The primary factor that makes magma thicker, or more viscous, is the amount of silica (silicon dioxide, SiO₂) it contains.
Here's a more detailed breakdown:
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Silica Content: Magma with a high silica content is more viscous (thicker) than magma with low silica content. This is because silica molecules link together to form complex structures, which resist flow. Think of it like adding flour to water – the more flour, the thicker the mixture becomes.
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Mafic vs. Felsic Magma:
- Mafic magmas are relatively low in silica and rich in magnesium and iron. They are less viscous and tend to flow more easily. Examples include basaltic lava.
- Felsic magmas are high in silica and contain minerals like quartz and feldspar. They are much more viscous and can be very resistant to flow. Rhyolitic lava is a prime example.
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Temperature: While silica content is the dominant factor, temperature also plays a role. Hotter magma is generally less viscous than cooler magma. As magma cools, it begins to crystallize, further increasing its viscosity.
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Dissolved Gases: The presence of dissolved gases can affect viscosity, but their impact is less significant than silica content. Gases can increase viscosity slightly in some cases, but their main impact is on the explosivity of eruptions.
In summary, the higher the silica content of magma, the more viscous (thicker) it is. This is a crucial factor in determining the style of volcanic eruption, with high-silica magmas generally leading to more explosive eruptions.
