• Volcanoes are vents (opening) in the Earth's crust through which material escapes to the surface.

• Volcanic zones are found on the surface of the Earth, but 60% of volcanoes are in the depths of the oceans at diverging or converging boundaries. Other volcanoes are found away from plate boundaries where Earth's crust is then and stretched.

• Eruptions from volcanoes my be effusive or dramatically explosive, depending upon the chemistry of the magma in the volcano and its interaction with water at the surface.

• Geochemist classify magma chemistry as ultramafic, mafic, intermediate or felsic, depending on SiO2 (silica) content.

• They are fundamental component of the Earth's cooling and recycling systems, and an important interface between the geosphere and atmosphere.

• Shield Volcanoes: Shield volcanoes are large, broad, and gently sloping volcanoes characterised by extensive lava flows. They are built by the accumulation of low viscosity basaltic lava, which spreads out in all directions from a central vent. Shield volcanoes typically have a low profile and can cover a vast area. Hawaiian Islands are a complex of overlapping and separate shield volcanoes rising over 17 km form the Pacific Ocean floor, making them the tallest mounts in the world from base to peak. They are predominanatly compose of a'a/pahoehoe lavas. They are commonly dotted with parasitic cinder cones and produce spatter, Pele's tears and hair and reticulite.

  • • Spatter refers to small fragments of molten lava that are ejected into the air during volcanic eruptions.

    • Pele's Tears are volcanic glass droplets that form when molten lava is ejected into the air and quickly cools and solidifies. They are tear-shaped and have a smooth, glassy texture. Pele's Tears are named after Pele, the Hawaiian goddess of fire and volcanoes. They are typically small, ranging in size from a few millimeters to a few centimeters in diameter.

    • Reticulite is a type of volcanic rock characterised by its unique texture and appearance. It is a frothy, highly vesicular rock that contains numerous small, interconnected voids or cavities. The texture of reticulite is often described as "net-like" or "honeycomb-like" due to the interconnectedness of the voids. Reticulite is typically associated with highly explosive eruptions that produce very gas-rich lava. It is one of the most lightweight volcanic rocks and can float on water.

• Stratovolcanoes (Composite Volcanoes): are tall and conical volcanoes composed of layers, or strata, of hardened lava, ash, and volcanic debris. These volcanoes are formed by alternating eruptions of lava flows and explosive eruptions. Stratovolcanoes have steep slopes and often exhibit a symmetrical shape. They are found along convergent plate boundaries around the " Ring of Fire". They form from intermediate composition magma with typically high viscosity and gas content. Fluctuations in magma input and degassing generate eruptions that alternate between explosive and effusive. The stratovolcanoes complex life yields steep sides covered by jagged lava blocks, pumice and tephra. Destabilisation and edifice collapse can result in catastrophic lateral blast and explosion - Mt St Helens, 1980 USA.

• Cinder Cones:  also known as scoria cones, are small, steep-sided volcanoes built from fragments of solidified lava called cinders or scoria. They usually form around a central vent and have a conical shape. Cinder cones often have a bowl-shaped crater at the summit. 

• Lava Domes: are rounded, steep-sided mounds formed by the slow extrusion of highly viscous lava. They are typically composed of thick, sticky lava that piles up around the volcanic vent. Lava domes can grow over time as new lava is added to the top, and they often form within the craters of larger volcanoes.

• Calderas: are large, basin-shaped depressions that form when a volcano collapses after a massive eruption or when magma is withdrawn from a shallow magma chamber. They can range in size from a few kilometers to tens of kilometers in diameter. Calderas often have steep walls and may contain lakes or smaller volcanic features within them.

• Fissure Eruptions:  occur when magma reaches the surface through a long, linear crack or fissure in the Earth's crust. Instead of a single vent, these eruptions can produce a series of vents that release lava flows over a wide area. Fissure eruptions are commonly associated with shield volcanoes.

• Volcanic Cones: are small to medium-sized mounds formed by the accumulation of volcanic material around a vent. They can be composed of loose ash and pyroclastic deposits or solidified lava flows. Volcanic cones often have a circular or oval shape.

• Is a measure of a fluid's resistance to flow. In the context of magmas and lava flows, viscosity refers to the molten material's ability to resist deformation and flow. It is a crucial property of magmas and lava flows that affects their behavior during volcanic eruptions. The viscosity of magmas and lava flows is influenced by several factors, including temperature, composition, and volatile content.

