Madagascar
Madagascar the Republic of Madagascar, known as the Malagasy Republic, is an island country in the Indian Ocean 400 kilometres off the coast of East Africa. The nation comprises the island of Madagascar and numerous smaller peripheral islands. Following the prehistoric breakup of the supercontinent Gondwana, Madagascar split from the Indian subcontinent around 88 million years ago, allowing native plants and animals to evolve in relative isolation. Madagascar is a biodiversity hotspot; the island's diverse ecosystems and unique wildlife are threatened by the encroachment of the growing human population and other environmental threats. The first archaeological evidence for human foraging on Madagascar may have occurred as much as 10,000 years ago. Human settlement of Madagascar occurred between 350 BC and 550 AD by Austronesian peoples, arriving on outrigger canoes from Borneo; these were joined around the 9th century AD by Bantu migrants crossing the Mozambique Channel from East Africa. Other groups continued to settle on Madagascar over time, each one making lasting contributions to Malagasy cultural life.
The Malagasy ethnic group is divided into 18 or more subgroups, of which the largest are the Merina of the central highlands. Until the late 18th century, the island of Madagascar was ruled by a fragmented assortment of shifting sociopolitical alliances. Beginning in the early 19th century, most of the island was united and ruled as the Kingdom of Madagascar by a series of Merina nobles; the monarchy ended in 1897 when the island was absorbed into the French colonial empire, from which the island gained independence in 1960. The autonomous state of Madagascar has since undergone four major constitutional periods, termed republics. Since 1992, the nation has been governed as a constitutional democracy from its capital at Antananarivo. However, in a popular uprising in 2009, president Marc Ravalomanana was made to resign and presidential power was transferred in March 2009 to Andry Rajoelina. Constitutional governance was restored in January 2014, when Hery Rajaonarimampianina was named president following a 2013 election deemed fair and transparent by the international community.
Madagascar is a member of the United Nations, the African Union, the Southern African Development Community, the Organisation Internationale de la Francophonie. Madagascar belongs according to the United Nations. Malagasy and French are both official languages of the state; the majority of the population adheres to traditional beliefs, Christianity, or an amalgamation of both. Ecotourism and agriculture, paired with greater investments in education and private enterprise, are key elements of Madagascar's development strategy. Under Ravalomanana, these investments produced substantial economic growth, but the benefits were not evenly spread throughout the population, producing tensions over the increasing cost of living and declining living standards among the poor and some segments of the middle class; as of 2017, the economy has been weakened by the 2009–2013 political crisis, quality of life remains low for the majority of the Malagasy population. In the Malagasy language, the island of Madagascar is called Madagasikara and its people are referred to as Malagasy.
The island's appellation "Madagascar" is not of local origin but rather was popularized in the Middle Ages by Europeans. The name Madageiscar was first recorded in the memoirs of 13th-century Venetian explorer Marco Polo as a corrupted transliteration of the name Mogadishu, the Somali port with which Polo had confused the island. On St. Laurence's Day in 1500, Portuguese explorer Diogo Dias landed on the island and named it São Lourenço. Polo's name popularized on Renaissance maps. No single Malagasy-language name predating Madagasikara appears to have been used by the local population to refer to the island, although some communities had their own name for part or all of the land they inhabited. At 592,800 square kilometres, Madagascar is the world's 47th largest country and the fourth-largest island; the country lies between latitudes 12°S and 26°S, longitudes 43°E and 51°E. Neighboring islands include the French territory of Réunion and the country of Mauritius to the east, as well as the state of Comoros and the French territory of Mayotte to the north west.
