Practically speaking -- and to any readers who are geologists, I will apologize for what will seem to you a drastic oversimplification -- there are three ways plates can move relative to one another:
- Divergent plate boundaries -- where the two plates are moving apart. This is what's happening in the middle of the Atlantic Ocean, where the Mid-Atlantic Rift Zone is upwelling magma that drags the North American and South American Plates toward the west and the European and African Plates toward the east. New divergent boundaries can tear a continent in half, which is currently happening in eastern Africa, where the East African Rift Zone is eventually going to rip the continent in two, sending most of Ethiopia, Kenya, and Tanzania, and all of Somalia, eastward, and creating a new ocean in between.
- Convergent plate boundaries -- where two plates are moving toward each other. If both are thick, cool continental plates, this causes a pile-up -- i.e., non-volcanic mountains, such as the Himalayas. If one or both is a thin oceanic plate, one will dive underneath the other and melt, creating a line of volcanoes more or less parallel to the plate boundary. Examples include Japan, Indonesia, and the Cascades.
- Strike-slip boundaries -- where two plates are moving alongside one another in opposite directions. An example is the famous San Andreas Fault and the other bits and pieces of the southern California fault system.
[Image is in the Public Domain]
This complicates the picture in the Atlantic, however. The conventional wisdom is that the divergent boundary in the middle of the Atlantic -- shown in red in the above picture -- is moving North and South America away from Europe and Africa, but if there's a convergent boundary off the coast of Portugal, that'll eat up bits of the oceanic plate off the coast and pull Europe closer to North America. (It also creates the possibility of Andes-type volcanoes in Portugal and Spain.) Geologists are still investigating how, and how fast, this new convergent zone is moving, and what its capacity is for generating earthquakes and tsunamis -- we really only have the one significant data point, the 1755 earthquake, to make a stab at what the potential for seismic activity is. And how this will affect the positions of the continents in the long term is at the moment anyone's guess.
It's endlessly fascinating to me how the face of the Earth can change -- for example, there have been at least three times that more or less all the land masses were fused together into one supercontinent (and the rest of the world was covered by one superocean). Mountains and oceans have been a symbol of something eternal, unchanging, but in reality everything is in flux. It recalls to mind the lines from Percy Shelley's evocative poem "Mont Blanc," which seems a fitting way to end:
Yet not a city, but a flood of ruin*************************************
Is there, that from the boundaries of the sky
Rolls its perpetual stream; vast pines are strewing
Its destin’d path, or in the mangled soil
Branchless and shatter’d stand; the rocks, drawn down
From yon remotest waste, have overthrown
The limits of the dead and living world,
Never to be reclaim’d. The dwelling-place
Of insects, beasts, and birds, becomes its spoil;
Their food and their retreat for ever gone,
So much of life and joy is lost. The race
Of man flies far in dread; his work and dwelling
Vanish, like smoke before the tempest’s stream,
And their place is not known. Below, vast caves
Shine in the rushing torrents’ restless gleam,
Which from those secret chasms in tumult welling
Meet in the vale, and one majestic River,
The breath and blood of distant lands, for ever
Rolls its loud waters to the ocean-waves,
Breathes its swift vapours to the circling air.
This week's Skeptophilia book recommendation of the week is about as cutting-edge as you can get, and is as scary as it is fascinating. A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution, by Jennifer Doudna and Samuel Sternberg, is a crash course in the new genetic technology called CRISPR-Cas9 -- the gene-editing protocol that Doudna herself discovered. This technique allows increasingly precise cut-and-paste of DNA, offering promise in not just treating, but curing, deadly genetic diseases like cystic fibrosis and Huntington's disease.
But as with most new discoveries, it is not without its ethical impact. The cautious are already warning us about "playing God," manipulating our genes not to eliminate disease, but to enhance intelligence or strength, to change personal appearance -- or personality.
A Crack in Creation is an unflinching look at the new science of gene editing, and tries to tease out the how much of what we're hearing is unwarranted fear-talk, and how much represents a genuine ethical minefield. Doudna and Sternberg give the reader a clear understanding of what CRISPR-Cas9 is likely to be able to do, and what it won't, and maps out a direction for the discussion to take based on actual science -- neither panic and alarmism, nor a Panglossian optimism that everything will sort itself out. It's a wonderful introduction to a topic that is sure to be much in the news over the next few years.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]