A satellite view of Marajó
|Area||40,100 km2 (15,500 sq mi)|
|Length||297 km (184.5 mi)|
|Width||204 km (126.8 mi)|
|Highest elevation||40 m (130 ft)|
|Highest point||Breves (city)|
|Largest settlement||Breves (pop. 99,223)|
Marajó (Portuguese pronunciation: [maɾaˈʒɔ]) is a large delta island in the state of Pará,  Brazil. The island is bordered by the mouth of the Amazon river to the west and northwest, the Atlantic Ocean to the northeast and by the Pará River, a distributary of the Amazon to the east. From approximately 400 BC to 1600 AD, Marajó was the site of an advanced Pre-Columbian society called the Marajoara culture, which may have numbered over 100,000 people at its peak. Today, the island is known for its large water buffalo population, as well as the pororoca tidal bore periodically exhibited by high tides overcoming the usual complex hydrodynamic interactions in the surrounding reaches of Amazon delta. It is the second-largest island in South America, and the 35th largest island in the world.
With a land area of 40,100 square kilometres (15,500 sq mi) Marajó is comparable in size to Switzerland. Its maximum span is 295 kilometres (183 mi) long and 200 kilometres (120 mi) in perpendicular width. 
The northeast coastline of Marajó faces the Atlantic Ocean. The outflow from the Amazon between January and July is so great that the sea at the mouth is made up of fresh water for some distance from shore. The city of Belém lies to the south across the southern fork (also called the Pará River) of the river's mouth.  The island sits almost directly on the equator.
Together with smaller neighboring islands, separated from Marajó by rivers, it forms the Marajó Archipelago, with an aggregate area of 49,602 square kilometres (19,151 sq mi).  The archipelago is contained in the 59,985 square kilometres (23,160 sq mi) Marajó Archipelago Environmental Protection Area, a sustainable-use conservation unit established in 1989 to protect the environment of the delta region. 
Large parts of the islands are flooded during the rainy season because of higher water levels of the Amazon River along the coast and heavy rainfall in the interior. Marajó is almost entirely flat. During the rainy season, much of the island becomes flooded as a large lake. 
There are 20 large rivers on the island. Because of the changing water levels and regular seasonal flooding, many settlements are built on stilts (Palafitas).
The east side of the island is dominated by savanna vegetation. There are large fazendas with animal husbandry.  This is also the location of Lake Arari, which has an area of 400 square kilometres (150 sq mi) but shrinks by 80% during the dry season.  There are large herds of domesticated water buffalo, which are technically invasive to the island; they now number about 450,000, higher than the island's human population.   The west side of the island is characterized by Várzea forests and small farms. Lumber and açaí are produced there.
The island is in the Marajó várzea ecoregion, an area of seasonally and tidally flooded várzea forest. To the north of the large savanna area are palm swamps, mainly with Buriti Palm ( Mauritia flexuosa) and Euterpe oleracea. During the rainy season, the swamps are flooded one meter high. Little is known about the ecology of these swamps.
The most important towns are in the southeast of the island: Soure, Salvaterra, and the largest city, Breves. They feature a basic touristic infrastructure and are popular because of the generous, lightly populated beaches. The city of Soure, on the island's Atlantic Coast, serves as an entry point to the island via its ferry link to Belém.
The island is shared by 16 municipalities of three microregions:
- Microregion of Arari:
- Microregion of Furos de Breves:
- Microregion of Portel:
The island was the site of an advanced pre-Columbian society, the Marajoara culture, which existed from approximately 400 BC to 1600 AD. The island has been a center of archaeological exploration and scholarship since the 19th century.  Scholars from the 1980s forward have divided the pre-Columbian period into the Ananatuba phase (c. 1100–c. 200 BC), the Mangueiras phase (c. 1000 BC–c. 100 AD), the Formiga phase (c. 100-400 AD), the Marajoará phase (c. 400-1200 AD), and the Aruã phase (1200-1500 AD). 
Since the 1990s, there has been debate over the origins and sophistication of Marajó's pre-Columbian society. Based on fieldwork in the 1940s and 1950s, the archaeologist Betty Meggers initially argued that the Marajoara culture had been founded by emigrants from the Andes, and that the society steadily declined until its final collapse in around 1400 AD, due to the Marajó's poor soil fertility and other environmental factors. Megger's hypotheses subsequently became associated with environmental determinism. However, her claims has since been rejected by the archaeologist Anna Curtenius Roosevelt, who re-excavated Marajó in the 1980s. According to Roosevelt, the Marajoara culture developed independently within the Amazon, and featured both intensive subsistence agriculture and major public works. 
Roosevelt estimated that Marajó had a population of possibly over 100,000 people at its peak.  The population lived in homes with tamped earth floors, organized themselves into matrilineal clans, and divided tasks by sex, age, and skill level.
The arrival of the Europeans in the 16th century was catastrophic to the indigenous population of the island; 90% died due to high mortality from Eurasian infectious diseases; they lacked immunity against these diseases, which had become endemic in European and Asian cities. 
The island is also the location of the Roman Catholic Territorial Prelature of Marajó.
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