The nearest mainland state is Mozambique, located to the west. The prehistoric breakup of the supercontinent Gondwana separated the Madagascar–Antarctica–India landmass from the Africa–South America landmass around 135 million years ago. Madagascar split from India about 88 million years ago during the late Cretaceous period allowing plants and animals on the island to evolve in relative isolation. Along the length of the eastern coast runs a narrow and steep escarpment containing much of the island's remaining tropical lowland forest. To the west of this ridge lies a plateau in the center of the island ranging in altitude from 750 to 1,500 m above sea level; these central highlands, traditionally the homeland of the Merina people and the location of their historic capital at Antananarivo, are the most densely populated part of the island and are characterized by terraced, rice-growing valleys lying between grassy hills and patches of the subhumid forests that covered the highland region. To the west of the highlands, the arid terrain slope
Jurassic
The Jurassic period was a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era known as the Age of Reptiles; the start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, the Tithonian event at the end; the Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations; the Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified. By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north, Gondwana to the south; this created more coastlines and shifted the continental climate from dry to humid, many of the arid deserts of the Triassic were replaced by lush rainforests.
On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, the evolution of therian mammals, including primitive placentals. Crocodilians made the transition from a terrestrial to an aquatic mode of life; the oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates. The chronostratigraphic term "Jurassic" is directly linked to the Jura Mountains, a mountain range following the course of the France–Switzerland border. During a tour of the region in 1795, Alexander von Humboldt recognized the limestone dominated mountain range of the Jura Mountains as a separate formation that had not been included in the established stratigraphic system defined by Abraham Gottlob Werner, he named it "Jura-Kalkstein" in 1799.
The name "Jura" is derived from the Celtic root *jor via Gaulish *iuris "wooded mountain", borrowed into Latin as a place name, evolved into Juria and Jura. The Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations known as Lias and Malm in Europe; the separation of the term Jurassic into three sections originated with Leopold von Buch. The faunal stages from youngest to oldest are: During the early Jurassic period, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana; the Jurassic North Atlantic Ocean was narrow, while the South Atlantic did not open until the following Cretaceous period, when Gondwana itself rifted apart. The Tethys Sea closed, the Neotethys basin appeared. Climates were warm, with no evidence of a glacier having appeared; as in the Triassic, there was no land over either pole, no extensive ice caps existed.
The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of that future landmass was submerged under shallow tropical seas. In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation; the Jurassic was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus common, along with calcitic ooids, calcitic cements, invertebrate faunas with dominantly calcitic skeletons; the first of several massive batholiths were emplaced in the northern American cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny. Important Jurassic exposures are found in Russia, South America, Japan and the United Kingdom.
In Africa, Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well studied in Africa. Late Jurassic strata are poorly represented apart from the spectacular Tendaguru fauna in Tanzania; the Late Jurassic life of Tendaguru is similar to that found in western North America's Morrison Formation. During the Jurassic period, the primary vertebrates living in the sea were marine reptiles; the latter include ichthyosaurs, which were at the peak of their diversity, plesiosaurs and marine crocodiles of the families Teleosauridae and Metriorhynchidae. Numerous turtles could be found in rivers. In the invertebrate world, several new groups appeared, including rudists (a reef-formi
Morocco
Morocco the Kingdom of Morocco, is a country located in the Maghreb region of North West Africa with an area of 710,850 km2. Its capital is the largest city Casablanca, it overlooks the Mediterranean Sea to the Atlantic Ocean to the west. Morocco claims the areas of Ceuta, Melilla and Peñón de Vélez de la Gomera, all of them under Spanish jurisdiction. Since the foundation of the first Moroccan state by Idris I in 788 AD, the country has been ruled by a series of independent dynasties, reaching its zenith under the Almoravid and Almohad dynasties, spanning parts of Iberia and northwestern Africa; the Marinid and Saadi dynasties continued the struggle against foreign domination, allowing Morocco to remain the only northwest African country to avoid Ottoman occupation. The Alaouite dynasty, which rules to this day, seized power in 1631. In 1912, Morocco was divided into French and Spanish protectorates, with an international zone in Tangier, it regained its independence in 1956, has since remained comparatively stable and prosperous by regional standards.