  • • Temperature: Higher temperatures lead to lower viscosity, making the magma or lava more fluid. As the temperature decreases, viscosity increases, causing the material to become more sluggish.

    • Composition: Silica content plays a significant role in determining magma viscosity. Silica-rich magmas, such as rhyolite, have higher viscosities due to the strong bonding between silica tetrahedra. Mafic magmas, like basalt, with lower silica content, have lower viscosities.

    • Volatile Content: Volatile components, such as water vapor and carbon dioxide, can lower magma viscosity by reducing the strength of the magma's chemical bonds. Higher volatile content generally leads to lower viscosity.

• Measurement: 

  • • CPS viscosity refers to the viscosity measurement in centipoise (cP). Centipoise is a common unit used to express dynamic viscosity, particularly for low-viscosity fluids. It is equal to one hundredth of a poise (P) in the CGS system.

    • Dyne second per square centimeter (dyn·s/cm²) is the correct unit for dynamic viscosity in the CGS system. It measures the resistance to flow in units of force per unit area and time.

• Viscosity Impact on Eruption Styles:

  • • Effusive Eruptions (fire fountains): Magmas with low viscosity, such as basaltic lavas, allow gases to escape more easily and tend to result in effusive eruptions. These eruptions are characterised by relatively gentle lava flows that can travel long distances before solidifying. The low viscosity promotes a fluid-like flow.

    • Explosive Eruptions: Magmas with high viscosity, like rhyolitic lavas, have difficulty releasing trapped gases, leading to explosive eruptions. The high viscosity restricts gas movement, causing pressure to build up until it's suddenly released, resulting in explosive fragmentation of different size tephra particles ( ash, tuff, lapilli and pumice - collectively known as pyroclastic flows.

  • Influence on Lava Flows:

    • • Pahoehoe: Low-viscosity basaltic lavas can form pahoehoe flows, characterised by smooth, ropy textures. The low viscosity allows the lava to flow easily and develop these distinctive features.

      • A'a: Higher-viscosity lavas, such as andesite and rhyolite, tend to form a'a flows. A'a flows have a rough, blocky texture due to the increased resistance to flow, causing the outer layer of the lava to break and tumble over the still-moving interior.

      • Block Lavas are slowly flowing, silica rich lavas generally form blocks with relatively smooth surfaces.

      • Lava Tubes: In some cases, low-viscosity lavas can form lava tubes. As the outer layers of the lava flow cool and solidify, the still-flowing interior creates a tunnel-like structure, allowing the lava to travel long distances.

      • Columnar jointing: refers to a pattern of cracks or fractures that develop in igneous rocks, particularly in volcanic or intrusive formations. It occurs when the rock cools and contracts, resulting in the formation of polygonal columns. This phenomenon is most commonly observed in basaltic lava flows.

      • Pillow basalts: are volcanic rocks that form when lava erupts underwater or flows into a body of water, such as the ocean. The rapid cooling of the lava due to contact with the water results in the formation of distinctive pillow-shaped structures.

Volcanic gases are a mixture of gases released during volcanic activity. These gases originate from the magma beneath the Earth's surface and are expelled into the atmosphere during volcanic eruptions. The composition of volcanic gases can vary depending on factors such as the type of volcano, the magma composition, and the stage of volcanic activity. Here are the primary gases commonly found in volcanic emissions:

• Pyroclastic Flows: These are fast-moving currents of hot gas, ash, and volcanic rock fragments that can reach speeds of hundreds of kilometers per hour. Pyroclastic flows are extremely destructive and can incinerate everything in their path. These devastating volcanic hazards can cause huge avalanches of searing hot ash, gas, and pumice fragments, which can travel at velocities exceeding 300 km/hours, over large distances greaten than 100 km, covering vast areas.

• Explosive volcanic eruptions eject large amounts of water vapour, ash and sulfuric acid, which contributes to the acid rain around most volcanic eruption locations.

• The sulfuric acid droplets freeze in the upper troposhere or lower straosphere and produce a sulfate aerosol tht is highly reflective of incoming solar radiaiton. This is known as radiative forcing. The amount of radiative forcing can change global climate in both the medium and long term. The difference between the amount of energy from the Sun raidating to Earth minus the amount of raidated back into space. If the amount is negative then the Earth is cooling.