Most earthquake occur when two pieces of Earth slide against each other. The epicenter is the location above the ground where the earthquake starts; the location below the surface where earthquakes begin is the hypocenter. The pieces of Earth are known as tectonic plates, and these plates make up the earth's four layers: inner core, outer core, mantle and crust. Tectonic plates are separate from each other and constantly moving below the Earth's surface. The boundaries of the plates are known as faults. Faults are where most earthquakes happen, since the edges of tectonic plates are rough and often get stuck to other tectonic plates.
Volcano eruptions start deep within the Earth, where temperatures are so hot that rocks melt into a fluid substance known as magma. Magma is lighter than solid rocks and settles into magma chambers. From the chambers, some magma shoots up to Earth's surface through vents known as volcanic mountains. Two type of volcanic eruptions occur. Gases are able to escape light and thin magma. If this is the case, when the volcano erupts, it results in a lava flow, or slow descension of magma down the volcano. However, thick magma does not allow gases to escape until it reaches the top of the volcano, which causes a turbulent explosion. These eruptions form large smoke clouds and magma debris called tephra. Tephra sizes range between ash-like particles to large boulders.
The majority of tsunamis begin when an earthquake happens at sea. Earthquakes create tsunamis when the energy of the Earth's movement causes a wave. The wave's speed and height increases as it moves toward a shoreline. Most tsunamis do not result in a large tidal wave, but are fast-moving tides that create a sudden rise in the local sea levels. Other catalysts for tsunamis are large underwater landslides, volcanic eruptions in the ocean and large meteors from outer space that crash into the ocean. Land volcanoes may cause tsunamis if debris from the eruptions slides or lands into a nearby ocean. The addition of the volcanic debris causes water displacement, which in turn forms a tsunami wave.
The largest earthquake in modern history occurred off the coast of South Central Chile in May 1960. This earthquake measured 9.5 on the Richter scale, which is a scale of 1 to 10. Tsunamis over 80 feet tall formed due to the earthquake and killed more than 200 people. One of the most destructive earthquakes in the United States happened in San Francisco in 1906. The 7.8 Richter scale earthquake leveled the Golden Gate City and started massive fires throughout the beleaguered metropolis; approximately 3,000 people died from this disaster. Mount St. Helens is an active volcano that erupted in Washington state in 1980. The eruption consisted of a loud explosion and created a crater at the mountain's peak.