The goal is to explain the differences between Basaltic, Rhyolitic, and Andesitic volcanic eruptions. This will be done by discussing the processes that cause them, the types of volcanoes they create, and the substances they release.
When magma, a scorching liquid rock within the earth’s core, rises to the surface, volcanic eruptions occur. This process creates gas bubbles in the magma, increasing pressure inside the volcano until it eventually erupts. The expelled magma is called lava. Different types of magma determine the characteristics of various volcanoes. Volcanoes with thin magma allow gas to escape easily, resulting in non-explosive eruptions where lava flows down the mountain like in Hawaii and Mount Etna. In contrast, thick and sticky magma traps gas, causing an accumulation of pressure that leads to explosions. These eruptions can trigger landslides and release massive clouds of burning rock and gas that devastate surrounding areas like Mount St Helens and Montserrat.
The effects of volcanic eruptions are diverse and include changes in weather patterns such as rain, thunderstorms, and lightning. They can also have long-term climate impacts by inducing global cooling. For example, in 1991 when Mount Pinatubo erupted, there was a 0.5 Celsius decrease in global temperatures the following year. Additionally, volcanic eruptions can directly cause fatalities through fast-moving lava flows known as pyroclastic flows that can kill individuals or make breathing difficult due to falling ash. People may also face death from related factors such as famine, fires, and earthquakes associated with volcanic activity.
The eruption of volcanoes can lead to extensive destruction, affecting homes, roads, and agricultural fields. The presence of lava poses a threat to plants and animals alike. A devastating event occurred in 1980 when Mount St Helens erupted, causing multiple deaths. An estimated number of around 24,000 animals lost their lives as a result, which included approximately 11,000 hares, 6,000 deer, 300 bobcats, 200 black bears, and 15 mountain lions.
In 1912, Alfred Wegener introduced the theory of plate tectonics, explaining volcano formation. The theory states that Pangaea was a Super Continent around 300 million years ago and later divided into two sections: Laurasia (north) and Gondwanaland (south). Over time, Laurasia became North America and Eurasia while Gondwanaland split into Africa, South America, Australia, India, and Antarctica. Eventually, India moved northward and collided with Eurasia to form the Himalayas.
Alfred Wegener provided evidence supporting the idea that the continents were once connected. One piece of evidence is that South America and Africa fit together like a puzzle. Additionally, coal and sedimentary deposits from 290 million years ago are found in India, Antarctica, South America, and Southern Africa, suggesting they were once joined. Similarly, rock formations in Scotland are identical to those in Canada. Biological evidence includes the presence of identical reptile fossils in Australia, India, South America, and Africa, as well as fossilized remains of tropical forests. However, despite this evidence, Wegener’s lack of understanding about the movement of continents led to his dismissal and ridicule.
Constructive plate boundaries can be found in various locations. Along mid-oceanic ridges in the Atlantic, Pacific, and Indian oceans, constructive plate boundaries are situated under the ocean. In northeast Africa, there is also a constructive plate boundary where a rift valley has developed as a result of the separation of Africa due to currents. A comparable event is happening at the Dead Sea.
Additionally, Oceanic Ridges are found in the Atlantic, Indian Ocean, and Pacific. They stretch from North to South and serve as locations where new rock is continuously formed due to the divergence of currents beneath the Earth’s crust. This separation of the oceanic plate results in the emergence of basalt and the creation of ridges. The height of these ridges varies depending on the speed at which the ocean floors are moving apart. When this rate is slow (1 – 1.5cm per year), the ridges significantly rise above the ocean floor. In contrast, a faster rate (up to 9cm per year) leads to a smooth and lower crest. The Pacific experiences fast rates while the Atlantic has slower rates.
Furthermore, beneath continents lie plate boundaries where occasional divergent currents occur. These currents cause continents to break apart and generate rift valleys like East Africa’s Great African Rift Valley. These rift valleys are characterized by lakes and volcanic activity, with Mount Kilimanjaro being an example. Over time, these rift valleys have potential for developing into separate oceans.
In conclusion, there are two types of plate margins: Destructive Plate Margins and conservative plate margins. Destructive Plate Margins involve the destruction and formation of the Earth’s crust. Oceanic Convergence occurs when oceanic and continental plates collide, causing subduction and the formation of land masses through melting sediments. Oceanic convergence, on the other hand, occurs when two oceanic plates meet, resulting in the formation of oceanic trenches and island arcs like Hawaii. Lastly, continental convergence happens when two continent plates come together, causing sediment to be forced up and form mountains. On the other hand, Conservative Margins involve the sliding of two plates without any creation or destruction. Although there is no subduction or volcanic activity, the different speeds at which the plates move create friction and cause shadow earthquakes.
Then Vulcanicity is typically linked to plate boundaries, but Hawaii is an exception as it is thought to have formed from a hot spot. A hot spot forms when there is a concentration of radioactive elements. Magma rises and erodes through the plate, resulting in active volcanoes when lava reaches the surface. The hot spot remains stationary while the Pacific plate moves over it, creating a line of volcanoes. The volcano above the hot spot is active, while the others form a chain of islands with extinct volcanoes.
Basaltic lava is created from low silica magma, resulting in a more fluid magma which allows for gas bubbles to expand during its ascent to the surface, preventing sudden explosive activity. Conversely, Rhyolitic and Andesitic lavas are formed from high silica magma that is highly viscous. Frequently, these lavas solidify prior to reaching the surface, resulting in a buildup of pressure and, ultimately, a violent explosion.
Basaltic lava flows primarily originate from shield volcanoes, fissure systems, scoria cones, and spatter cones. These types of volcanic landforms are commonly found in lava plateaus and shield volcanoes. Lava plateaus form when fissure eruptions occur, allowing extensive basaltic lava flows to travel long distances across the ground. Shield volcanoes, like Mauna Loa in Hawaii, are also formed by the free-flowing nature of basaltic lava. These volcanoes have gentle slopes and cover large areas. The Hawaiian shield volcanoes serve as famous examples of this type of formation.
