Land and Sea Change

Land–sea change

Land and sea on Earth's surface continually change: in some places sea becomes land, while elsewhere land becomes sea. This is the ancient idea of 'seas turning into fields”.

Causes of land–sea changeCrustal movement and sea-level change are natural causes. Human activities such as reclamation also alter coastlines, on a smaller scale.

Marine fossils on the mountain

Rocks thousands of meters above sea level in the Himalayas contain fossils of ammonites, trilobites and ichthyosaurs. These marine fossils show that the region was once an ocean.

About 70 million years ago, India and Asia were separated by the Tethys Ocean, where sediments containing marine remains accumulated on the seabed.

The Indian Plate drifted north, narrowing the ocean. About 50 million years ago, the continents collided.

Seafloor sediments were compressed, folded and uplifted to form the Himalayas, carrying their fossils into the mountains.

Crustal movementPlate-driven uplift and subsidence are major causes of land–sea change. The Himalayas are still rising slowly.

Ancient river channel under the strait

On the seabed of the Taiwan Strait, researchers have found ancient river channels. Fishers also recover ancient elephant fossils and human remains and artifacts, showing the area was once land.

During the last ice age, water stored in land glaciers lowered global sea level by more than 120 m.

The Taiwan Strait averages only about 60 m deep. Lower sea level exposed much of the seabed, joining Taiwan to the mainland.

After the ice age, glaciers melted and sea level rose, flooding the land to form today's strait.

Sea-level changeClimate-driven sea-level changes alter land–sea distribution. Submerged human sites and forests provide evidence.

continental drift hypothesis

In 1912, German scientist Alfred Wegener noticed the matching outlines on opposite Atlantic shores, especially eastern South America and western Africa. He proposed that the continents had once formed a single landmass, then split and drifted apart to form today's arrangement.

Breakup of Pangaea

About 250 million years ago, the continents formed Pangaea, surrounded by the vast Panthalassa Ocean. It split into northern Laurasia and southern Gondwana, then drifted into today's seven continents.

Six major plates

In the 1960s, building on continental drift and seafloor spreading, scientists developed plate tectonic theory. Earth's rocky outer layer consists of six major plates in this model, rather than one solid shell. These plates are in constant motion.

The Pacific Plate is almost entirely oceanic; the others include both continental and oceanic areas.

In general, plate interiors are relatively stable, while plate boundaries have more active crustal movement.

Volcanoes and Earthquakes

Most volcanoes and earthquakes occur along plate boundaries, especially in two belts: the Circum-Pacific Belt and the Mediterranean–Himalayan Belt.

Japan lies at the Eurasian–Pacific plate boundary and experiences frequent volcanoes and earthquakes.

China's Taiwan, southwestern regions (such as Sichuan) experience many earthquakes associated with plate convergence.

Iceland lies on the divergent Mid-Atlantic Ridge and has frequent volcanic activity.

Rupture and collision

Plate motion shapes Earth's relief. Divergence often produces rift valleys or oceans; plate convergence often produces mountain ranges. Collision between oceanic and continental plates also creates ocean trenches.

The Red Sea lies between the separating African and Indian Ocean plates. It is widening and may become a new ocean.

The Mediterranean lies where the African and Eurasian plates converge. It is shrinking.

In-class testing