Tsunamis are a series of very long waves generated by any rapid, large-scale disturbance of the sea. Most are generated by seafloor displacements from large undersea earthquakes; [however, tsunamis can also be caused by volcanic eruptions, oceanic landslides, or any other event that displaces a large amount of water.] Tsunamis can cause great destruction and loss of life within minutes on shores near their source, and some tsunamis can cause destruction within hours across an entire ocean basin.
Most tsunamis occur in the Pacific region, but they [have been] known to happen in every ocean and sea. Although infrequent, tsunamis are a significant natural hazard with great destructive potential. They can only be dealt with effectively through programs of warning, mitigation, and education."1
Tsunamis only affect coastal regions. The best thing to do if you suspect a tsunami is about to occur is to get on higher ground. One sign of a tsunami is a strong earthquake that lasts twenty seconds or longer and occurs in or near the ocean. The sign that Tilly Smith noticed was the water rising. When a tsunami occurs, the water does not rise in the same way as tidal waters do, with closely spaced waves that one could surf on; instead, the water rises rapidly and is often very turbulent and contains a lot of debris. Sometimes the first sign observed is a rapid receding of the ocean followed by a wall of rising water. When this happens, it is time to run!
You can get an idea of how a tsunami occurs by watching this video animation. As you watch it, you will see the initial displacement of the water as water sprays into the air, followed by the wave that becomes bigger as it travels through the ocean.
Calderas are formed when volcanoes collapse, usually after much of the material in the magma chamber has been blown out. Calderas formed at the summit of stratovolcanoes may range in depth from a few hundred feet to several thousand feet. Formation of a caldera usually takes place late in the history of the volcano and often follows a long pause in activity during which the cone of the volcano may become deeply eroded. The caldera results not from explosive decapitation of the mountain, but when partial drainage of an underlying magma chamber removes support from beneath the top of the mountain, causing it to collapse.
Although the destructive effects of volcanism (volcanic action or activity) are obvious, volcanoes also provide many benefits to mankind. They are the major contributors to the building of continents. All oceanic islands owe their origin directly or indirectly to volcanism. Over the billions of years of Earth's existence, volcanoes and hot springs near volcanic intrusions have released hot water from the earth's interior. This steam and hot water can be used to produce geothermal energy. Geothermal energy produces electricity inexpensively and with low environmental impact.
Lava provides fertile soil in which crops such as pineapples, sugar cane, and coffee thrive. Lava erodes quickly in areas with adequate rainfall. In some cases, revegetation can begin in less than one year after the eruption. Lava flows are very fertile, especially if covered by ash. The fine ash particles retain water within reach of plant roots and release minerals, such as potassium, which plants need. Vegetation destroyed by falling ash often returns in a more luxuriant form. However, in areas where there is little rainfall, the erosion and breakdown of lava flows to form fertile soils can take thousands of years.