Chapter 2 Land and Ocean Section 2: Changes in Sea and Land

Section 2: Changes in Sea and Land The Sea Becomes the Land

There are many examples of such changes as the sea becoming a mulberry field and the mulberry field becoming a sea. Modern scientific research shows that there are many causes of sea and land changes, such as changes in the earth's crust and the rise and fall of sea level. Human activities can also cause changes in sea and land, such as reclamation of the sea to create land.

Insights from the world map

Looking at the world map, it is interesting to note that the contours of the two sides of the Atlantic Ocean, especially the west coast of Africa, are very similar to those of the east coast of South America. The convex part of the South American continent and the concave part of the African continent almost coincide. If we cut these two continents out of the map, they can be put together as a whole. In response, the German scientist Wegener proposed the continental drift theory. The continental drift theory suggests that 200 million years ago, the earth's continents were interconnected as a single land mass surrounded by a vast ocean. Later, the original continents split into several pieces and slowly drifted apart, gradually forming the present distribution of the seven continents and four oceans.

The movement of the plates

In the 1960s, on the basis of the theory of continental drift, scientists proposed the theory of plate tectonics. According to the plate tectonics theory, the earth's surface is not composed of rocks as a whole, but is made up of plates that are assembled together. The world is roughly divided into six major plates and several smaller plates, which are in constant motion. Generally speaking, the crust inside the plates is more stable; the crust is more active in the areas where the plates meet. The world's volcanoes and earthquakes are also concentrated in the junction of plates.

At the junction of plates, two plates are rifted, often forming rift valleys or oceans; two plates collide, often forming mountains on land.

Alps a Himalayan system, Cordillera mountain system, etc. are mainly distributed in the zone of plate collision and extrusion. The world's volcanoes and earthquakes are also concentrated in the junction zones of plates where the earth's crust is more active, such as the Pacific Rim Volcanic Seismic Zone and the Mediterranean Himalayan Zone.

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