Morocco claims the non-self-governing territory of Western Sahara Spanish Sahara, as its Southern Provinces. After Spain agreed to decolonise the territory to Morocco and Mauritania in 1975, a guerrilla war arose with local forces. Mauritania relinquished its claim in 1979, the war lasted until a cease-fire in 1991. Morocco occupies two thirds of the territory, peace processes have thus far failed to break the political deadlock; the unitary sovereign state of Morocco is a constitutional monarchy with an elected parliament. The King of Morocco holds vast executive and legislative powers over the military, foreign policy and religious affairs. Executive power is exercised by the government, while legislative power is vested in both the government and the two chambers of parliament, the Assembly of Representatives and the Assembly of Councillors; the king can issue decrees called dahirs. He can dissolve the parliament after consulting the Prime Minister and the president of the constitutional court.
Morocco's predominant religion is Islam, its official languages are Arabic and Berber. E; the Moroccan dialect of Arabic, referred to as Darija, French are widely spoken. Moroccan culture is a blend of Berber, Sephardi Jews, West African and European influences. Morocco is a member of the Union for the Mediterranean and the African Union, it has the fifth largest economy of Africa. The full Arabic name al-Mamlakah al-Maghribiyyah translates to "Kingdom of the West". For historical references, medieval Arab historians and geographers sometimes referred to Morocco as al-Maghrib al-Aqṣá to distinguish it from neighbouring historical regions called al-Maghrib al-Awsaṭ and al-Maghrib al-Adná; the basis of Morocco's English name is Marrakesh, its capital under the Almoravid dynasty and Almohad Caliphate. The origin of the name Marrakesh is disputed, but is most from the Berber words amur akush or "Land of God"; the modern Berber name for Marrakesh is Mṛṛakc. In Turkish, Morocco is known as a name derived from its ancient capital of Fes.
However, this was not the case in other parts of the Islamic world: until the middle of the 20th century, the common name of Morocco in Egyptian and Middle Eastern Arabic literature was Marrakesh. The English name Morocco is an anglicisation of the Spanish "Marruecos", from which derives the Tuscan "Morrocco", the origin of the Italian "Marocco"; the area of present-day Morocco has been inhabited since Paleolithic times, sometime between 190,000 and 90,000 BC. A recent publication may demonstrate an earlier habitation period, as Homo sapiens fossils discovered in the late 2000s near the Atlantic coast in Jebel Irhoud were dated to 315,000 years before present. During the Upper Paleolithic, the Maghreb was more fertile than it is today, resembling a savanna more than today's arid landscape. Twenty-two thousand years ago, the Aterian was succeeded by the Iberomaurusian culture, which shared similarities with Iberian cultures. Skeletal similarities have been suggested between the Iberomaurusian "Mechta-Afalou" burials and European Cro-Magnon remains.
The Iberomaurusian was succeeded by the Beaker culture in Morocco. Mitochondrial DNA studies have discovered the Saami of Scandinavia; this supports theories that the Franco-Cantabrian refuge area of southwestern Europe was the source of late-glacial expansions of hunter-gatherers who repopulated northern Europe after the last ice age. Northwest Africa and Morocco were drawn into the wider emerging Mediterranean world by the Phoenicians, who established trading colonies and settlements in the early Classical period. Substantial Phoenician settlements were at Chellah and Mogador. Mogador was a Phoenician colony as early as the early 6th century BC. Morocco became a realm of the Northwest African civilisation of ancie
Asia
Asia is Earth's largest and most populous continent, located in the Eastern and Northern Hemispheres. It shares the continental landmass of Eurasia with the continent of Europe and the continental landmass of Afro-Eurasia with both Europe and Africa. Asia covers an area of 44,579,000 square kilometres, about 30% of Earth's total land area and 8.7% of the Earth's total surface area. The continent, which has long been home to the majority of the human population, was the site of many of the first civilizations. Asia is notable for not only its overall large size and population, but dense and large settlements, as well as vast populated regions, its 4.5 billion people constitute 60% of the world's population. In general terms, Asia is bounded on the east by the Pacific Ocean, on the south by the Indian Ocean, on the north by the Arctic Ocean; the border of Asia with Europe is a historical and cultural construct, as there is no clear physical and geographical separation between them. It has moved since its first conception in classical antiquity.
