PLATES

About 40 to 50 million years ago, two large land masses, India and Eurasia, collided to begin the formation of the most visible product of plate tectonics – the Himalayas. Since subduction is impossible between two colliding continental plates, pressure is released by pushing the crust upward and forming the Himalayan peaks. Also, collision of continental plates with shallow earthquake activities.

Divergence of Plates

Formation of rift valleys and oceanic ridges are indications that the crust is spreading or splitting apart. In this case, the plates are forming divergent plate boundaries wherein they tend to move apart. Most divergent boundaries are situated along underwater mountain ranges called oceanic ridges. As the plates separate, new materials from the mantle ooze up to fill the gap. These materials slowly cool to produce new ocean floor. The spreading rate at these ridges may vary from 2 to 20 cm per year. Although, a very slow process, the divergence of plates ensures a continuous supply of new materials from the mantle. The Mid-Atlantic Ocean ridge is an example of spreading center which causes the divergence of the South American plate and the African plate.

When a spreading center develops within a continent, the crust may break into several segments. The breaking leads to the formation of down faulted valleys called rift valleys. It is also associated with the rising of hot materials from the mantle. The rift valleys increases its length and depth as the spreading continues. At this point, the valleys develops into a linear sea, similar to the Red Sea today.

Transform Fault Boundaries

If the blocks of wood in Activity 7 were to represent the lithospheric plates, you will notice that there were two sets of divergent plate boundaries (between blocks 1 and 2, and blocks 3 and 4). But since the plates were adjacent to each other, a new type of boundary. Most transform fault join two segments of a mid-ocean ridge (represented by the gaps between 1 and 2, and between 3 and 4). Remember that the presence of a ridge is an indication of diverging plates, and as the plates diverge between the two segments of the mid-ocean ridge, the adjacent slabs of crust are grinding past each other (blocks 2 and 3, block 1, and 2 and 4). Although most transform faults are located within the Ocean basins, there are few that cut through the continental crust. An example of this is the San Andreas fault. The immediate concerns about transform fault boundaries are earthquake activities triggered by movements along the fault system.

It was stated at the beginning of the module that majority of tectonic activities like earthquake, mountain formations, and volcanic activities happen along Or near plate boundaries. But there are SOme Cases wherein activities take place in the middle of a plate. Let's take the Case of the Hawaiian islands. Here, we can find some of the largest and most active volcanoes of the world. If we're going to look at Hawaii, it is situated right in the middle of Pacific plate and not along the boundaries. What Causes the formation of this chain of volcanic islands? The answer Iies in an area called hot spot. of Extensive mapping seafloor volcanoes in the Pacific revealed a chain of structures volcanic the from extending Islands to Hawaiian Islands. When Midway geologists determined the age of each volcanic island radiometric through dating, they noticed that the farther the volcano from Hawaii is, the older and less active it is. Scientists suggested that there is a source of molten materials from the mantle called nmantle plume that formed the volcanic island chains. As the Pacific plate moves, different parts of it will be on top of the mantle plume to receive the molten materials, thus creating the volcanic islands. Continuing plate movement eventually carries the island beyond the hot spot, cutting it off from the magma sOurce, and volcanism ceases. As island volcano one becomes extinct, another develops over the hot spot, and the cycle is repeated. This process of volcano growth and death, over many millions of years, has left a long trail of volcanic islands and seamounts acrosS the Pacific Ocean floor.