Starkey begins with the exciting stuff — destruction! — focusing on terrestrial phenomena: lava flows, pyroclastic eruptions spewing everything from boulders to ash, tsunamis, supervolcanoes and large igneous provinces.
She then goes a little into the geochemistry of magmas, looking at the different kinds of rocks in the Earth's mantle and their different viscosities, and at the role of volatiles and lubricants.
"We know now that increasing the silica content of a lava has far-reaching consequences on the style of eruptions that occur, making them much more explosive and dangerous. ... All magmas contain gases known as volatiles. These can be in the form of water and carbon dioxide but also sulphur, chlorine or fluorine, to name just a few of the more common ones."
The Earth appears to be unique in the Solar System in having plate tectonics. Starkey describes the construction of crust, through spreading ridges or magma plumes, and its destruction by subduction.
"Continental crust is simply a more advanced type of planetary surface. In fact, we could look at the Earth as a more evolved version of the other terrestrial planets. Because Earth has developed and maintained plate tectonics over millennia, it has continued to change, forming different types of crust, whereas the other planets are stuck in an earlier era of evolution with simpler — although still not fully explored and understood — surfaces."
Volcanism plays a key role in providing the requirements for life. Starkey looks at the evidence from Venus (phosphine) and Mars (methane), at hydrothermal vents on Earth, and at possibilities elsewhere. (Strangely, she doesn't mention the popular theory that life on Earth actually originated in hydrothermal vents.)
We have a range of tools for probing the interior of planetary bodies: xenoliths or chunks "from possibly great depths", seismic recordings, magnetic fields, meteorites, geysers, and sampling of other planets.
Planetary bodies have several sources of heat: accretion during creation, sinking metal (as on Earth) or helium descending through hydrogen (Jupiter), nuclear heating (with isotopes with different half-lives providing heat at different periods), and tidal heating (which makes Io "one of the hottest and most volcanically active places in the Solar System").
Other factors control how planets cool down: size and location, and the volcanic processes that transfer heat to the surface, leaving distinctive signatures. Mars, Venus, the Moon, and the Earth all have different histories.
"basaltic flows [with] a relatively low viscosity were able to flow and enter depressions in the lunar surface, filling up more ancient craters... These flows represent the Moon's secondary crust. ... The secondary crusts on the Moon, Mars, Earth and Venus were all formed after the crystallisation of primary crusts from their magma oceans."
Three final chapters survey volcanic activity on different planetary bodies: "Fiery Moons", "Fiery Planets", and "Ice Worlds".
"If Titan's magmas are a mix of ammonia and water, then cryovolcanism on this alien world is thought to behave in much the same way as basaltic volcanism does on Earth. This means that features such as shield volcanoes and runny lava flows could form, and that the potential for explosive volcanism is limited. This is partly due to Titan's dense atmosphere, which would prohibit explosive activity. Yet, there are a lot of unknowns, and it is hard to predict how Titan's lavas would react to the surface environmental conditions because we still know relatively little about this seemingly Earth-like world."
Fire and Ice is accessible and informative. It doesn't assume any background in chemistry or geology or planetary science — it includes brief explanations of plate tectonics, for example, and of the discoveries of the most important space probes. It is not at all repetitive, however, and there are no ventures into biography or discovery narratives, so it fits a good deal of science in, including much that was new to me. Eight pages provide a nice assortment of colour photographs.
February 2022
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