The division of Eurasia into two continents reflects East–West cultural and ethnic differences, some of which vary on a spectrum rather than with a sharp dividing line. The most accepted boundaries place Asia to the east of the Suez Canal separating it from Africa. China and India alternated in being the largest economies in the world from 1 to 1800 CE. China was a major economic power and attracted many to the east, for many the legendary wealth and prosperity of the ancient culture of India personified Asia, attracting European commerce and colonialism; the accidental discovery of a trans-Atlantic route from Europe to America by Columbus while in search for a route to India demonstrates this deep fascination. The Silk Road became the main east–west trading route in the Asian hinterlands while the Straits of Malacca stood as a major sea route. Asia has exhibited economic dynamism as well as robust population growth during the 20th century, but overall population growth has since fallen. Asia was the birthplace of most of the world's mainstream religions including Hinduism, Judaism, Buddhism, Taoism, Islam, Sikhism, as well as many other religions.
Given its size and diversity, the concept of Asia—a name dating back to classical antiquity—may have more to do with human geography than physical geography. Asia varies across and within its regions with regard to ethnic groups, environments, historical ties and government systems, it has a mix of many different climates ranging from the equatorial south via the hot desert in the Middle East, temperate areas in the east and the continental centre to vast subarctic and polar areas in Siberia. The boundary between Asia and Africa is the Red Sea, the Gulf of Suez, the Suez Canal; this makes Egypt a transcontinental country, with the Sinai peninsula in Asia and the remainder of the country in Africa. The border between Asia and Europe was defined by European academics; the Don River became unsatisfactory to northern Europeans when Peter the Great, king of the Tsardom of Russia, defeating rival claims of Sweden and the Ottoman Empire to the eastern lands, armed resistance by the tribes of Siberia, synthesized a new Russian Empire extending to the Ural Mountains and beyond, founded in 1721.
The major geographical theorist of the empire was a former Swedish prisoner-of-war, taken at the Battle of Poltava in 1709 and assigned to Tobolsk, where he associated with Peter's Siberian official, Vasily Tatishchev, was allowed freedom to conduct geographical and anthropological studies in preparation for a future book. In Sweden, five years after Peter's death, in 1730 Philip Johan von Strahlenberg published a new atlas proposing the Urals as the border of Asia. Tatishchev announced; the latter had suggested the Emba River as the lower boundary. Over the next century various proposals were made until the Ural River prevailed in the mid-19th century; the border had been moved perforce from the Black Sea to the Caspian Sea into which the Ural River projects. The border between the Black Sea and the Caspian is placed along the crest of the Caucasus Mountains, although it is sometimes placed further north; the border between Asia and the region of Oceania is placed somewhere in the Malay Archipelago.
The Maluku Islands in Indonesia are considered to lie on the border of southeast Asia, with New Guinea, to the east of the islands, being wholly part of Oceania. The terms Southeast Asia and Oceania, devised in the 19th century, have had several vastly different geographic meanings since their inception; the chief factor in determining which islands of the Malay Archipelago are Asian has been the location of the colonial possessions of the various empires there. Lewis and Wigen assert, "The narrowing of'Southeast Asia' to its present boundaries was thus a gradual process." Geographical Asia is a cultural artifact of European conceptions of the world, beginning with the Ancient Greeks, being imposed onto other cultures, an imprecise concept causing endemic contention about what it means. Asia does not correspond to the cultural borders of its various types of constituents. From the time of Herodotus a minority of geographers have rejected the three-continent system on the grounds that there is no substantial physical separation between
Mesozoic
The Mesozoic Era is an interval of geological time from about 252 to 66 million years ago. It is called the Age of Reptiles and the Age of Conifers; the Mesozoic is one of three geologic eras of the Phanerozoic Eon, preceded by the Paleozoic and succeeded by the Cenozoic. The era is subdivided into three major periods: the Triassic and Cretaceous, which are further subdivided into a number of epochs and stages; the era began in the wake of the Permian–Triassic extinction event, the largest well-documented mass extinction in Earth's history, ended with the Cretaceous–Paleogene extinction event, another mass extinction whose victims included the non-avian dinosaurs. The Mesozoic was a time of significant tectonic and evolutionary activity; the era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would move into their current positions during the next era. The climate of the Mesozoic was varied, alternating between cooling periods. Overall, the Earth was hotter than it is today.
