Rivers of Southeast Australia

Having watched two seasons of Secret City on Netflix, I naturally wondered about the source of the scenic lake in Canberra.  With a bit of online research, I found out that Lake Burley Griffin lies along the course of the Molonglo River; this stream rises in Tallaganda State Forest, on the west flank of the Great Dividing Range, southeast of Canberra.  After flowing northwestward to the city, it angles westward to merge with the Murrumbidgee River, the second longest in Australia.

Rising in the Snowy Mountains of New South Wales, the Murrumbidgee flows northward past Canberra ( in the Australian Capitol Territory) and then westward across the plains of southern New South Wales.  Almost 925 miles from its source, this river joins the Murray River (Australia's largest and longest) near Boundary Bend.  Unfortunately, a large portion of the Murrumbidgee's headwaters have been diverted into reservoirs, significantly reducing the flow through this ancient and once powerful river; nevertheless, severe floods have continued to plague valley towns during wet years (see La Nina and the Australian Floods).

The Murray River, 1558 miles in length, rises in the Australian Alps, ENE of Melbourne.  Flowing northwestward through the "Breadbasket of Australia," the Murray forms the boundary between New South Wales and Victoria.  At Mildura, the river angles westward, entering South Australia, and, at Morgan, it turns southward, flowing toward Lake Alexandrina (southeast of Adelaide) and thence to the Southern Ocean.  While Canberra lies relatively close to the Pacific Coast of Southeast Australia, its lake and river waters flow a very long distance before reaching the sea.

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 Stable Continent

Though it appears static during our brief human life spans, Earth's surface is continually molded by natural forces and many of these ongoing events place the lives of humans at risk.  Those who live in geologically active regions are at most risk; such areas include subduction zones, continental collision zones, rift valleys, transverse faults and active volcanic hotspots.

Most of the Continents harbor a number of these high risk areas.  Subduction zones (where oceanic plates dip beneath the Continents), which produce earthquakes, tsunamis and volcanism, line the Pacific rim, including the west coasts of North and South America and the eastern edge of Asia; they are also found along the southeast coast of Asia and in isolated areas of southern Europe and southwest Asia.  Collision zones are prominent across southern Asia (at the northern edge of the Indian Subcontinent), across the Middle East, through the Alps of southern Europe and in eastern Siberia.  Rift Valleys, sites of earthquakes and volcanism, are found in East Africa, in the Rio Grande Valley of the American Southwest and across Iceland.  Transverse faults, where one tectonic plate is scraping past another, include the San Andreas Fault of southern California and faults along the east and west sides of the Arabian Plate.  Potentially catastrophic volcanic hotspots are found in North America (Yellowstone) and Indonesia.

Though it is subject to floods, droughts, tropical storms and wildfires, Australia, which sits near the center of its tectonic plate (far from active margins), is relatively immune from earthquakes and volcanism; in that respect, it might be considered the most stable Continent on which to live (excluding Antarctica which, at least for now, does not invite human habitation).  On the other hand, old suture lines, potential sources for earthquakes, criss-cross the bedrock of all continents, new volcanic hotspots (resulting from mantle plumes) may develop almost anywhere and no place on Earth is immune from cosmic collisions with asteroids.

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.

Australia's Devonian Reef

The Kimberley Plateau of Australia, which forms the northwest corner of that Continent, is composed of ancient Precambrian rock.  During the Devonian Period (some 350 million years ago), when sea levels were significantly higher than today, the Plateau was ringed by a limestone barrier reef; lime-secreting bacteria and algae and primitive coral produced the reef as the first sharks and boney fish were evolving in Earth's oceans.  As sea levels fell and erosional debris accumulated, sections of that reef were incorporated into the mainland of Australia, now outcropping as limestone ranges along the southern edge of the Kimberley Plateau.

Several streams, rising on the Plateau, have cut spectacular canyons through the Devonian Reef complex (the Napier, Oscar and smaller ranges) as they flow southward into the Canning Basin; the latter is a layer cake of Upper Paleozoic and Mesozoic sediments, eroded from adjacent highlands.  Windjana Gorge (cut by the Lennard River), Gelkie Gorge (sculpted by the Fitzroy River) and Tunnel Creek are protected within National Parks and offer the best locations to explore the Devonian Reef.

