Education Links
Leaving Cert
Maths
French
English
Chemistry
Physics
Biology
Economics
Spanish
Geography
History
Junior Cert
2. Mountains under the sea
The longest mountain chains are not found on land, they are found on the ocean floor where plates are separating (diverging). At spreading boundaries, magma rises up thorough the crust and spreads over the surface. This material generally flows easily and when it solidifies forms a black rock called basalt. If the magma is forced upward through continental crust it will flood the surrounding terrain and is termed 'flood basalt'. Eruptions of magma at these locations produce spectacular shows as fiery magma spouts into the air.However, eruptions are rarely explosive. These features are thought to be the beginning of continental breakup. As the basalt solidifies the continental crust is forced apart. As it separates, water fills the space and new ocean floor is formed between the plates. The divergent plate boundary is now found beneath the water level.
Divergent plate boundaries are found in all oceans of the world. In the Atlantic an enormous seam extends through its middle and separates the Euro-Asian and African Plates from the South American, Caribbean and North American Plates. All along the seam when magma emerges it piles up along the rift. Because the magma is cooled quickly it forms a ridge near the rift.
One of the most important pieces of evidence for Plate Tectonics was found in the 1960's when scientists collected rock samples from the Atlantic Ocean floor. They discovered that the rocks nearest the mid-oceanic ridge were the youngest. The conclusion drawn was that the ridge represented the site of new ocean floor. Along this seam there is a place where the mid-oceanic ridge extends above the ocean surface. Iceland strides this ridge. The youngest part of Iceland lies along a north-south axis and the oldest part of the island is to be found in the east and west. Iceland is continually growing!
