*Introduction to the Volcanology DOI: http://dx.doi.org/10.5772/intechopen.102771*


**Figure 12.** *Dyke at Ponza Island (Italy) [30].*

**Figure 13.** *Lava flow plateau (author's collection).*

rifts (**Figure 13**). The most important manifestations of effusive volcanism are lava flows. The mobility of the flow depends on the viscosity of the lava: (1) Basaltic lavas have high temperatures ( 1100°C) and less silica (<50%), and therefore, they have a low viscosity. Due to these characteristics, they can flow for tens of kilometers. (2) Andesitic lavas have temperatures of 900– 800°C and silica ( 57%), and therefore, have a high viscosity, which prevents excessive flow so that the lava tends to break as it flows. (3) The rhyolitic lavas have temperatures of about 700°C and silica >60%. They are so viscous that the lava accumulates at the volcanic mouth giving dome shapes or even spire-like extrusions. The lava flow can be defined in different forms—(1) pahoehoe lava: Characteristic surface form of basaltic lava. It is formed by high temperature and low viscosity. Characteristic form of lava poor in silica (**Figure 14**). (2) AA lava: They are lava morphologies typical of basaltic magmas that give the form of scoria type (**Figure 15**). They are characterized by a lower temperature and a higher viscosity than the basaltic magmas that give the pahoehoe lavas. Flow lava levée: The AA lava and pahoehoe while flowing build levée for fast cooling of the external parts of the flow (**Figure 16**). Lava-tube flow: It can happen that even when the surface of a large basaltic flow has solidified, the inner part continues to flow into a channel below the solid surface of the flow. If the lava flows go out from this channel, an empty space is formed that produces a lava tunnel (**Figure 17**). Block lava flow: A typical form of high viscosity rhyolitic andesitic acid lavas form block lava-like as Paricutin volcano (Mexico) (**Figure 18**). Cooling structures in subareal lava flows: During the final stages of cooling, the inner part of a lava flow contracts in almost hexagonal-shaped columns acquiring a

**Figure 14.** *Pahoehoe lava flow (author's collection).*

**Figure 15.** *Lava flow AA (author's collection). A solidified flow from the last eruption of Vesuvius (1944 AD).*

**Figure 16.** *Lava flow with levee (author's collection). Levee grew during a Hawaiian eruption.*

typical columnar crack (**Figure 19**). Submarine lava flows: In underwater basaltic flows, the lava cools much faster, and therefore solidifies and forms a bubble of vitreous lava or cushions that temporarily blocks the advance of the flow. After a while, the internal pressure lava breaks the crust, and a new bubble comes out. The process is repeated several times, forming layers of pillows on top of each other (**Figure 20**).

d. *Explosive volcanism*: The development of explosive volcanism requires viscous magma with a high content of silica and water. These characteristics are

**Figure 17.** *Lavatube flow with a growth stalactite melt (unknown volcano).*

**Figure 18.** *Block lava flow. Modified from Schmincke [14].*

generally acquired by processes of differentiation of basaltic magmas in the magma chamber or by direct melting of the continental crust by basaltic magmas or by the direct mixing with external water. Headings are from **Table 2**.

**Figure 19.** *Columnar lava (author's collection).*

**Figure 20.** *Pillow lava formed under the sea (author's collection).*


#### **Table 3.**

*Types of explosive volcanism.*
