1. Why do only oceansget subducted?

2. Why are oceansdeep?

Isostasy IsostasyGeorge Everest discoverederrors whilst surveyingIndia by triangulation in1850s

Errors through caused bygravitational attraction ofHimalayas on surveyor'splumb bob

Archdeacon John Prattcalled in to investigate –discovered thatgravitational attraction ofmountain range was LESSthan theory predicted

IsostasyImplication of unexpectedly lowgravitational attraction of Himalayasis that there is a MASS DEFICIT atdepth.

PRATT ISOSASTY: Mountain rangesupported by density variationswithin the crust.

AIRY ISOSTASY: Mountain rangesupported by changes in crustalthickness; crustal density relativelyconstant.

Either way, the plate is relatively lightand 'floats' on a fluid layer beneath.

PRATT

AIRY

1. Why do only oceans get subducted?

2. Why are oceans deep?

1. Continental Drift:Geological observationsimply Lateral Motions

2. Isostasy:Implies vertical motions

Problem . . .

Mantle must be rigid because

it transmits seismic waves

But isostasy concept implies

that mantle is fluid

. . . Timescale