Exercises: Magmatism and igneous rocks

Define magma and explain the three factors that can cause rock inside the planet to melt: an increase in temperature, a decrease in pressure, and the addition of water.

Explain why most of the planet's active volcanoes are aligned with lithospheric plate boundaries, and cite a volcanic region associated with a divergent boundary and another with a convergent boundary.

Match each magmatic setting — mid-ocean ridge, subduction zone, and hotspot — with the type of igneous rocks it produces, choosing between basic rocks (basalt, gabbro) and intermediate or felsic rocks (andesite, granite), and cite a famous archipelago formed by a hotspot.

The cross-section diagram shows three settings where magmas are generated (1 to 3). a) Identify each one. b) Explain the melting mechanism operating in each setting: pressure reduction, addition of water, or thermal mantle anomaly. c) State what type of igneous rock results in each case.

A magma of the same composition can give rise to either a plutonic or a volcanic rock. Explain, based on their cooling history, the difference between a granite and a rhyolite, and what feature of their texture allows them to be distinguished with the naked eye.

Hotspots generate chains of volcanic islands aligned by age, such as the Hawaiian archipelago. Explain how this chain forms as the plate moves over the hotspot and why the age of the islands serves as a record of plate motion.

A volcanology team analyzes samples from two recent submarine eruptions: volcano A emits fluid lavas with 49 % $\text{SiO}_2$ and less than 0.5 % $\text{H}_2\text{O}$; volcano B, located behind an oceanic trench, ejects viscous lavas and pyroclasts with 68 % $\text{SiO}_2$ and 4.2 % volatiles. a) Justify the thermal and compositional origin of the magma in volcano A, deducing why adiabatic decompression melting at mid-ocean ridges barely generates differentiation. b) Justify the two geochemical processes that have transformed the initial melt in volcano B into an explosive felsic magma, detailing the role of the overlying lithospheric mantle and the fluids released by the subducting plate.

A popular science report states the following: "The Aleutian island arc exhibits quiet and effusive volcanism because the surrounding seawater cools the magma, making it stable; in contrast, Iceland suffers continuous devastating Plinian eruptions because it is located on an open mid-ocean ridge." a) Refute both statements on a reasoned basis: classify the real tectonic setting of each area and justify what type of eruption predominates in each one according to viscosity, silica content, and magma degassing. b) Predict and compare the two main direct volcanic hazards (lahars, lava flows, pyroclastic flows, ash) that threaten the population in each of these two scenarios.

In 2021, the eruption of Tajogaite (La Palma, intraplate setting) emitted basaltic lavas that advanced down the slope, burying buildings but allowing orderly evacuations. In contrast, in 2008, the Chaitén volcano (Chile, subduction zone) woke up abruptly with a plinian column of rhyolitic ash over 20 km high and the collapse of pyroclastic domes. a) Based on mantle melting conditions and magmatic evolution in each geodynamic setting, justify why the rheology (viscosity and gases) of both magmas was so disparate. b) As an advisor to Civil Protection, justify why the evacuation and exclusion zone delimitation measures applied in La Palma would have been useless and lethal in an event like the one at Chaitén.