Marine Geology Phần 2 doc

pumped nutrients into the sea, fueling booms of marine plankton, which

increased the food supply for higher creatures.The number of genera of mol￾lusks, brachiopods, and trilobites dramatically increased, because organisms with

abundant food are more likely to thrive and diversify into different species.

During the formation of Laurasia, island arcs between the two land￾masses were scooped up and plastered against continental edges as the oceanic

crustal plate carrying the islands subducted under Baltica. This subduction

rafted the islands into collision with the continent and deposited the formerly

submerged rocks onto the present west coast of Norway. Slices of land called

terranes residing in western Europe drifted into the Iapetus from ancient

Africa. Likewise, slivers of crust from Asia traveled across the ancestral Pacific

Ocean called the Panthalassa to form much of western North America.

North America was a lost continent around 500 million years ago. Dur￾ing that time, the continental landmass and a few smaller continental frag￾ments drifted freely on their own. South America,Africa,Australia,Antarctica,

and India had assembled into Gondwana by continental plate collisions.At this

time, North America was situated a few thousand miles off the western coast

of South America, placing it on the western side of Gondwana. Eventually,

North and South America collided (Fig. 15), placing what would be present￾day Washington, D.C., near Lima, Peru. A limestone formation in Argentina

Figure 15 North and

South America might

have collided at the

beginning of the

Ordovician 500 million

years ago.

20

Marine Geology

AFRICA

SOUTH

AMERICA

NORTH

AMERICA

AUSTRALIA

INDIA

BALTICA

contains a distinctive trilobite species typical of North America but not of

South America, suggesting the two continents once had much in common.

THE PANTHALASSA SEA

Throughout geologic history, smaller continental blocks collided and merged

into larger continents. Millions of years after assembling, the continents rifted

apart, and the chasms filled with seawater to form new oceans. However, the

regions presently bordering the Pacific basin apparently did not collide.

Rather, the Pacific Ocean is a remnant of an ancient sea called the Panthalassa.

It narrowed and widened in response to continental breakup, dispersal, and

reconvergence in the area occupied by today’s Atlantic Ocean. So, while

oceans have repeatedly opened and closed in the vicinity of the Atlantic basin,

a single ocean has existed continuously at the site of the Pacific basin.

When Laurentia fused with Baltica to form Laurasia, island arcs in the

Panthalassa Sea began colliding with the western margin of present-day North

America. Erosion leveled the continents. Shallow seas flowed inland, flooding

more than half the land surface.The inland seas and wide continental margins,

along with a stable environment, encouraged marine life to flourish and

spread throughout the world.

From 360 million to 270 million years ago, Gondwana and Laurasia con￾verged into Pangaea (Fig. 16), which straddled the equator and extended

almost from pole to pole.This massive continent reached its peak size about

210 million years ago with an area of about 80 million square miles or 40 per￾cent of Earth’s total surface area. More than one-third of the landmass was

covered with water. An almost equal amount of land existed in both hemi￾spheres. In contrast, today two-thirds of the continental landmass is located

north of the equator. South of the equator, the breakdown is 10 percent land￾mass and 90 percent ocean. A single great ocean stretched uninterrupted

across the planet, while the continents huddled to one side of the globe.

The sea level fell substantially after the formation of Pangaea, draining

the interiors of the continents and causing the inland seas to retreat. A con￾tinuous shallow-water margin ran around the entire perimeter of Pangaea.As

a result, no major physical barriers hampered the dispersal of marine life.

Moreover, the seas were largely restricted to the ocean basins, leaving the con￾tinental shelves mostly exposed.

The continental margins were less extensive and narrower than they are

today due to a drop in sea level as much as 500 feet.This drop confined marine

habitats to the nearshore regions. Consequently, habitat areas for shallow￾water marine organisms were limited, resulting in low species diversity.

Permian ocean life was sparse, with many immobile animals and few active

21

The Blue Planet

predators. Ocean temperatures remained cool following a late Permian ice

age. Marine invertebrates that managed to escape extinction lived in a narrow

margin near the equator.

THE TETHYS SEA

When Laurasia occupied the Northern Hemisphere and its counterpart

Gondwana was located in the Southern Hemisphere, the two landmasses were

separated by a large shallow equatorial body of water called the Tethys Sea

(Fig. 17) that was named for the mother of the seas in Greek mythology.After

the assembly of Pangaea, the Tethys became a huge embayment separating the

northern and southern arms of the supercontinent, which resembled a gigan￾tic letter C straddling the equator.

The Tethys was a broad tropical seaway extending from western Europe

to southeast Asia that harbored diverse and abundant shallow-water marine

life. Reef building in the Tethys Sea was intense, forming thick deposits of

limestone and dolomite laid down by prolific lime-secreting organisms.The

tropics served as an evolutionary cradle.This is because they had a greater area

22

Marine Geology

EUROPE

and

ASIA

AFRICA

ANTARCTICA

INDIA

AUSTRALIA

SOUTH

AMERICA

NORTH

AMERICA

Figure 16 The

supercontinent Pangaea

extended almost from pole

to pole.