Lava | Science for ACT PDF Download

Introduction

• Lava, the molten or partially molten rock known as magma, emerges from the depths of terrestrial planets or moons, such as Earth, onto their surfaces. This expulsion of lava can occur through volcanic eruptions or fractures in the crust, whether on land or underwater, typically at temperatures ranging from 800 to 1,200 °C (1,470 to 2,190 °F). Subsequent to cooling, the resulting volcanic rock is often referred to as lava.
• During an effusive eruption, lava flows out, constituting a lava flow. In contrast, an explosive eruption generates a mixture of volcanic ash and other fragments known as tephra, distinct from lava flows. With a viscosity comparable to ketchup, typically 10,000 to 100,000 times that of water, lava possesses the capacity to travel considerable distances before solidifying due to cooling. This phenomenon occurs because when lava is exposed to air, a solid crust forms swiftly, insulating the underlying liquid lava, thereby sustaining its high temperature and low viscosity, facilitating continued flow.
• The term "lava" originates from Italian and likely traces back to the Latin word "labes," meaning a fall or slide. Early usage of the term in the context of magma extrusion from beneath the surface dates back to a brief account of the 1737 eruption of Vesuvius by Francesco Serao. In his narrative, he likened the flow of "fiery lava" to the movement of water and mud down the volcano's flanks after heavy rainfall, akin to a lahar.

Lava Properties

Composition

• In a video capturing the eruption of Litli-Hrútur in 2023, lava is depicted agitating and bubbling, showcasing its dynamic nature. Upon solidification on the Earth's crust, lava predominantly consists of silicate minerals, encompassing feldspars, feldspathoids, olivine, pyroxenes, amphiboles, micas, and quartz. 
• Occasionally, nonsilicate lavas form through localized melting of nonsilicate mineral deposits or separation of magma into immiscible silicate and nonsilicate liquid phases.

Silicate Lavas

• Silicate lavas represent molten blends mainly composed of oxygen and silicon, the primary elements abundant in the Earth's crust. They also contain trace amounts of aluminum, calcium, magnesium, iron, sodium, and potassium, alongside minor quantities of various other elements. 
• Petrologists typically express the composition of silicate lava in terms of the weight or molar mass fraction of the oxides of major elements other than oxygen present in the lava.
• The presence of silica significantly influences the physical properties of silicate magmas. Silicon ions in lava form strong bonds with four oxygen ions in a tetrahedral arrangement. The degree of polymerization, characterized by the clustering or chaining of silicon ions via bridging oxygen ions, affects lava viscosity. Aluminum, in conjunction with alkali metal oxides like sodium and potassium, further contributes to polymerization. Conversely, cations such as ferrous iron, calcium, and magnesium weaken bonding with oxygen, reducing polymerization. Higher silica content results in increased viscosity, distinguishing lava high in silica from lava with lower silica content.
• Silicate lavas are categorized into four chemical types based on their silica content: felsic, intermediate, mafic, and ultramafic.

Felsic Lava

• Felsic or silicic lavas, with silica content surpassing 63%, include varieties such as rhyolite and dacite lavas. Due to their high silica content, felsic lavas exhibit extreme viscosity, ranging from 108 cP (105 Pa⋅s) for hot rhyolite lava at 1,200 °C (2,190 °F) to 1011 cP (108 Pa⋅s) for cool rhyolite lava at 800 °C (1,470 °F). 
• This high viscosity often leads to explosive eruptions, yielding pyroclastic deposits. However, felsic lavas can also erupt effusively, forming lava spines, domes, or "coulees." These lavas frequently fragment upon extrusion, giving rise to block lava flows, often containing obsidian.

Intermediate Lava

• Intermediate or andesitic lavas, with silica content ranging from 52% to 63%, are lower in aluminum and richer in magnesium and iron compared to felsic lavas. They typically erupt at temperatures between 850 to 1,100 °C (1,560 to 2,010 °F) and possess lower viscosity, making them less explosive. 
• Intermediate lavas tend to form andesite domes and block lavas, commonly found on steep composite volcanoes.

Mafic Lava

• Mafic or basaltic lavas are characterized by relatively high magnesium oxide and iron oxide content, with silica content ranging from 52% to 45%. Erupting at temperatures between 1,100 to 1,200 °C (2,010 to 2,190 °F), these lavas have lower viscosities, allowing them to flow for long distances from the vent. Mafic lavas often produce low-profile shield volcanoes or flood basalts, with lava types like ʻaʻā or pāhoehoe.

Ultramafic Lava

• Ultramafic lavas, exemplified by komatiite and highly magnesian magmas like boninite, possess silica content under 45%. These lavas erupt at extreme temperatures, with komatiites thought to erupt at 1,600 °C (2,910 °F). 
• With practically no polymerization of mineral compounds at such high temperatures, ultramafic lavas exhibit high mobility. Although no modern komatiite lavas are known due to the Earth's mantle cooling, these lavas are significant in understanding ancient volcanic activity.

Alkaline Lavas

• Certain silicate lavas exhibit elevated alkali metal oxide content, particularly in regions of continental rifting, overlying deeply subducted plates, or intraplate hotspots. 
• These lavas vary in silica content, from ultramafic to felsic, and are more likely to originate at greater depths in the mantle than subalkaline magmas. Olivine nephelinite lavas, both ultramafic and highly alkaline, are believed to originate from deeper mantle regions compared to other lavas.

