Planet Earth and Earth systems - Physical Processes in Earth and Environmental Sciences - page 7

Fig. 1.10 Pyroclastic flow descending Montserrat volcano, West
Indies. Flow moving to left: note various scales of mixing eddies on
upper surface shear layer with the atmosphere.

Fig. 1.11 Black smokers venting metal sulfide at Monolith vent site
on the Juan de Fuca ocean ridge.

1.3.6 Black smoker at mid-ocean ridge

water through vents: it represents about 35 percent of the
total heat input from crustal rocks into the oceans. Chemical
oxidation reactions between the hot waters and cool ambi-
ent seawater cause metal sulfide production (the black
smoker particles visualizing the flows in Fig. 1.11).
Chemical reactions (sulfur reduction) in the hot (17-40

Black smokers were discovered as recently as 8 years after
the first lunar landing, emphasizing human ignorance of
the very basic cycling of Earth's hydrosphere and litho-
sphere. Eruption of lava from volcanic vents along the
mid-ocean ridges attests to magma melt present at shallow
levels and thus to high geothermal gradients. Seawater in
the cracks and interstices of the surrounding ocean crust is
drawn in to the ridge crest where it reissues as superheated

C)
vented waters provide energy for chemoautotrophic bacteria
that form the basic member of a food chain reaching to
abundant specialized metazoan life, the famous giant worms
and clams of the so-called vent community.

1.4 Measuring Earth

Humans have for long measured the features on Earth,
beginning with rod and knotted rope and ending with
satellite GPS and Total Station Surveying. According to
Herodotus ( c .2,484- c .2,420 ka) and later authors, the
word geometry, literally meaning “measuring the Earth,”
originated from the necessity of accurately and rapidly sur-
veying land-holding boundaries destroyed by the annual
Nile flood. Nowadays we use a huge array of techniques to
remotely measure natural features, like ocean currents,
atmospheric phenomena, and lithospheric plates, to name
but a few. Yet there are still a lot of things we do not know
about Earth. Here we briefly review the progress of whole
Earth measurements over the past millennia.

(2) all other celestial bodies are of this shape (Babylonian
discovery); (3) during lunar eclipses, the Earth's shadow is
curved (Hellenistic discovery); (4) star constellations vary
slowly according to latitude, some rising, some falling as
position shifts for a given time (Hellenistic discovery).

1.4.2 Earth's diameter/circumference

Knowing Earth to be spherical and with a thorough
understanding of Euclidean geometry, Eratosthenes of
Cyrene (Fig. 1.13; Egypt, died 2.198 ka) observed that at
summer solstice, Syene in Upper Egypt lay directly under
the Sun. He then determined that at his workplace in the
great library of Alexandria, a shadow of angle 7.5 was cast

by a vertical pole at solstice. He reasoned that if the sun's
rays were parallel, the Earth's circumference must lie in
proportion to the longitudinal distance between Cyrene and
Alexandria, as 7.5

1.4.1 Earth's shape

Earth's surface was deduced to be curved everywhere and
the planet essentially spherical (Fig. 1.12) because
(1) large ships can be seen to gradually disappear from the
hull upward as they travel toward a distant sea horizon;

. His logic was impeccable and
despite longitude being a little off, the circumferential
estimate was accurate to c .10 percent.

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