Difference Between Magma And Lava

Delving Deep: Understanding the Difference Between Magma and Lava

The fiery heart of our planet, responsible for volcanic eruptions and the creation of stunning geological formations, holds two key components: magma and lava. While often used interchangeably, these terms represent distinct stages in the life cycle of molten rock. This article delves deep into the fundamental differences between magma and lava, exploring their composition, behavior, and the geological processes that govern their transformation. Understanding this distinction is crucial to comprehending volcanism and the dynamic processes shaping our Earth.

What is Magma?

Magma is molten rock found beneath the Earth's surface. It's a complex mixture of molten silicate minerals, dissolved gases (primarily water vapor, carbon dioxide, and sulfur dioxide), and crystals. The composition of magma is highly variable, depending on the source rock it melts from, the degree of melting, and the pressure conditions within the Earth. This variability directly influences the type of volcanic eruption and the resulting lava. The key characteristic of magma is its location: it's underground. High pressure keeps the dissolved gases contained within the molten rock.

Factors Affecting Magma Formation:

• Temperature: The temperature within the Earth increases with depth (geothermal gradient). When temperatures reach the melting point of rocks, magma forms. This melting can be caused by various factors, including decompression melting (reduction in pressure), addition of water (flux melting), or heat transfer from rising magma.
• Pressure: Pressure plays a critical role in determining the melting point of rocks. A decrease in pressure lowers the melting point, facilitating magma formation. This is particularly important at mid-ocean ridges where tectonic plates are diverging.
• Water Content: The presence of water lowers the melting point of rocks. Water, released from subducting plates, can trigger melting in the overlying mantle wedge, creating magma.
• Rock Composition: The chemical composition of the source rock significantly affects the composition of the resulting magma. Different rock types melt at different temperatures and produce magmas with varying chemical compositions.

Types of Magma:

Magma is classified based on its silica (SiO2) content:

• Basaltic Magma: Low in silica (around 45-55%), high in iron and magnesium. This type of magma is typically less viscous (thinner) and produces effusive (gentle) eruptions.
• Andesitic Magma: Intermediate silica content (around 55-65%). It has a moderate viscosity and can produce both effusive and explosive eruptions.
• Rhyolitic Magma: High in silica (around 65-75%), high in potassium and sodium. This type of magma is very viscous (thick) and tends to produce highly explosive eruptions.

What is Lava?

Lava is molten rock that has reached the Earth's surface. Essentially, lava is magma that has erupted from a volcano. The process of eruption dramatically alters the behavior of the molten rock. The release of pressure as magma reaches the surface causes the dissolved gases to expand rapidly, leading to the characteristic explosive or effusive eruptions. The key difference is location: lava is above ground.

Lava Flows and Eruptive Styles:

The viscosity (thickness) of lava significantly affects its flow behavior and the resulting volcanic landforms.

• Pahoehoe Lava: This type of lava is relatively low in viscosity, forming smooth, ropy flows. It's typically associated with basaltic magmas.
• Aa Lava: This lava is more viscous than pahoehoe, resulting in a blocky, jagged flow with a rough surface. It's also commonly associated with basaltic magmas, but can occur with other types.
• Pillow Lava: This type of lava forms underwater, where the rapid cooling of the lava causes it to solidify into pillow-like shapes.
• Block Lava: This lava is very viscous and flows slowly, forming large, angular blocks. It's often associated with andesitic and rhyolitic magmas.

The Transformation from Magma to Lava: The Eruptive Process

The journey of magma from its subsurface reservoir to its emergence as lava is a complex process influenced by several factors:

1. Magma Generation: The initial stage involves the melting of rocks in the Earth's mantle or crust to form magma.
2. Magma Ascent: The buoyant magma rises through the Earth's crust, often utilizing existing fractures and weaknesses. The ascent is influenced by pressure differences and the magma's density.
3. Magma Chamber: Magma often accumulates in underground reservoirs called magma chambers. These chambers can act as temporary storage zones before eruption.
4. Eruption: The eruption occurs when the pressure within the magma chamber exceeds the strength of the overlying rocks. The release of pressure allows dissolved gases to expand rapidly, propelling the magma upwards. The style of eruption (effusive or explosive) depends largely on the magma's viscosity and gas content.
5. Lava Flow: Once the magma reaches the surface, it's now called lava. The lava flows outwards, cooling and solidifying to form volcanic rocks.

Comparing Magma and Lava: A Summary Table

Frequently Asked Questions (FAQs)

• Can lava turn back into magma? No, once magma reaches the surface and becomes lava, it cannot revert back to its subsurface state. The release of pressure and cooling process are irreversible.

• What is the difference between volcanic rock and magma/lava? Volcanic rock is the solidified form of lava. Once lava cools and crystallizes, it forms igneous rock.

• Are all volcanic eruptions explosive? No, volcanic eruptions can be effusive (gentle flows of lava) or explosive, depending on the magma's viscosity and gas content. Basaltic magmas often produce effusive eruptions, while rhyolitic magmas tend to produce highly explosive eruptions.

• How does the composition of magma affect the type of volcanic eruption? The silica content of magma is a key factor. High-silica magmas are more viscous and tend to trap gases, leading to explosive eruptions. Low-silica magmas are less viscous and allow gases to escape more easily, leading to effusive eruptions.

• What are some of the hazards associated with magma and lava flows? Hazards associated with magma include earthquakes, ground deformation, and the potential for explosive eruptions. Lava flows can destroy property, infrastructure, and vegetation, as well as pose a threat to human life.

Conclusion

The distinction between magma and lava is fundamental to understanding volcanic processes. While both are molten rock, their location and behavior are vastly different. Magma, the subterranean molten rock, is under immense pressure and contains dissolved gases. Upon reaching the Earth's surface, it transforms into lava, releasing its gases and flowing as a molten stream. The characteristics of lava, particularly its viscosity and gas content, dictate the style of volcanic eruption and the resulting landforms. Understanding these differences offers a deeper appreciation for the powerful forces shaping our planet and the dynamic interplay between the Earth's interior and its surface. Further research into magma and lava compositions and behaviors continues to refine our understanding of volcanic activity and its impacts on the environment.