Australia's Great Barrier Reef, the latest reincarnation of which has been forming over the past 8000 years, will eventually become part of the mainland as well.  By then, Australia, which is slowly drifting to the north, will be much closer to (if not contiguous with) Southeast Asia.

Australia's Great Central Ranges

While the Great Dividing Range runs up the east edge of Australia and various highlands are scattered along its periphery, the Continent is dominated by flat terrain; there are certainly no ranges comparable to the Andes, the Rockies, the Alps or the Himalayas in Australia.  However, that has not always been the case.

Back in the Precambrian Era, about 1 billion years ago, a massive, snow-capped range was lifted across the center of Australia, stretching east to west; while erosion began to demolish this lofty range as soon as it formed, subsequent uplifts occurred 500 million years ago (MYA) and 300 MYA.  Today, the remnants of those orogenies, known as the MacDonnell Ranges, are a swath of relatively low ridges and hills (with maximum elevations below 5000 feet) that stretch for 400 miles across the southern edge of the Northern Territory, just north of Alice Springs.  Another great mountain range, the Petermann Ranges, crumpled skyward about 600 MYA and its residual swath of hills and ridges (all below 4000 feet) now stretch about 200 miles, from the east-central border of Western Australia to the northwestern corner of South Australia.

These great ranges were forced up by continental collisions back when Australia was attached to Antarctica and, at times, to other land masses.  Once Australia broke from Antarctica (by 80 MYA) and drifted into prolonged isolation, it was no longer subjected to such tectonic forces and the great central ranges gradually eroded, spreading vast aprons of sediment across the Red Center of the Continent.  Today, Australia sits near the center of its vast tectonic plate, far from the subduction and collision zones that ring the edge of the plate.  Those who hope to see a towering alpine range in Australia will have to wait until one of its bordering oceans stops spreading and completely subducts, allowing the Continent to collide with Antarctica, Asia or, perhaps, the new Continent of East Africa.

Australia's Largest River System

Most of Earth's Continents are known for their major river systems; there is the Amazon Basin of South America, the Mississippi Watershed of North America, the Nile and Congo systems of Africa, the Rhine of Europe and the Ganges-Brahmaputra and Yangtze of Asia, among many others.  When it comes to Australia, however, a nation Continent known for its vast desert landscapes and sunny climate, rivers do not immediately come to mind.

The Murray-Darling River System of southeast Australia is the largest on that Continent.  The Murray River, which rises on the western slope of the Great Dividing Range in New South Wales and Victoria, is Australia's longest river; flowing westward to form the border between those two States, it then angles to the southwest across the southeastern corner of South Australia, entering the Southern Ocean south of Adelaide to complete its 1476 mile course.  The major tributary of the Murray is the Darling River, which merges with the Murray in western New South Wales; the Darling rises as numerous tributaries along the western slope of the Great Dividing Range, from southeastern Queensland through much of New South Wales, and along the southern flank of the Expedition Range, northwest of Brisbane.  While the Darling itself is "only" 915 miles long, the area of its watershed is larger than that of the Murray above their junction.

Since the Great Dividing Range lies close to the eastern coast of Australia and since rain-producing weather systems arrive from the east, the Murray-Darling watershed drains the dry-side of those highlands; numerous coastal rivers, rising on the east slope of the Divide discharge far more water to the sea than does the massive Murray-Darling River System.  Finally, this system, like the Mississippi-Missouri network of North America, highlights the fact that our longest continuous waterways do not always correspond to the course of our "longest rivers."  The latter is determined by how we choose to name the segments of any given watershed; the longest waterway in North America begins at the uppermost headwaters of the Missouri River and ends at the tip of the Mississippi Delta while the longest waterway in the Murray-Darling System begins in the mountains of southeast Queensland and ends at the mouth of the Murray River, on the coast of South Australia.

Australian Volcanism

Most of the active volcanoes on Earth occur along subduction zones of the Pacific Rim and western Indonesia.  Others rise above hotspots (both oceanic and terrestrial), atop mid-oceanic ridges (Iceland) and at continental rift zones (e.g. the East African Rift and Rio Grande Rift).  Examples of these geophysical processes can be found in parts of the Americas, Europe, Asia, Africa and across the major oceans of planet Earth; however, one does not generally associate volcanism with Australia, which lies far from active plate boundaries.