Dinosaurs first appeared in the Mid-Triassic, became the dominant terrestrial vertebrates in the Late Triassic or Early Jurassic, occupying this position for about 150 or 135 million years until their demise at the end of the Cretaceous. Birds first appeared in the Jurassic; the first mammals appeared during the Mesozoic, but would remain small—less than 15 kg —until the Cenozoic. The flowering plants arose in the Triassic or Jurassic and came to prominence in the late Cretaceous when they replaced the conifers and other gymnosperms as the dominant trees; the phrase "Age of Reptiles" was introduced by the 19th century paleontologist Gideon Mantell who viewed it as dominated by diapsids such as Iguanodon, Megalosaurus and Pterodactylus. Mesozoic means "middle life", deriving from the Greek prefix meso-/μεσο- for "between" and zōon/ζῷον meaning "animal" or "living being"; the name "Mesozoic" was proposed in 1840 by the British geologist John Phillips. Following the Paleozoic, the Mesozoic extended 186 million years, from 251.902 to 66 million years ago when the Cenozoic Era began.
This time frame is separated into three geologic periods. From oldest to youngest: Triassic Jurassic Cretaceous The lower boundary of the Mesozoic is set by the Permian–Triassic extinction event, during which 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct, it is known as the "Great Dying" because it is considered the largest mass extinction in the Earth's history. The upper boundary of the Mesozoic is set at the Cretaceous–Paleogene extinction event, which may have been caused by an asteroid impactor that created Chicxulub Crater on the Yucatán Peninsula. Towards the Late Cretaceous, large volcanic eruptions are believed to have contributed to the Cretaceous–Paleogene extinction event. 50% of all genera became extinct, including all of the non-avian dinosaurs. The Triassic ranges from 252 million to 201 million years ago, preceding the Jurassic Period; the period is bracketed between the Permian–Triassic extinction event and the Triassic–Jurassic extinction event, two of the "big five", it is divided into three major epochs: Early and Late Triassic.
The Early Triassic, about 252 to 247 million years ago, was dominated by deserts in the interior of the Pangaea supercontinent. The Earth had just witnessed a massive die-off in which 95% of all life became extinct, the most common vertebrate life on land were lystrosaurus and euparkeria along with many other creatures that managed to survive the Permian extinction. Temnospondyls would be the dominant predator for much of the Triassic; the Middle Triassic, from 247 to 237 million years ago, featured the beginnings of the breakup of Pangaea and the opening of the Tethys Sea. Ecosystems had recovered from the Permian extinction. Algae, sponge and crustaceans all had recovered, new aquatic reptiles evolved, such as ichthyosaurs and nothosaurs. On land, pine forests flourished, as did groups of insects like mosquitoes and fruit flies. Reptiles began to get bigger and bigger, the first crocodilians and dinosaurs evolved, which sparked competition with the large amphibians that had ruled the freshwater world mammal-like reptiles on land.
Following the bloom of the Middle Triassic, the Late Triassic, from 237 to 201 million years ago, featured frequent heat spells and moderate precipitation. The recent warming led to a boom of dinosaurian evolution on land as those one began to separate from each other, as well as first pterosaurs. During the Late Triassic, some advanced cynodonts gave rise to the first Mammaliaformes. All this climatic change, resulted in a large die-out known as the Triassic-Jurassic extinction event, in which many archosaurs, most synapsids, all large amphibians became extinct, as well as 34% of marine life, in the Earth's fourth mass extinction event; the cause is debatable. The Jurassic ranges from 200 million years to 145 million years ago and features three major epochs: The Early Jurassic, the Middle Jurassic, the L
Ordovician
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated
