Comprehensive Guide to Volcanism and Volcano Types

Magma is defined as hot molten rock that originates within the Earth's interior and moves upwards.
Magma does not always reach the Earth's surface; it often cools and solidifies into underground rock formations at various depths.
Volcanism occurs when magma successfully reaches the Earth's surface.
Lava is the term used for magma once it has reached the surface.
The process of volcanic rock formation involves lava erupting from a magma chamber inside a volcano and subsequently cooling.

Volcanic activity and the physical properties of the materials exposed on the surface are governed by the chemical composition of the magma.
Chemical composition varies, but the most important constituent influencing behavior is silica ( $SiO_2$ ).
Differences in silica concentration dictate the viscosity, color, and eruption style of the volcano.

Distribution: Approximately $\frac{4}{5}$ (four-fifths) of all magma on Earth is formed at divergent plate boundaries.
Locations of Divergence:
- Rift valleys located on land.
- Mid-ocean ridges at the bottom of the sea.
Physical and Chemical Properties of Divergent Magma:
- The molten rock originates from the great depths of the asthenosphere.
- Temperature: The magma has a high temperature, typically ranging from $1100-1200\,^\circ\text{C}$ .
- Metallic Content: The magma is rich in metallic alloys, specifically magnesium and iron.
- Silica Content: It is relatively poor in $SiO_2$ .
- Fluidity: The basaltic lava produced is highly fluid because it is poor in expansive (feszítő) gases.

Due to the high fluidity of the lava at divergent boundaries, the lava spreads out easily, leading to specific landforms:
- Gently sloping shield volcanoes.
- Large, flat basalt plateaus.
Fissure Volcanoes (hasadékvulkán):
- This type of volcano is formed when lava reaches the surface through long, narrow gaps or gaps in the crust.
Geographic Example: The Mid-Atlantic Ridge, specifically in Iceland.

Formation Process:
- Volcanic activity occurs along deep-sea trenches where the lithosphere is converging and subducting.
- Melting occurs, but only a very small fraction of the subducting rock mass actually melts.
Compositional Characteristics:
- The resulting magma has a different composition from the subducting plate because only a fraction reaches melting point.
- Silica Content: It contains more silica ( $SiO_2$ ) than divergent boundary magma.
- Metallic Content: It contains less metal, which causes the resulting rocks to be lighter in color.
Eruption Dynamics:
- Viscosity: The lava flows more densely and is more viscous (thick) than basaltic lava.
- Slope: Due to the high viscosity, the lava does not travel far, resulting in volcanic cones with steeper slopes.
- Gas Content: The magma has a higher gas content, which frequently results in violent, explosive eruptions.

Explosive Mechanisms:
- Explosive eruptions involve both lava flows and the intense dispersal of volcanic debris.
- These debris deposits form rocks known as tuffs.
Stratovolcanoes (Composite Volcanoes):
- Repeated eruptions result in the deposition of alternating layers of solidified lava rock and volcanic debris rocks.
- This type of volcano is also known as a conduit or piped volcano (csatornás/kürtős).
Anatomy of a Stratovolcano:
- It features a long tube or magma channel through which the magma is forced upwards from the magma chamber.
- The eruption occurs through a central vent.
- The vent often terminates in a bowl-shaped depression called a crater.
Geographic Examples:
- Fuji (Japan).
- Mt. St. Helens (USA).
- Vesuvius (Italy).