Nevertheless, volcanic terrain is found throughout eastern Australia, from northern Queensland to Tasmania and South Australia.  The many volcanic centers, now extinct or dormant, were produced by a cluster of hotspots over which the Continent has moved during the past 40 million years.  Since the Australian Plate is moving slowly to the NNE, the oldest volcanic sites are in the north while the youngest are near the southern coast; indeed, the most recent volcanic eruption on the Continent occurred more than 4000 years ago, in South Australia.  Some of the southernmost volcanoes are dormant (not completely extinct) and may erupt again.

While active volcanism is not currently found on the Australian Continent, it will eventually return when a dormant volcano reactivates or in the form of a new hotspot or rift zone, either of which would be triggered by a new mantle plume.  In the meantime, Australian Territory does possess active volcanism at the Heard and McDonald Islands, a volcanic island group in the Southern Ocean, some 2500 miles southwest of Perth and 1000 miles north of Antarctica.

Human Colonization of Australia

Current DNA evidence suggests that humans evolved in East Africa about 130,000 years ago and did not migrate from our home continent until 80,000 years ago; at that time, expansion of the Sahara would have discouraged migration to the north and it is thought that they followed the coast of the Red Sea (or crossed a shallow portion of that channel) and then the southern coast of Asia, reaching Indonesia about 70,000 years ago.

By that time, the Wisconsin Glaciation was underway and sea levels were much lower than they are today.  In fact, the islands of Indonesia were joined to form a broad peninsula and New Guinea was continuous with the Australian Continent.  While the oldest human fossils found in Australia date from 40,000 years ago, artifact evidence suggests that humans first reached that Continent between 60,000 and 55,000 years ago.  Since there is no evidence of human seafaring before the Phoenician and Polynesian cultures developed, some 4000-3500 years ago, it is presumed that the first human Australians, having crossed at least 50 miles of open sea, arrived accidentally, perhaps swept southward by a tropical storm.  Of course, others may have set out on rafts to find them and met the same fate.

Until additional evidence is discovered, the timing and circumstance of man's initial colonization of Australia will remain uncertain.  It is known (via DNA evidence) that native populations of New Guinea and Australia share a common ancestry (though the islands separated 8000 years ago as sea levels rose) and that humans occupied all regions of Australia, including Tasmania, by 30,000 years ago.  Of course, we also know that the earliest European settlement was established in 1788, at least 53,000 years after the first humans set foot on the Continent.

The Evolution of Corals

Corals are a diverse group of marine invertebrates that first appeared in the Cambrian Period, some 550 million years ago (MYA).  Fossils of these primitive tabulate corals are found in sedimentary rocks throughout the Paleozoic Era; a second group of ancient corals, the rugose corals, arose in the Silurian (400 MYA) and both groups apparently died out during the Permian Extinction, 225 MYA.

Early Triassic rocks are devoid of coral fossils but they reappear during the mid-late Triassic, some 210 MYA.  These scleractinian corals are the ancestors of all modern corals, having undergone cycles of expansion and near extinction as well as major periods of diversification, especially during the Jurassic (150 MYA) and the Miocene (25 MYA) Periods.  It was during the latter Period that the Great Barrier Reef began to form off the northeast coast of Australia, now composed of almost 3000 reefs that harbor at least 500 species of coral.

Marine biologists have cataloged about 70,000 species of coral across the globe.  While the great majority of corals are found in shallow, clear, warm waters of tropical and subtropical seas, there are coral species that occupy deep water and cold water habitats as well.  Unfortunately, these diverse communities are significantly threatened by human activity, primarily due to pollution and the effects of global warming.  At least 10% of modern coral reefs are dead and an increasing concentration of carbon dioxide in our oceans, which acidifies the seawater, impairs the formation of calcium carbonate shells, thus threatening corals and other shell-forming marine life.

