VOLCANOES OF THE GEOLOGICAL PAST
The question is often asked whether the volcanic eruptions of the
geological past were not much more violent and destructive than the
volcanoes of the present time. Now, while this is a matter that
properly belongs to the subject of geology, and will be treated at
greater length in the Wonder Book on Geology, yet a short mention
should be made of it here.
It is the opinion of Dana that while there have been volcanoes during
the different geological ages, yet volcanic activity has increased
through the geological past until the age that immediately preceded
the appearance of man on the earth. He thinks there is no reason for
believing that there were any very great volcanic eruptions during the
earliest geological time known as the Archæic. Dana speaks as follows
concerning this:
"In this connection it is an instructive fact that in
eastern North America, at epochs when there was the
greatest amount of friction and crushing ... those of the
making of the Green Mountains and the Appalachians ... no
volcanoes were made, and little took place in the way of
eruptions through fissures."
On the other hand, Prestwich seems inclined to think that the absence
of well-marked cones of volcanic material in the rock of the older
geological ages is not to be regarded as proof that no eruptions then
took place, since the very great amount of erosion that occurred
between that time and the Tertiary Age before the appearance of man,
would, probably, have completely obliterated any cones, and even the
volcanic materials would have undergone such changes as completely to
alter their general character. He agrees, however, with Dana that,
probably, the most violent and explosive volcanoes of the geological
ages have been those of the Tertiary Age.
Without, however, attempting anything more than a brief reference to
the volcanoes of the geological past, it may be said that many of the
more important of the active volcanoes of the earth's present time
were begun in the Tertiary Age. Mt. Etna, Vesuvius, and Mt. Hecla are
believed to have commenced at this time.
There is an interesting region of geological volcanoes in the
neighborhood of Auvergne in Central France. Here they occur in three
separate groups that extend over a high granite platform from north
to south for a distance of about 100 miles, and from twenty to eighty
miles from east to west. The eruptions began in the earlier portions
of the Tertiary Age, and continued down to the latter periods of
prehistoric times. Some of these volcanic craters remain to-day almost
as unaffected by erosion as if they had been formed but recently.
Other regions of geological volcanoes are to be found in parts of
Spain near the foot of the Pyrenees Mountains, in parts of Italy and
Germany, as well as in regions in the Caucasus Mountains.
In Asia Minor there exists a group of almost thirty extinct volcanoes
in the neighborhood of the Gulf of Smyrna. Both Little and Great
Ararat contain volcanic cones: that in the latter mountain was active
during historical times. There are also extensive volcanic districts
in the Taurus Mountains. In addition to these there are groups of
extinct volcanoes in portions of Central Asia.
Aden, on the Red Sea, is the centre of an extensive volcanic district.
Indeed, on both shores of the Red Sea there are a few volcanoes that
are still active, while in Sinai, and in the districts of the south,
there are several extinct craters.
But it is in the New World, especially on the Pacific coast of
North America, that volcanic activity was especially great during
the geological past. There is a district containing volcanic rocks
that extends through various parts of western North America, from
New Mexico and North California, to Oregon and British Columbia.
This district has a width of from eighty to 200 miles, and a length
of not quite 800 miles. This great area of nearly 150,000 square
miles is covered with great sheets of volcanic rocks except where
mountain ranges rise from them, or where the rivers have cut deep
valleys through them. In portions of California and New Mexico these
plateaus rise to heights of from 8,000 to 10,000 feet, while in parts
of Colorado, where they form huge dome-like mountains, they reach a
thickness of 14,000 feet. In Oregon the sheet of lava is 2,000 feet
thick, and, indeed, in some places, is estimated to have a depth of
7,000 feet.
In the opinion of nearly all American geologists these great lava
flows in western North America were not of the type known as crater
eruptions, but were what are called fissure eruptions. Some of them
are believed to have occurred during geological times as early as the
Eocene. Prestwich, however, is of the opinion that the eruptions of
the past in these portions of the world were not confined to fissure
eruptions, but that crater eruptions also occurred; and that it was
towards the close of the Tertiary Age that crater eruptions occurred
with great lava flows. Indeed, as we have seen, in portions of Utah
and the neighborhood the remains of true craters can be found.
Besides the above there are evidences of geological volcanoes of still
older times. In portions of Deccan, in southern Hindostan, there is an
immense plateau formed of trap rock, that extends from east to west
for a distance of 400 miles, and from north to south through from 700
to 800 miles. This district, with an area of almost 200,000 square
miles, is covered with a vast lava sheet. It was, in the opinion of
Prestwich, from whom many of the facts of the geological volcanic
eruptions have been obtained, probably still more extensive. The
plateau of Deccan rises gradually from the east to the west, where, in
some parts of the Ghauts Mountains, it reaches a height of from 4,000
to 5,000 feet.
One of the greatest of these prehistoric volcanoes of Scotland was a
volcano in the Isle of Mull in the Hebrides. This volcano was probably
nearly thirty miles across at its base, and was from 10,000 to 12,000
feet high. It is now only 3,172 feet in height.
According to Judd the Island of Skye in Inverness-shire is the remains
of a volcano that was active in Tertiary times, probably many millions
of years ago. This volcano was very large, probably about thirty miles
across at its base, with a height of perhaps as great as 12,000 or
15,000 feet. Now there are only left some granite and other similar
rocks that form the Red Mountains and Coolim Hills of Skye that reach
about 3,000 feet above the sea level.
There are many other parts of the world containing volcanoes that were
active during the geological past. The above, however, is as far as we
can describe such volcanoes in this book.