CAT scanning Earth

Originally published 24 June 1985

In geol­o­gy, before the 1960s, we were taught the Earth was “as sol­id as a rock.” And we were told the sur­face of the Earth had always looked more or less the way it looks today, the same con­ti­nents, the same ocean basins. Oh yes, there had been changes on the sur­face, crin­klings and fold­ings that lift­ed moun­tains or cracked the crust, ver­ti­cal move­ments most­ly, like the wrin­kles on the skin of an orange.

Today, every school child knows this isn’t so. The “drift of con­ti­nents” is now a com­mon­place, and a new the­o­ry — the the­o­ry of plate tec­ton­ics — describes the changes that affect the crust of the planet.

In the new geol­o­gy, the move­ments that shape the crust are not ver­ti­cal, but hor­i­zon­tal. The Earth is more like an egg than a bil­liard ball. The rigid crust is thin, as thin rel­a­tive­ly speak­ing as the shell of an egg, and the crust floats on man­tle rocks that are hot and soft. The crust is cracked into sheets called plates, and the plates slip and slide across the face of the Earth col­lid­ing and diverging.

The the­o­ry of mov­ing plates has had impres­sive suc­cess in explain­ing the major fea­tures of the Earth­’s sur­face. Moun­tains are locat­ed where plates col­lide, ocean trench­es where one plate slips beneath anoth­er, and sea floor rift val­leys where plates diverge. Earth­quakes and vol­ca­noes are over­whelm­ing­ly con­fined to plate boundaries.

CAT scan of earth’s body

But a nag­ging ques­tion remains: What dri­ve the plates? What is the force that drags the con­ti­nents and sea floors hith­er and yon, now crunch­ing them togeth­er, now pulling them apart?

Most geol­o­gists have assumed that the dri­ving force of plate tec­ton­ics is con­vec­tion cur­rents in the hot man­tle of the Earth; great loops of ris­ing and sink­ing plas­tic rock — hot rock ris­ing, cool rock sink­ing — mov­ing an inch or so a year, like pud­ding in a pan on the stove. Accord­ing to the gospel, it is this tur­moil under the crust that breaks and moves the plates.

But all of that was con­jec­ture. Earth­’s inte­ri­or is hid­den from direct obser­va­tion. Now, a tech­nique called seis­mic tomog­ra­phy is pro­vid­ing crude maps of con­vec­tion loops.

Seis­mic tomog­ra­phy does for Earth­’s body what the diag­nos­tic tech­nol­o­gy called CAT scan­ning does for the human body. CAT stands for “com­put­er-aid­ed tomography.”

In med­i­cine, X‑rays are pro­ject­ed through the body at dif­fer­ent angles and record­ed, and the degree of absorp­tion of the X‑rays depends upon the den­si­ty of the organs they pass through. An ordi­nary X‑ray pho­to­graph shows a two-dimen­sion­al image. In a CAT scan, the criss­cross­ing X‑rays are ana­lyzed math­e­mat­i­cal­ly by a com­put­er to con­struct a three-dimen­sion­al image that can be dis­played on the screen of the computer.

Support for convection theory

In seis­mic tomog­ra­phy, the source of the waves that pass through the Earth­’s body is an earth­quake. The waves are record­ed at seis­mic sta­tions across the face of the plan­et. By record­ing the shocks of many earth­quakes, seis­mol­o­gists can get the same criss­cross­ing effect employed in a CAT scan.

The veloc­i­ty of an earth­quake wave depends upon the tem­per­a­ture and den­si­ty of the mate­r­i­al it pass­es through. By math­e­mat­i­cal­ly ana­lyz­ing the arrival times of earth­quake waves at many sta­tions from many quakes, it is pos­si­ble to assem­ble three-dimen­sion­al tem­per­a­ture and den­si­ty maps of the inside of the Earth. The tech­nique requires a world­wide net­work of seis­mo­graphs able to record data in a dig­i­tal form that can be read by a com­put­er. And the huge bulk of the cal­cu­la­tions requires a pow­er­ful high-speed computer.

Although the maps so far are rough, they do seem to con­firm the belief con­vec­tion cur­rents dri­ve the plates. There is hot, ris­ing rock close to the sur­face under places where plates diverge, and under “hot spots” like Hawaii and Ice­land. These plumes orig­i­nate deep in the man­tle, but not nec­es­sar­i­ly direct­ly below the places where they reach the sur­face. Where plates con­verge, cold slabs of old crust can be detect­ed far below the sur­face. Some mod­est progress has been made map­ping the hor­i­zon­tal flow of rock that com­pletes the con­vec­tion loops.

The key to improv­ing the maps of the Earth­’s inte­ri­or is the emplace­ment of a wider net­work of dig­i­tal seis­mo­graph­ic sta­tions. Seis­mol­o­gists from near­ly 50 uni­ver­si­ties and research insti­tu­tions recent­ly formed a non­prof­it cor­po­ra­tion with the objec­tive of mod­ern­iz­ing and dig­i­tal­iz­ing seis­mo­graph­ic sta­tions, and increas­ing the num­ber of sta­tions that are dis­trib­uted across the face of the Earth. With that net­work they hope to pre­pare detailed maps of the plan­et’s inte­ri­or and of the cur­rents of mov­ing rock that dri­ve the sur­face plates.