Australia's Great Basin

The Lake Eyre Basin of east-central Australia is a vast topographic bowl within which streams drain toward the lowest part of the basin, never reaching the sea.  Covering 440,000 square miles from southwestern Queensland to South Australia and from the southeastern corner of the Northern Territory to the northwestern edge of New South Wales, most of it is dry, desert landscape through which ephemeral streams lead to Lake Eyre, in the southwest corner of the Basin.  Nearly dry and coated with salt flats most of the time, the lake fills only twice each Century (on average); composed of a large northern basin connected to a smaller southern basin by the Goyder Channel, Lake Eyre covers 3700 square miles and has an average depth of less than 10 feet (when full)  The lowest point of the lake basin, in Belt Bay of the northern portion, is 50 feet below sea level while the rim of the lake is 30 feet below the level of the sea.

The Lake Eyre Basin began to form about 200 million years ago, when Australia was part of Gondwanaland.  Tectonic forces caused the crust of this region to subside and, within another 100 million years, an arm of the sea invaded the basin; when uplift occurred along the northern and eastern margins of the basin, the sea drained away and rivers flowed across the region, depositing sediments on their way to the ocean.  During the middle of the Pleistocene, about 1 million years ago, uplift along the southern rim closed off the basin and all streams fed Lake Diers, the much larger predecessor of Lake Eyre (as Lake Bonneville preceded the Great Salt Lake in the U.S.).  As the climate became warmer and drier late in the Pleistocene and into the Holocene, the flow through the rivers diminished and eventually became sporadic.  Today, what little water reaches the lake is via three primary river systems: the Georgina River from the north, the Diamantia River from the northeast and Cooper Creek from the east.  Most streams from the west and northwest dry up before reaching Lake Eyre.

During those rare periods when monsoon rains or tropical storms fill Lake Eyre, this remote oasis attracts huge flocks of shorebirds, terns and Australian Pelicans that nest on the islands and feed in the shallows; how these birds know that the distant lake is full remains a mystery.  Lake Eyre National Park stretches along the east shore of the northern lake, just a short 435 mile drive north from Adelaide.  Major towns within the Lake Eyre Basin include Alice Springs, Mt. Isa, Longreach and Broken Hill.

Geology of Australia's Blue Mountains

The beautiful city of Sydney, Australia, sits on a thick slab of Triassic Hawkesbury sandstone, some 200 million years old. Below this bedrock are older Triassic sediments, shales, mudstones and conglomerates of the Narrabeen Group. All of these deposits, swept into the Sydney Basin by ancient rivers, lie atop Permian strata; containing seams of coal, these latter rocks were emplaced about 250 million years ago, when Earth's land masses had merged into the mega-continent of Pangea.

Just west of the Sydney metropolitan area are the scenic Blue Mountains, a broad, dissected plateau that rose during the Jurassic Period, some 170 million years ago, when Australia was still part of Gondwanaland. The geologic strata of the plateau is identical to the bedrock below Sydney; Triassic sandstone, capped at high points by Miocene basalt, forms massive cliffs which sit atop the older Narrabeen Group and underlying Permian deposits. Metamorphosed Paleozoic rock, from the Silurian and Devonian Periods, lies at the base of the Mountains.

As this elongated block of crust warped upward, vertical fractures developed in the sandstone cap, setting the stage for magnificent canyons to erode through the plateau as streams and rock falls gradually widened the gaps. Now home to a spectacular diversity of plant and animal life, much of the plateau, a component of the Great Dividing Range and named for the blue haze produced by its eucalytus forests, is protected as a World Heritage Area.

A Message from Christchurch

Today's tragic earthquake, in Christchurch, New Zealand, is just the latest reminder that the surface of the Earth continues to evolve and that we who live upon its moving plates are potential victims of the tectonic forces that mold our planet. Those of us who reside along the active margins of these plates, where collision or subduction are occuring, are at the greatest risk of earthquakes but the presence of old suture lines, aborted rifts and buried faults within the interior of continental plates make us all susceptible to some degree.

Active zones of volcanism and earthquakes are spaced along the Pacific Rim, popularly known as the Ring of Fire. In most of these areas, the Pacific Plate and its smaller associated plates are subducting beneath the South American, North American, Eurasian and Australian Plates, producing volcanic mountain ranges and triggering earthquakes that eminate from both the oceanic trenches and the rising peaks; the Andes, the Mexican Volcanic Belt, the Cascades, the Aleutians, the Japanese Islands, Taiwan, the Philippines, and the North Island of New Zealand have all formed (and continue to form)in this manner.

