Volcanic eruptions on Hawaii usually are relatively non-explosive because:

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Question: Volcanic eruptions on Hawaii usually are relatively non-explosive because:

Option 1: 'Hawaiian-type magmas contain no dissolved gases' This is incorrect. Hawaiian lavas are typically basaltic and do contain dissolved volatiles, including water and carbon dioxide, but their low viscosity allows these gases to escape more readily as magma ascends and lava flows. The claim that there are no dissolved gases misstates the volatile content and the mechanism of degassing.

Option 2: 'lavas on Hawaii never flow into the sea, so there are no explosions when lava contacts water and turns it into steam' While lava–sea interactions can produce steam explosions, they are not the primary reason Hawaiian eruptions are typically non-explosive. The emphasis of Hawaiian-style activity is on low-viscosity magma that can degas gradually before reaching the surface, rather than on the absence of sea contact explosions. The statement also ignores how lava can still interact with water and not generate large explosive events simply due to the magma’s properties.

Option 3: 'the magma chambers are so shallow that rising magma is never under great pressure; thus the magma has no explosive potential' This is not accurate. Shallow magma storage can influence eruptive style, but the key to Hawaiian eruptions’ relatively gentle nature lies mainly in magma viscosity and gas content/degassing, not solely in chamber depth. Eruptions can still be vigorous if gas pressure builds, and other factors can contribute to explosions.

Option 4: 'Hawaiian-type lavas are sufficiently poor in silica that they are very fluid; thus, dissolved gases escape relatively easily once the magma reaches surface pressures' This statement captures the essential reasoning. Low-silica, low-viscosity basalts permit volatiles to exsolve and escape more efficiently as magma ascends and approaches the surface, reducing the buildup of explosive pressure. The result is effusive eruptions with flowing lava rather than highly explosive fragmentation.

In summary, the non-explosive or effusive nature of Hawaiian eruptions is best explained by the low silica content and low viscosity of the lava, which promote efficient degassing of dissolved gases as magma rises and reaches surface conditions. The other options misstate gas content, the role of sea interactions, or the influence of chamber depth on explosivity.

Volcanic eruptions on Hawaii usually are relatively non-explosive because:单项选择题

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