Evening star., 1952-12-24
Earth S 'ReuI Surface
20 Miles Down Probed
Soundings Show Top Crust
Rests on Irregular Bose
By Thomos R. Henry

"A Adob 4IOIlClCI, Tile STceu.ClNCeQ
plains and mountains 20 to 80 miles
under the earth. is being explored by
geophysicists of the Carnegie Institu-
ton Of Washington.
They are using probes of sound waves
set of by half-ton blasts of TINT which
are reflected like light from mirror off
the real surface of the planet, which is
covered by thin skin of granitelike
rock. the crust of the earth as man
knows it.
The echoes Of these blasts, according
to the report of DR.. Merle Tuve, director
of the Carnegie's terrestrial magnetism
department, and his associate, DR.. H. E.
Tatel, show that the earth'S basic surface
is not smooth like billiard ball but has
its own deep-buried mountains and
great plains. The depth varies greatly
from place to place and in some places
the surface apparently is so rough. like
badly cracked mirror. that there are
no clear reflections.
The kind of rock which forms this
basic surface is not known with cer-
tainty but is believed to be very similar
to dense greenish yellow mineral, dun
mite, Of which considerable outcroppings
have been found in Australia and in
North Carolina. It belongs to the same
family as Divine, some forms of which
are valued as gem stones. Stripped of
its crust, the planet might have looked
a good deal like a half-ripe orange.
Crust is Changing Constantly.
Built over this dunnite surface are
far lighter rockS-granites and sand
StoneS-which form the continents. In
some places they are piled into noun
tains miles high. In other places, COW
cred with sand or clay as in the great
deserts and plains, they are quite
smooth. This crust constantly is chang
ing. 1t is as inconstant as the surface
OF the sea. allowing for long time
periods. The Appalachians, for example,
several times have stood east or west of
their present line in the past half billion
years, while the space they now occupy
was sea bottom.
A basic purpose of the Carnegie sound
probe experiments is to throw light on
the mechanisms by which this crust
mat Is, the continents-is built up.
The rough surface of dunnite below
the 20 or 80 miles of crust is known as
the Mohorovicic discontinuity, after a
Croatian scientist who postulated it's
existence in 1908. He apparently made
an inspired guess. say the Carnegie SCI
entlsts. One of their most priced pos
sessions is copy of Mohorovicic's orig
lnal report in the Croat language. Now
they call this last frontier "the moho."
They have carried out their blast
probes under parts of the Appalachians
and Atlantic Coast line, a large area of
Minnesota and sections of the Pacific
Coast running west of Seattle and Los
Angeles. Other blasts in Colorado and
in Mexico have given doubtful results.
In all these places the depth of "the
moho' is calculated from mean sea level.
In the middle of the Appalachians, the
blast reflections show, the dunnite sur
face is about % miles Deep Along the
Atlantic Coast it is much closer to the
surface OF the crust. Under Minnesota
1t is at about the same depth as under
the Appalachians, and most of Minne-
sofa is at about the same mean sea level.
Under all these areas the "moho" ap-
pears to be quite smooth over consider
able areas. But under parts of the Pa
clf1c Coast the Carnegie scientists can
get hardly any reflections. Their inter-
probation is that this indicates rather
rough surface-too rough for the ref1ec-
tlonS OF sound waves with wave lengths
OF approximately half mile.
1t is believed the bottoms of the oceans
beyond the continental shelves are parts
OF the "moho" They have been covered
through the millenniums with thin
layer Of sediments from the land but it
is doubtful if these can be. at the most,
more than or 3 miles thick.
May Be Several Layers.
1t has been postulated from inter-
pretations of seismograph records of
earthquake waves that there are several
layers of light crust itself above the
"moho." The denser the rock the more
rapidly sound waves or shock waves go
through it. From their blast experiments.
DR.. Tuve and his associates can find no
evidence of this. The speed of the waves
increases the deeper they go, but with
no clearly marked discontinuitieS. This.
they say, is to be expected. The more
weight is piled on rocks. the denser they
become.
But when they reach the "moho" the
break is very sharp There appears to be
no gradual shading of one kind of rock
into the other. The speed of the waves
goes up about mile second It is OBJ
vious that new realm has been reached.
The ups and downs of the earthS tenn
poral crust have been left behind.
The "moho," they say, might be COM
pared to the surface of the sea-a sur
face of very heavy, somewhat plastic