On the South Island of New Zealand, the Australian and Pacific Plates are colliding and scraping against one another, forcing up the scenic mountains of that island and setting the stage for catastrophic earthquakes. Today's quake, measuring 6.3 on the Richter scale, is thought to be an aftershock from the 7.1 quake last September; unfortunately, this one was both shallow (less than 3 miles deep) and close to Christchurch, resulting in extensive damage and at least 65 deaths. The message is clear: devastating quakes have and will continue to affect major urban centers across the globe as our planet evolves beneath our feet.

La Nina & the Australian Floods

The massive flooding across eastern Australia, which began in November, has been associated with the La Nina phenomenon, which tends to peak every 3 to 7 years. Produced by high pressure over the eastern Pacific and low pressure over the western Pacific, this weather pattern results in strong Pacific trade winds, which bring relatively warm ocean waters to the southeast coast of Asia and the northeast coast of Australia. This spawns strong cyclones and excessive rainfall in these areas, generally during an autumn to autumn cycle in the Southern Hemisphere.

Coinciding with a high Southern Oscillation Index, which measures the seasonal variance of sea surface pressure between Tahiti and Darwin, La Nina episodes trigger excessive precipitation across northern and eastern Australia. This year's flooding has been especially severe, disrupting transportation, stranding inland towns, inundating coal mines and wiping out much of the region's wheat crop. The Great Barrier Reef may also be affected, as plumes from the rivers of northeast Australia sweep particulates and pollutants toward that fragile ecosystem.

The current Australian flooding may prove to be the worst in recorded history. Unfortunately, some climatologists project that the La Nina and the opposite El Nino patterns will intensify with the advance of global warming. For eastern Australia, that could mean an alternating pattern of severe floods and prolonged drought.

New Zealand's Quake

Mention earthquake zones and most of us would think of Japan, Sumatra, Chile or Southern California; others might include western China, Pakistan and Iran, where the Indian and Arabian Plates are crunching into Eurasia. Few would include New Zealand but, as we learned yesterday, this is also earthquake country.

Deposited along the edge of future Antarctica throughout the Paleozoic Era, the sedimentary rocks of New Zealand rifted from the other Southern Continents about 85 million years ago, some 30 million years before Australia began its long isolation.
Lying along the boundary of the Australian and Pacific Plates, New Zealand straddles both of them; the North Island and the northwest section of the South Island lie on the Australian Plate while the remainder of the South Island lies on the Pacific Plate. Pressure and friction between the plates has lifted the "Southern Alps" of the South Island and subduction of the Pacific Plate beneath the Australian Plate produced the volcanic summits of the North Island.

Yesterday's 7.0 magnitude quake occurred on the South Island, west of Christchurch, where the Australian Plate is slipping northeastward along the Pacific Plate; this movement is triggered by sea floor spreading as the ocean between Australia and Antarctica continues to expand. Indeed, hundreds of earthquakes occur in New Zealand each year though most are too deep or too weak to feel at the surface. But, as in all earthquake zones, the next "big one" could strike anytime.

Uluru

The most recognizable natural symbol of Australia, Uluru, also known as Ayers Rock, rises more than 1100 feet above the arid terrain of the Continent's Red Center. Composed of arkosic sandstone (rich in feldspar), this spectacular formation developed over the course of a half a billion years.

As the Precambrian gave way to the Paleozoic Era, some 600 million years ago (MYA), ancient mountains to the west and southwest of Uluru were eroding into the Amadeus Basin, producing vast alluvial fans of debris. By the Ordovician, 500 MYA, seas invaded the region, compacting these deposits beneath layers of ocean sediment. During the Silurian Period, about 400 MYA, the region underwent compression, likely related to collision with other continents, uplifting and tilting the various layers of rock. Over the past 300 million years, softer overlying and surrounding sediments have eroded away, leaving the massive Uluru monolith, almost 6 miles in circumference. As large as it is, Uluru is but the tip of a sandstone formation which extends outward and downward for several miles.

