The broad and comprehensive definition that comes to mind is that a volcano is a more or less extensive point on the surface of a planet which crops out lava, gases and / or hot or incandescent rock fragments, in a more or less continuous way. No matter if there is a mountain or not, or the shape of the terrain, if at a certain point there is expulsion of lava and / or projection of incandescent rock fragments (pyroclasts), this is a volcano.
Of course we all have in mind an image of a volcanic eruption. However, not ever we know what is its origin, what processes trigger an eruption. Before continuing, we need to know the difference between magma and lava, and have a basic idea of the internal structure of a volcano.
Magma is just molten rock. This molten rock contains numerous substances and compounds. Lava is magma that has lost part or all of its gases (substances which are gaseous under conditions of pressure and temperature of the earth's surface). The volcanic eruptions always crops out lava, never magma. Regarding the internal structure, simplifying a lot, all volcanoes presents a shallow (hundreds of meters to some kilometer) magma chamber, a "reservoir" of magma. This reservoir is connected to bigger ones on the depths of the lithosphere and / or the lithosphere-mantle limit, where magma generated by partial melting of rocks accumulates slowly.
It is usually in the magma chamber where processes that produce eruptions occur. They are basically changes in pressure, temperature and/or composition of the magma.
Imagine a pressure cooker. Water at high temperature (above 100 ° C) is held in liquid form under pressure. If we uncover abruptly, something not recommended to put into practice, the pressure drop will make water boil suddenly. In a magma chamber, a quick pressure drop can occur, for example, by the occurrence of a crack in its surroundings. Then much of the gases dissolved in the magma could move away from it (the technical term would be "exolution"), and this circumstance could trigger a volcanic eruption. On the other hand, a sudden increase in pressure in the magma chamber would push the magma out of it, or lead to the appearance of a crack in the volcano, or both, also triggering an eruption. Thus, pressure changes are a first factor which can result in a volcanic eruption.
Now imagine that we can inject boiling oil at 170 ºC in our pressure cooker full of hot water. I'm not sure if the pot would explode, but since then the water would become superheated, and would reach a temperature at which it will evaporate quickly, despite the pressure. If a more or less stable magma chamber receives a new input of magma at higher temperatures, it is likely that the original magma suffers a sudden degassing, again triggering our exciting (but do not forget, potentially destructive) volcanic eruption. In our pot, the injection of a substance at a lower temperature than water simply would lower the temperature, pressure and therefore the risk of explosion. But in a magma chamber, in the unlikely event of the entry of a magma at a lower temperature than the existing, it would cause the new magma to heat up again and sudden degassing, causing the eruption.
The changes in the magma composition deserve a separate chapter, and enter fully into the field of the Igneous Petrology. Suffice it to say that the mixture of different composition magmas undoubtedly trigger chemical reactions, many of which in turn generate changes in temperature, pressure, and of course may give rise to gas and, again, be the source of a volcanic eruption. And do not forget that the magma mixing typically occurs between melts that not only have different compositions but different starting pressures and temperatures.
In the end, the best of all is that walking on this region we can find, with the help of a geologist, evidence of the causes of volcanic eruptions that generated this little paradise on earth which is the Cabo de Gata Natural Park.