Shield volcanoes mainly consist of highly fluid basalt lava that easily runs downhill upon eruption. This fluidity prevents steepness in the volcano’s shape. Explosive eruptions at shield volcanoes are rare unless water enters the vent. Generally, eruption activity is characterized by low-explosivity fountaining that creates cinder cones and spatter cones at the vent. Despite this, approximately 90% of a shield volcano is composed of lava rather than pyroclastic material.
In Hawaii’s hot spots where underground water is present, explosive activity can occur near shield volcanoes. However, these occurrences are not common since shield volcanoes form due to high rates of magma supply and the lava remains hot and relatively unchanged since its formation.Shield volcanoes are typically formed by hotspot volcanism, but they can also be found in volcanic arcs like the Galápagos Islands. Basaltic volcanoes, which are usually located near constructive plate margins, have fluid magma that is not viscous. These types of volcanoes primarily release lava and have limited amounts of lava bombs. As a result, fast pyroclastic flows are common.
Lastly, andesitic eruptions are characterized by the viscosity in the flow of lava. Andesitic lava is commonly found in the Andes and other North American mountains. It is primarily composed of the rock andesite, which is the second most abundant volcanic rock, following basalt. This type of lava consists of coarse crystals. Although andesitic lava flows more easily than rhyolitic lava, it is not as fluid as basaltic lava. Eruptions of andesitic volcanoes typically give rise to composite volcanoes, which are characterized by a cone-like shape, representing the classic volcano form.
Rhyolite volcanoes, which are the most explosive and complex of Earth’s volcanoes, do not typically resemble typical volcanoes. They undergo extreme gas eruptions, resulting in a massive explosion that removes the summit of the cone, leaving behind a several kilometer-wide opening. The resulting caldera may become flooded by the sea or form a lake within it. These volcanoes are so explosive that they collapse inward rather than forming tall structures. The sunken areas are known as calderas, indicating the presence of vast magma chambers associated with the eruptions. Ash layers from these calderas often extend over thousands of square kilometers in all directions. One notable eruption occurred in 83 AD when Taupo erupted in New Zealand. Smaller eruptions from these complexes may occur along the ring faults of the main caldera, but this is not always the case. The origin of these rhyolite complexes is still not fully understood. Examples of rhyolite caldera complexes include Yellowstone, La Primavera, Rabaul, Taupo, Toba, and others. Additionally, rhyolitic volcanoes are typically found near destructive plate margins and their magma is thick and viscous. Lava bombs are common, while lava flows are rare occurrences.
Two major volcanic explosions have occurred in Sicily (Mt Etna) and Montserrat (Soufriere Hills) within the past twenty-two years. Mt Etna stands at an impressive height of 3,323 meters and dominates the landscape above Catania, the second largest city in Sicily. A notable feature of Mt Etna is the Valle del Bove, a horseshoe-shaped Caldera spanning 5 to 10 km, which is completely open to the east. This Caldera was formed when the volcano collapsed catastrophically during an eruption, resulting in a massive landslide. Although the volcano can be destructive, it is not considered particularly dangerous. Numerous people reside on its slopes and nearby areas, cultivating the fertile volcanic soils.
In late 1991, lava began to flow from Bents Hi, located on the volcano’s eastern flank. Various protective measures were implemented to halt the progression of the lava flow. One measure involved constructing a substantial earth barrier across the Val Calanna, located at the southern end of the Valle del Bove. This barrier stood several tens of meters high and stretched 400 meters in length, effectively holding back the lava for several months. The intention was to temporarily slow down the advance of the lava while implementing additional protective measures.
By the spring of 1992, the accumulated lava began overflowing from this barrier and cascading into the valley leading to Zafferna. Smaller barriers constructed across the valley were quickly overwhelmed by the advancing lava, resulting in the destruction of orchards and a few small buildings. As a solution, it was decided to cut off the flow by blocking the primary feeder channel.In an attempt to redirect the lava flow on Mount Etna, engineers initially dropped concrete blocks from helicopters through the roof of the upper lava tube. However, it was not until May 1992 that they resorted to blasting openings in the lava tube to encourage a new direction of flow, partially on top of the existing globe rather than feeding it. This approach, however, does not always prove successful in halting lava flows on Mount Etna. As evidenced in 2002, a more severe eruption resulted in the complete destruction of the ski station in multiple resorts, along with damage to buildings and ash rain in the surrounding areas. Given that Mount Etna is a shield volcano, it is most likely to have experienced an andesitic volcanic eruption.
In July 1995, the volcano in the Soufriere Hills on Montserrat Island in the Caribbean became active after centuries of dormancy. The initial activity included ash emissions, steaming explosions, and earthquakes, with steam and ash reaching heights exceeding 2500 m. In early 1997, there was ongoing growth with small explosions and projectile eruptions. The most significant event occurred on June 25 when large explosions generated extensive pyroclastic flows, releasing 4 to 5 million cubic meters of material from the eruption. This material flowed down the northern flank of the volcano, causing damage to houses and fatalities. Only 40 km of the island’s 100 km was deemed safe for habitation. The capital city, Plymouth, was eventually buried under more than 10 m of ash and mud. During this period, the British government assisted in evacuating 7000 residents out of a total population of 11,000 to neighboring islands like Antigua or offering resettlement in the UK. Financial aid was provided for all resettlement efforts. The eruption originated from a rhyolitic volcano. As a result, Plymouth, which once had a population of 3000, now stands as a deserted town due to the significant ash and mud deposition caused by the explosion.
