Magma

What is Magma and How Does it Form?

Magma, a fascinating substance that lies beneath our feet, is much more than just molten rock. It’s like a hidden treasure chest of Earth’s secrets, containing suspended crystals and gas bubbles. Imagine magma as the raw material from which mountains are built and volcanoes erupt – but how does it come to be? Where do these incredible forces originate?

The Formation of Magma

Magma is born in various tectonic settings, such as subduction zones and hotspots. These areas experience intense heat and pressure that cause rocks to melt. The Earth’s interior temperature increases with depth, driven by radioactive decay and heat loss, with an average geothermal gradient of 25 °C/km. How does this process work exactly?

Decompression Melting

When rock rises through the mantle, it cools slightly as it expands but eventually reaches its solidus temperature and melts. This is known as decompression melting. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800 °C in the presence of excess water, but near 1,500 °C in its absence. Water is driven out of the oceanic lithosphere in subduction zones and causes melting in the overlying mantle.

Hydrous Magmas

Hydrous magmas with the composition of basalt or andesite are produced directly and indirectly as results of dehydration during the subduction process. The addition of water is a crucial factor, lowering the solidus temperature of rocks and creating magma. Can you imagine how this process shapes our planet?

The Composition of Magma

Magma consists of liquid rock with suspended solid crystals. As it approaches the surface, dissolved gases bubble out, resulting in a mixture that can be both solid, liquid, and gas phases. Most magma is rich in silica, forming silicate magmas dominated by oxygen and silicon.

Types of Silicate Magmas

Silicate magmas are divided into four chemical types based on silica content: felsic (silica > 63%), intermediate (52-63% silica), mafic (32-52% silica), and ultramafic (less than 32%). Felsic magmas, with their high silica content, are extremely viscous. They erupt explosively to produce pyroclastic deposits but can also form lava spines or block lava flows.

Intermediate Magmas

Intermediate magmas contain 52-63% silica and are lower in aluminium, richer in magnesium and iron. They’re commonly hotter than felsic magmas and less viscous, forming andesite domes and block lavas. Can you picture the diverse landscapes these magmas create?

Mafic and Ultramafic Magmas

Mafic and ultramafic magmas have lower silica content and higher eruptive temperatures, with viscosities around 10 to 100 Pa⋅s. Mafic magmas are typified by their high ferromagnesian content and generally erupt at temperatures of 1,100 to 1,200 °C (2,010 to 2,190 °F). Ultramafic magmas have a silica content under 45%, with komatiites containing over 18% magnesium oxide. These magmas tell us stories of Earth’s deep history and evolution.

Magma’s Behavior

The viscosity of magma is crucial, varying over seven orders of magnitude between 104 cP (10 Pa⋅s) for mafic lava to 1011 cP (108 Pa⋅s) for felsic magmas. Silica tetrahedra play a significant role in determining viscosity, forming chains, sheets, and clumps that increase it. Water acts as a network modifier, reducing melt viscosity, while carbon dioxide neutralizes it. Higher-temperature melts are less viscous due to increased thermal energy.

Thixotropic Behavior

Magma is typically viscoelastic, flowing under low stresses but fracturing when stressed beyond a critical value. This thixotropic behavior hinders crystal settling, resulting in plug flow. Crystal content above 60% causes the magma to behave like a solid.

Temperature and Density of Magma

Magma temperatures range from 700 to 1,400 °C (1,300 to 2,600 °F). Carbonatite magmas can be as cool as 490 °C (910 °F), while komatiite magmas may reach 1,600 °C (2,900 °F). Densities of magma are influenced by iron content, expanding slightly with lower pressure or higher temperature as gases bubble out.

The Journey from Magma to Lava

Magma cools quickly on the surface during eruptions, forming lava. Lava’s fast cooling prevents crystal growth, resulting in rocks like obsidian and scoria. Volatile exsolution from magma leads to explosive eruptions when massive amounts of steam are released.

Modern Applications

The Iceland Deep Drilling Project reached magma at 2,100 m in 2009, generating the world’s first magma-enhanced geothermal system. This project not only provides insights into Earth’s internal processes but also opens new avenues for renewable energy.

Understanding magma is like peeling back the layers of an onion, revealing the complex and dynamic nature of our planet. From its formation deep within the Earth’s mantle to its explosive emergence at the surface, magma continues to fascinate us with its raw power and beauty.