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A NATURAL seep of bitumen ignited to burn a bush for Moses. Lot’s wife may
have glanced back at Sodom and Gomorrah, but the pillar of salt mourned by her
husband was just an extruded body of the mineral halite. Geology pops up again
and again in the Bible, but to discover that these seemingly miraculous events
were actually the fault of flowing rocks, you’ll have to turn to the
textbooks.

Let’s begin with Lot’s wife and the world of salt. John Warren’s
Evaporites, Their Evolution and Economics is illuminating. Common salt, or
halite, is a most unusual material. It is one of a group of minerals called
evaporites which, as their name suggests, are left behind by evaporating water.
Many of the world’s sedimentary basins contain such deposits, and they play a
key role in controlling the build-up of oil, gas and metal ores.

But halite makes rock layers that are always on the move. It is one of the
least dense of all minerals so it forms layers that are more buoyant than other
sedimentary rocks such as sandstone and limestone. What’s more, halite is one of
the weakest minerals, especially if it’s even slightly damp. Put those two
attributes together, and you have a rock that just can’t stay still. Buried
beneath other sedimentary layers, halite tries to rise because of its buoyancy.
Squeeze it, and over time it flows like treacle.

Such salt glaciers move at the staggering speed of half a metre a day after
rainy periods. Although that’s ten times slower than the fastest ice glaciers,
it’s thousands of times faster than any other rock flow.

Evaporites are rarely found at the Earth’s surface because they tend to
dissolve away. It’s equally rare for them to get a mention in textbooks or
courses. But there are no excuses now for a salt-free diet—Warren’s book
is an authoritative account that is bang up to date.

And you need a metaphorical pinch of salt to explain that biblical tale of
the flattening of Jericho. Slipping faults are part of plate tectonics and they
echo motion deep in the mantle below.

Seismic imaging allows geophysicists to trace flow cells in the mantle,
providing there are hints of how turmoil deep in the Earth makes things happen
at the surface. Until now, finding out about this work meant wading through
journals and hefty research papers. Geoffrey Davies has changed all that with
his superb book, Dynamic Earth: Plates, Plumes and Mantle Convection.

It reveals, for example, how hot plumes of mantle flowing up from the outer
edge of the Earth’s core take tens of millions of years to rise, then cause the
eruption of millions of cubic kilometres of lava in only a few hundred thousand
years.

An advanced text, Dynamic Earth contains mathematics that might
intimidate less numerate readers. However, Davies has helpfully marked the more
difficult sections as intermediate or advanced, so you can follow the story.

Perhaps some of the readers of these two books will answer the problems
Davies deals with. We’ll just have to go with the flow.