; ancestral penguins apparently found that swimming and diving for their food was more efficient than flying. Likely resembling loons and cormorants in their earliest forms, penguins gradually lost their flight muscles and their wings regressed to shorter, more functional flippers; they also developed streamlined, fat-insulated bodies, equipping them for life in the sea.

When penguins first evolved, earth's climate was much warmer than it is today and there was no Antarctic glaciation. By the mid Eocene, 25 million years later, these marine birds had diversified, spreading northward to the South American coasts, to islands of the Indian Ocean, to southern Australia and to the southern coast of Africa. In the late Eocene and early Oligocene, a dramatic cooling of our planet's climate produced ice formation on Antarctica and forced penguin populations to adapt to local climate conditions. This broader range of regional climate led to the evolution of at least 40 penguin species by the Miocene Period (20 million years ago); seventeen species remain today, living in conditions ranging from the harsh, frozen world of Antarctica to the permanent summer of the Galapagos Islands.