DEFINITION OF FLOOD BASALT

Deposits include vast plateaus of basalts that

cover large provinces of some continents and are also known

as continental flood basalts, plateau basalts, large igneous

provinces, and traps. They have a tholeiitic basalt composition,

but some show chemical evidence of minor contamination

by continental crust. They are similar to the anomalously

thick and topographically high seafloor known as oceanic

plateaus, and some volcanic rifted passive margins. At several

times in the past several hundred million years, these vast

outpourings of lava have accumulated forming thick piles of

basalt, representing the largest known volcanic episodes on

the planet. These piles of volcanic rock represent times when

the Earth moved more material and energy from its interior

than during intervals between the massive volcanic events.

Such large amounts of volcanism also released large amounts

of volcanic gases into the atmosphere, with serious implications

for global temperatures and climate, and may have contributed

to some global mass extinctions.

The largest continental flood basalt province in the United

States is the Columbia River flood basalt in Washington,

Oregon, and Idaho. The Columbia River flood basalt

province is 6–17 million years old and contains an estimated

1,250 cubic miles (4,900 km3) of basalt. Individual lava flows

erupted through fissures or cracks in the crust, then flowed

laterally across the plain for up to 400 miles (645 km).

The 66-million-year-old Deccan flood basalts, also

known as traps, cover a large part of western India and the

Seychelles. They are associated with the breakup of India

from the Seychelles during the opening of the Indian Ocean.

Slightly older flood basalts (90–83 million years old) are

associated with the breakaway of Madagascar from India.

The volume of the Deccan traps is estimated at 5 million

cubic miles (2,008,000 km3), and the volcanics are thought to

have been erupted in about one million years, starting slightly

before the great Cretaceous-Tertiary extinction. Most workers

now agree that the gases released during the flood basalt

volcanism stressed the global biosphere to such an extent that

many marine organisms became extinct, and many others

were stressed. Then the planet was hit by the massive Chicxulub

impact, causing the massive extinction including the end

of the dinosaurs.

The breakup of East Africa along the East African rift

system and the Red Sea is associated with large amounts of

Cenozoic (less than 30 million years old) continental flood

basalts. Some of the older volcanic fields are located in East

Africa in the Afar region of Ethiopia, south into Kenya and

Uganda, and north across the Red Sea and Gulf of Aden into

Yemen and Saudi Arabia. These volcanic piles are overlain by

younger (less than 15-million-year-old) flood basalts that

extend both farther south into Tanzania and farther north

through central Arabia, where they are known as Harrats,

and into Syria, Israel, Lebanon, and Jordan.

An older volcanic province also associated with the

breakup of a continent is known as the North Atlantic

Igneous Province. It formed along with the breakup of the

North Atlantic Ocean at 62–55 million years ago and

includes both onshore and offshore volcanic flows and intrusions

in Greenland, Iceland, and the northern British Isles,

including most of the Rockall Plateau and Faeroes Islands. In

the South Atlantic, similar 129–134-million-year-old flood

basalts were split by the opening of the ocean and now are

comprised of two parts. In Brazil the flood lavas are known

as the Parana basalts, and in Namibia and Angola of West

Africa as the Etendeka basalts.

These breakup basalts are transitional to submarine

flood basalts that form oceanic plateaus. The Caribbean

Ocean floor represents one of the best examples of an oceanic

plateau, with other major examples including the Ontong-

Java Plateau, Manihiki Plateau, Hess Rise, Shatsky Rise, and

Mid Pacific Mountains. All of these oceanic plateaus contain

between six and 25-mile (10–40-km) thick piles of volcanic

and subvolcanic rocks representing huge outpourings of lava.

The Caribbean seafloor preserves five to 13-mile (8–21-km)

thick oceanic crust formed before about 85 million years ago

in the eastern Pacific Ocean. This unusually thick ocean floor

was transported eastward by plate tectonics, where pieces of

the seafloor collided with South America as it passed into the

Atlantic Ocean. Pieces of the Caribbean oceanic crust are

now preserved in Colombia, Ecuador, Panama, Hispaniola,

and Cuba, and some scientists estimate that the Caribbean

oceanic plateau may have once been twice its present size. In

either case, it represents a vast outpouring of lava that would

have been associated with significant outgassing with possible

consequences for global climate and evolution.

The western Pacific Ocean basin contains several large

oceanic plateaus, including the 20-mile (32-km) thick crust of

the Alaskan-sized Ontong-Java Plateau, which is the largest

outpouring of volcanic rocks on the planet. It apparently

formed in two intervals, at 122 million and 90 million years

ago, entirely within the ocean, and represents magma that rose

in a plume from deep in the mantle and erupted on the

seafloor. It is estimated that the volume of magma erupted in

the first event was equivalent to that of all the magma being

erupted at mid-ocean ridges at the present time. Sea levels rose

by more than 30 feet (9 m) in response to this volcanic outpouring.

The gases released during these eruptions are estimated

to have raised average global temperatures by 23°F (13°C).

See also IGNEOUS ROCKS; MASS EXTINCTIONS; OCEANIC

PLATEAU.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar