Formation and Types of Volcanoes: A Comprehensive Overview

Volcanoes generally form along the edge of the tectonic plate which prevails over a subduction zone. Cracks in the crust caused by the instability of the overlying plate allow magma to move towards the surface and form a magma chamber. Further cracks in the crust allow magma to be ejected from the chamber and form a volcano.

Lava can be divided into three general groups: basaltic, andesitic, and rhyolithic. Basaltic lava contains 48% – 58% silica, is basic and is very fluid, with a temperature of about 1160 °C. Rhyolitic lava contains 65% silica and more and is acidic and viscous with a temperature of about 900 °C. Andesite is intermediate between these types of lava.

Shield Volcano

Shield volcanoes consist of fluid basalt lava. Lava contains low levels of silica and flows down the sides of the volcano, causing its base to widen without increasing its height. They have a very broad base.


Calderas are the remains of large volcanic craters formed by the collapse of a volcano. The magma chamber empties causing the overlying ground to collapse. The crater may eventually fill with water, forming a lake.

Dome Volcano

Dome volcanoes are formed from silica-rich viscous rhyolithic magma. This magma traps a large amount of gas. The pressure caused by the accumulation of gas is not easily released because the magma is so viscous, so the eruptions are often extremely violent.

Strato Volcano

Volcanic strata tend to have steep upper slopes. The ejected lava is commonly andesitic and relatively viscous, containing moderate levels of silica. This causes lava to accumulate on the upper slopes. This produces an area that is seismically and volcanically extremely active. About three-quarters of the world’s active and dormant volcanoes are found around the edge of the ring, and nearly 90% of all seismic activity is found there.

Multilayer structure of the Earth


Evidence comes from the analysis of:

  • external rocks along mountain ranges
  • lava ejected
  • washes in diamond vent


Evidence comes from the study of:

  • gravitational field
  • magnetic field
  • analysis of methods
  • propagation pattern of seismic waves

CONTINENTAL CRUST: 25-70 km thick below the continents, it is made up of igneous, metamorphic, and sedimentary rocks. It is not recycled within the Earth as often as the oceanic crust, so some continental rocks are up to 4 billion years old. Igneous and sedimentary rocks; Base in curved and metamorphic granite; continental crust; Highly metamorphic granite; Moho cloak

OCEANIC CRUST: about 10 km thick under the oceans, it constitutes more than two-thirds of the earth’s surface, is continuously formed by the mantle material and is therefore young. Even the oldest parts of the ocean floor are no more than 200 million years old. Sediment, Oceanic Crust, Moho Mantle

The crust is rich in lighter minerals such as silicon, calcium, and aluminum and is less dense than the mantle. He rides it causing oceans, mountains, and volcanoes to form.


The refraction of the wave is a change in the direction of movement and the area in which it occurs is called a discontinuity. The refraction of the seismic wave is due to a change in the state and composition of the rock.

WAVES P: primary, they travel in the solids and fluids that make the propagation. WAVES S: secondary, they travel only in the solids.

WAVES L: longitudinal, which slow down start from the epicenter. WAVES R: almost circular movements.


The study of the propagation of seismic waves across the planet made it possible to identify some discontinuities between different chemical and physical areas.

COAT (mantello)

The mantle is a layer under the crust, where the slow movements of convection currents represent the engine of plate tectonics.

Plate boundaries

  • Divergent or accretionary boundaries 10Km: the two plates move apart: birth of the oceans, the oceans are born and deep ocean ridges and the new lithosphere are created
  • Transform or conservative boundaries 25Km: the plates slide side by side and on the surface creates a fault
  • Convergent or destructive boundaries are found along subduction zones where plates converge and one plate sinks down under the other and is consumed in the asthenosphere. +100Km: one of the two plates goes over the other, the oceans give way to the emerged lands and the mountains are born

The lithosphere comprises the earth’s crust and is rigid and hard, including the upper part of the mantle. The asthenosphere follows the lithosphere and the rocks are in the molten state.

One evidence of the sea floor spreading: magnetic anomalies of the ocean floor

Magnetic anomaly is related to a different magnetic orientation of the rocks compared to the main earth magnetic field. Magnetic orientation is memorized by the rocks when they had been cooled down and along the ridge it appears equal as a sort of several bands.

Another evidence of sea floor spreading: the age of oceanic sediments

The age of the sediments that rest on the oceanic crust increases with distance from the axis of the ridge.

Oceanic sediments are formed from the accumulation of minute shells of planktonic organisms.

A hotspot is a large plume of hot mantle material rising from deep within the Earth.

The passage of a lithospheric plate over a hot spot leaves a line of volcanoes as a trace, assuming that the hot spots are immobile or almost motionless, it’s possible to reconstruct plate movement based on the alignment of the volcanoes.

Convective motions are mainly due to the following phenomenon: a fluid, if its temperature increases, tends to expand (or to increase pressure), decreasing its density; while if it cools, its density increases and the liquid contracts (or its pressure decreases). Between portions of fluid at different temperatures, and therefore with different densities, hydrostatic equilibrium is lacking and the force of gravity causes denser portions of liquid to move downwards, and warmer portions are pushed upwards.