It is difficult to come across a universally accepted definition of a mountain; with some definitions stressing on the elevation and some stressing on the gradient of slope as the main factor which has to be taken into consideration. While none is perfect, a near-perfect definition of mountain would be 'a huge landform projecting well above the surrounding area and typically characterized by the presence of a peak. Even though a hill is a similar landform, the two are distinguished from each other on the basis of gradient - with a mountain being steeper of the two. As with the definition, even the classification of mountains is an issue of debate - with experts finding it difficult to come up with a proper method of classifying them. Of the currently used methods of classification, one of the apt methods is to classify mountains into various types on the basis of the geological process involved in their formation. On the basis of underlying formation process, mountains are classified into four different types i.e. the fold mountains, fault-block mountains, volcanic mountains and dome mountains - each of which are discussed with examples below. As the name suggests, fold mountains are mountains which are formed as a result of folding of layers within the upper portion of Earth's crust which can be attributed to the movement of tectonic plates. When two continental plates (or one continental and one oceanic plate) move towards each other, they apply tremendous force on the sedimentary rock bed and push it from either sides in such a manner that it buckles and folds, and thus fold mountains are formed. As continental plates brush against each other to form fold mountains, you often get to see such mountains at the edge of the continents. One of the best examples of fold mountains is the Himalayan Mountain Range in the Indian Subcontinent. Furthermore, fold mountains are classified into two types - young fold mountains (such as the Rockies) and old fold mountains (such as the Appalachians). Fault-block mountains - at times referred to as fault mountains, are those mountains which are formed due to vertical displacement of the Earth's crust as a result of break up in bedrock caused by faults. As opposed to fold mountains which are formed as a result of continental plates moving towards each other, fault-block mountains are formed when faults in the Earth's crust displace rock layers on either side. Simply put, fault-block mountains are formed as a result of tremendous pressure exerted on the Earth's crust when it is stretched and extended along the faults. While the uplifted block is referred to as the horst (block mountain), the dropped block is referred to as the graben (a low rift valley). Fault-block mountains are further classified into tilted type block mountains (typically characterized by one steep side) and lifted type block mountain (characterized by two steep sides). The Sierra Nevada mountains in the continent of North America are perhaps the best examples of fault-block mountains. Those mountains which are formed as a result of volcanic activity on the planet are known as the volcanic mountains. (While the term volcano is actual sense refers to the opening in the Earth's crust from where magma oozes out onto the surface, the landform formed as a result of this activity is referred to as volcanic mountain.) These mountains start off as volcanoes which are nothing but small cracks in the Earth's crust that are formed as a result of tectonic movement. When volcanic eruption takes place, hot magma (in form of lava), ash, rocks, etc., are thrown out of the vent - all of which eventually settles around the vent. As eruptions continue, the buildup of this material continues and results in formation of a volcanic mountain. The entire process which can take several thousands of years. More importantly, it can also occur at the ocean floor and the resulting mountain may breach water in course of time. Some of the most prominent examples of volcanic mountains are Mt. Fuji in Japan, Mount Rainier in the USA, etc. As in case of volcanic mountains, even dome mountains are formed as a result of cooling and solidification of magma. The difference though is the fact that this process takes place beneath the Earth's crust and not above it as in case of volcanic mountain formation. Basically, volcanic mountains are formed when magma makes it way to the surface of the Earth wherein it cools and solidifies. In some cases however, the magma does make its way up but doesn't actually come to the surface of the Earth, but instead it stops right under the sedimentary layer of the Earth's crust - where it eventually solidifies. This - in turn, uplifts the sedimentary layer thus resulting in formation of a dome-shaped mountain. The name dome mountain can be attributed to the characteristic dome shape of this landform. In some classifications, you are likely to come across yet another a concept referred to as the 'plateau mountains'. Unlike the four types of mountains mentioned above, these landforms are not formed by any internal activity of the Earth's crust, but are formed as a result of agents of erosion working upon them over a course of time. Even though they are not true mountains, they are referred to as 'mountains' owing to their gigantic size.