A volcano is an opening in Earth’s crust through which molten rock and gases erupt from deep below. Most volcanoes look like mountains. Their cone shape is formed as layers of ash and other materials build up around the main vent. This can take thousands of years.There are many different kinds of volcanoes, but they are all born in the same way — when magma erupts though a weak place in Earth’s crust.
Magma usually forms deep below Earth’s surface. Here, it mixes with gases that make the magma lighter and help it to rise. As the magma draws closer to the crust, it collects in a chamber, which is under great pressure from the surrounding rock. Eventually, the magma is forced up through the crust and into a conduit that has been made by the intense heat.
Magma that has erupted from a volcano is called lava. Most lava explodes through the main vent at the top of the volcano. The force is so great that it can shoot the molten material high into the air. Lava can also seep through smaller side vents before flowing down the side of the volcano.
After an eruption, the volcano starts to collapse, and a dish like hole called a crater forms at the top. Over time, the crater may fill with water, and a lake forms.
The short answer is that earthquakes are caused by faulting, a sudden lateral or vertical movement of rock along a rupture (break) surface. Here\'s the longer answer: The surface of the Earth is in continuous slow motion. This is plate tectonics--the motion of immense rigid plates at the surface of the Earth in response to flow of rock within the Earth. The plates cover the entire surface of the globe. Since they are all moving they rub against each other in some places (like the San Andreas Fault in California), sink beneath each other in others (like the Peru-Chile Trench along the western border of South America), or spread apart from each other (like the Mid-Atlantic Ridge). At such places the motion isn\'t smooth--the plates are stuck together at the edges but the rest of each plate is continuing to move, so the rocks along the edges are distorted (what we call "strain"). As the motion continues, the strain builds up to the point where the rock cannot withstand any more bending. With a lurch, the rock breaks and the two sides move. An earthquake is the shaking that radiates out from the breaking rock. People have known about earthquakes for thousands of years, of course, but they didn\'t know what caused them. In particular, people believed that the breaks in the Earth\'s surface--faults--which appear after earthquakes, were caused *by* the earthquakes rather than the cause *of* them. It was Bunjiro Koto, a geologist in Japan studying a 60-mile long fault whose two sides shifted about 15 feet in the great Japanese earthquake of 1871, who first suggested that earthquakes were caused by faults. Henry Reid, studying the great San Francisco earthquake of 1906, took the idea further. He said that an earthquake is the huge amount of energy released when accumulated strain causes a fault to rupture. He explained that rock twisted further and further out of shape by continuing forces over the centuries eventually yields in a wrenching snap as the two sides of the fault slip to a new position to relieve the strain. This is the idea of "elastic rebound" which is now central to all studies of fault rupture.
Most earthquake-related deaths are caused by the collapse of structures and the construction practices play a tremendous role in the death toll of an earthquake. In southern Italy in 1909 more than 100,000 people perished in an earthquake that struck the region. Almost half of the people living in the region of Messina were killed due to the easily collapsible structures that dominated the villages of the region. A larger earthquake that struck San Francisco three years earlier had killed fewer people (about 700) because building construction practices were different type (predominantly wood). Survival rates in the San Francisco earthquake was about 98%, that in the Messina earthquake was between 33% and 45%) (Zebrowski, 1997). Building practices can make all the difference in earthquakes, even a moderate rupture beneath a city with structures unprepared for shaking can produce tens of thousands of casualties.
The causes of landslides are usually related to instabilities in slopes. It is usually possible to identify one or more landslide causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time. Landslide causes are listed in the following table, and include geological factors, morphological factors, physical factors and factors associated with human activity.
Causes may be considered to be factors that made the slope vulnerable to failure, that predispose the slope to becoming unstable. The trigger is the single event that finally initiated the landslide. Thus, causes combine to make a slope vulnerable to failure, and the trigger finally initiates the movement. Landslides can have many causes but can only have one trigger as shown in the next figure. Usually, it is relatively easy to determine the trigger after the landslide has occurred (although it is generally very difficult to determine the exact nature of landslide triggers ahead of a movement event).
Occasionally, even after detailed investigations, no trigger can be determined - this was the case in the large Mount Cook landslide in New Zealand 1991. It is unclear as to whether the lack of a trigger in such cases is the result of some unknown process acting within the landslide, or whether there was in fact a trigger, but it cannot be determined. Perhaps this is because the trigger was in fact a slow but steady decrease in material strength associated with the weathering of the rock - at some point the material becomes so weak that failure must occur. Hence the trigger is the weathering process, but this is not detectable externally. In most cases we think of a trigger as an external stimulus that induces an immediate or near-immediate response in the slope, in this case in the form of the movement of the landslide. Generally this movement is induced either because the stresses in the slope are altered, perhaps by increasing shear stress or decreasing the effective normal stress, or by reducing the resistance to the movement perhaps by decreasing the shear strength of the materials within the landslide