Kata Tjuta (the Olgas), 16 miles west of Uluru, formed in a similar sequence but is composed of conglomerate rock. Both formations, sacred sites for regional Aboriginal tribes, are protected within the Kata Tjuta-Uluru National Park, a World Heritage Site; the Park is about 280 miles southwest of Alice Springs.

Isolation Lost

The land that would become New Zealand formed along the western coast of Gondwanaland throughout the Paleozoic Era. Beginning 500 million years ago and ending 100 million years ago, recurrent episodes of deposition, compression and uplift added this land to the great Southern Continent. Then, about 85 million years ago, future New Zealand was torn from Gondwanaland as the Tasman Sea began to open, triggering the Island's long period of isolation.

From 55 to 25 million years ago, the ocean gradually opened between Australia and Antarctica; this process increased tectonic forces in New Zealand, which lies along the boundary of the Australian and Pacific Plates. The North Island sits above the edge of the Australian plate; subduction of the Pacific Plate below the Australian Plate in this area has produced volcanism across the North Island. New Zealand's South Island, on the other hand, straddles the two plates and the prominant Alpine Fault marks the boundary; compression east of the fault forced up the Southern Alps while the western side of the Island creeps northeastward along the fault. South of the South Island, the Pacific Plate subducts beneath the Australian once again. There is certainly no mystery why New Zealand is prone to earthquakes!

Since New Zealand rifted into isolation 85 million years ago (a time when Tyrannosaurus rex ruled the land), its only native terrestrial mammals have been bats, which flew in more than 30 million years later; neither has it ever been home to snakes, which evolved after the rift occured. Indeed, New Zealand's native fauna has been limited to invertebrates, fish, amphibians, lizards, birds, bats and marine mammals (pinnipeds, whales and dolphins). Since they had no land predators, some of the birds became flightless, represented today by the kiwi and kakapo (a flightless parrot). Polynesian explorers finally arrived in 800 AD, making New Zealand the last land mass (other than Antarctica) to be colonized by man; bringing in dogs and rats, the Mauri soon altered the flora and fauna forever!

Island Refuge

Marsupials are pouch-bearing mammals such as opossums, kangaroos and wombats. After fertilization, the fetus initially grows within the uterus where it is nourished by a yolk sack (rather than a placenta); within 4-5 weeks, the immature fetus must leave the uterus and make its way to the maternal pouch where it attaches to a nipple for another month or more of development. This system imposes a much higher risk for the marsupial fetus compared with the fetal risk of a placental mammal; thus, in areas where they both exist, placental mammals tend to be more successful and often displace the marsupials.

Marsupials first appeared in the Cretaceous Period, some 100 million years ago. Whether they first evolved in North or South America remains a subject of controversy; nevertheless, the dirth of placental mammals in Gondwanaland (the attached land masses of South America, Antarctica, Australia, Madagascar and India) allowed the marsupials to thrive and disperse.
When marsupials first appeared, Africa had just separated from South America and the latter would break from Gondwanaland in another 25 million years. India also broke away about 85 million years ago, drifting northward with its cargo of marsupial fauna. Australia separated from Antarctica about 55 million years ago, moving toward the northeast and beginning its long history as an island continent.

Antarctica drifted to the South Pole, killing off the vegetation and all terrestrial fauna. India would begin crunching into southern Asia about 65 million years ago, forcing up the Himalayas and permitting an interchange of species between these land masses; as discussed above, the placental mammals would soon dominate. About 3 million years ago, South America connected with North America as the Isthmus of Panama drifted into place; the marsupials of South America would soon compete with eutherians (placental mammals) moving down from the north.

Australia thus remained the lone refuge for marsupials, allowing them to spread and diversify across the continent without competition. It was not until man arrived, about 60,000 years ago, that conditions changed; a combination of hunting and the use of fire to clear scrub are thought to have triggered the demise of the larger marsupial species. An additional threat was the introduction of dingos, which likely arose from domesticated dogs, within the last 10,000 years. The spread of urban centers, ranching and other human activities continue to pressure the native fauna, as evidenced by the extinction of the marsupial wolf in the 1950s. Nevertheless, Australia remains the last great refuge for marsupial life.