The most obvious trait of a shield volcano is its shape, which is gently sloping with an immensely wide base. A shield volcano is one from which mainly lava flows, and its shape relates directly to the acidity of its lava, which is alkaline.
Along the lines where tectonic plates meet, molten rock (magma, containing hot gases and sometimes other materials) is able to expand upwards and sometimes reach the surface of the Earth. Its eruption at the surface will form a volcano. As the magma emerges from the volcano’s crater, the hot gases will be given off and the remaining pure molten rock is the lava.
The behaviour of the lava will be determined by its acidity, as this will affect its thickness. For example acid lava is extremely thick and viscous and therefore slow moving. Imagine the thickest possible porridge, or toffee. Their viscosity causes them to be difficult to pour, and so it is with acid lava. Emerging from the crater, its slow movement tends to cause steep sided and dumpy shaped volcanic cones.
On the other hand imagine a very thin sauce, or boiling milk. If either of these boil over they will flow quickly down the sides of a pan and make a real mess of the hob ( or stove-top, for Americans) as they are able to move a long way quite easily. This is exactly how basic lava behaves – it has a relatively thin consistency and is able to run out over long distances, down the gentle gradient of the volcanic cone it creates.
This describes the production of a shield volcano, and imagining a circular medieval shield lying on the ground, its outer part upwards, explains the origin of the word shield when described to a volcano – circular, gently sloping and with its highest point, more or less, in the centre.
The largest examples of shield volcanoes are much larger than is immediately apparent, as they have erupted from the sea bed. Volcanoes of the Hawaiian Islands, for example, produce extremely basic lava which begins to spread out from submarine cracks beneath the Pacific Ocean. Mauna Loa has a base which is 480 km (300 miles) in diameter on the sea bed, and a maximum height of 9,750m (32,000 feet) from its base. This makes it the tallest mountain in the world, although it only reaches about 4,168 m (13,675 feet) above sea level. It also makes it one of the most extensive volcanic cones in the world.
The main traits of a shield volcano are its size, shape and the acidity of its lava. They always have gently sloping cones, and can extend over huge areas. Both of these factors are caused by the alkalinity of the lava which flows from a shield volcano.