Non-Silicate Lavas

Uncommon compositions of lava have erupted onto the Earth's surface, exemplified by:

Carbonatite and Natrocarbonatite Lavas

• Found at Ol Doinyo Lengai volcano in Tanzania, carbonatite and natrocarbonatite lavas constitute a distinctive example of an active carbonatite volcano. These lavas typically contain about 75% carbonate minerals, accompanied by lesser amounts of silica-undersaturated silicate minerals like micas and olivine, as well as apatite, magnetite, and pyrochlore. The original composition of the lava may have included sodium carbonate, subsequently removed by hydrothermal activity. 
• Carbonatite lavas exhibit stable isotope ratios indicative of derivation from highly alkaline silicic lavas, likely through separation of an immiscible phase. Natrocarbonatite lavas, prevalent at Ol Doinyo Lengai, consist primarily of sodium carbonate, with lesser amounts of calcium carbonate and potassium carbonate, along with minor halides, fluorides, and sulfates. These lavas exhibit remarkable fluidity, with viscosities slightly greater than water and temperatures ranging from 491 to 544 °C (916 to 1,011 °F).

Iron Oxide Lavas

• Considered the source of iron ore at Kiruna, Sweden, these lavas formed during the Proterozoic era. Pliocene-age iron oxide lavas are also found at the El Laco volcanic complex on the Chile-Argentina border. These lavas are believed to result from the immiscible separation of iron oxide magma from a parental magma of calc-alkaline or alkaline composition.

Sulfur Lavas

• Sulfur lava flows, up to 250 meters (820 feet) in length and 10 meters (33 feet) in width, occur at Lastarria volcano in Chile. Formed by the melting of sulfur deposits at relatively low temperatures, as low as 113 °C (235 °F), these unique lava flows represent an uncommon phenomenon.
• Additionally, the term "lava" extends to refer to molten "ice mixtures" in eruptions observed on the icy satellites of the Solar System's gas giants.

Rheology

• The behavior of lava flows is predominantly determined by lava viscosity. While the temperature of typical silicate lava ranges from approximately 800 °C (1,470 °F) for felsic lavas to 1,200 °C (2,190 °F) for mafic lavas, lava viscosity spans over seven orders of magnitude, from 1011 cP (108 Pa⋅s) for felsic lavas to 104 cP (10 Pa⋅s) for mafic lavas. The viscosity of lava depends on its composition, temperature, and shear rate.
• Lava viscosity dictates the type of volcanic activity upon eruption, with higher viscosity lavas exhibiting a greater tendency for explosive eruptions. Consequently, most lava flows on Earth, Mars, and Venus consist of basalt lava. Earth's lava flows primarily comprise mafic or ultramafic lavas, while intermediate and felsic lavas constitute a smaller proportion.
• The speed and aspect of lava flows, as well as their surface characteristics, are influenced by viscosity. Highly viscous lavas often erupt as high-aspect flows or domes when effusive, while intermediate lavas tend to form steep stratovolcanoes. Mafic lavas, characterized by lower viscosity, generate relatively thin flows that can travel long distances, forming shield volcanoes with gentle slopes.

Temperature

• The temperature of most molten lava types ranges between about 800 °C (1,470 °F) to 1,200 °C (2,190 °F), varying according to chemical composition. Lava exhibits greater fluidity when first erupted, gradually becoming more viscous as it cools. As lava cools, it forms an insulating crust of solid rock, with cooling occurring primarily through slow heat conduction. 
• Crystallization within the lava flow creates vesicles at its boundaries, contributing to its characteristic texture and structure. Flow banding and other features are common in lava flows, dependent on their composition and cooling dynamics.

Lava Formations

Lava and volcanic eruptions sculpt a diverse array of landforms, ranging from large-scale features to intricate formations.

Volcanoes

• Volcanoes represent the principal landforms resulting from successive lava and ash eruptions over time. They vary in morphology, from shield volcanoes with gentle slopes formed by effusive basaltic lava flows to steep-sided stratovolcanoes, composed of alternating layers of ash and viscous lava typical of intermediate and felsic lavas.

Calderas

• A caldera, a large subsidence crater, may form within a stratovolcano when the magma chamber undergoes significant emptying due to explosive eruptions. This results in the collapse of the summit cone, sometimes creating features like volcanic crater lakes and lava domes.

Cinder and Spatter Cones

• These are small-scale features formed by the accumulation of lava around a vent. Cinder cones consist of tephra or ash and tuff, while spatter cones form from molten volcanic slag and cinders ejected in a more fluid state.

Kīpukas

• Kīpukas are elevated areas like hills or ridges surrounded by active volcanic terrain. They often appear as forested islands amidst barren lava flows.

Lava Domes and Coulées

• Lava domes result from the extrusion of viscous felsic magma, forming prominent rounded structures. When formed on inclined surfaces, lava domes can flow in thick, short streams known as coulées.

Lava Tubes

• These form when the upper surface of a lava flow cools to create a crust, allowing molten lava to flow beneath it. Over time, this can create tunnel-like structures known as lava tubes, which can transport lava for considerable distances.

Lava Lakes

• In rare instances, volcanic cones may fill with lava without erupting, resulting in lava lakes. These formations are transient, often draining back into the magma chamber or erupting as lava flows or pyroclastic explosions.

Lava Deltas

• Lava deltas form when subaerial lava flows enter bodies of water, cooling and breaking apart as they encounter the water. Fragments fill in the seabed, allowing the flow to extend further offshore.

Lava Fountains

• A volcanic phenomenon where lava is forcefully ejected from a crater or vent, lava fountains are commonly associated with Hawaiian eruptions. They can reach impressive heights and occur as short pulses or continuous jets.

Hazards

• Lava flows pose significant hazards to property and lives in their path, though casualties are rare due to their slow movement. However, injuries and deaths can occur if escape routes are cut off or if individuals get too close to the flow. Other volcanic hazards include pyroclastic flows, lahars, toxic gases, and explosions triggered by interactions with water. 
• Areas affected by recent lava flows remain hazardous due to unstable terrain and the risk of collapse or injury from sharp lava formations.