MID-OCEAN VOLCANIC ISLANDS
Besides the volcanoes we have already described, there are many others
situated in mid-ocean far from any continent. A brief description will
be given of a few of these.
All the three great central oceans, the Pacific, the Atlantic, and the
Indian, contain numerous volcanic islands, some of which rise many
thousands of feet above the general level.
We will begin with a description of some of the more important
volcanic islands of the Pacific. It was first pointed out by Kotzebue,
and afterwards by Darwin, that all the islands of the Pacific Ocean
can be divided into two great classes, the _high islands_ and the
_low islands_. All the high islands are of volcanic origin, while
the low islands are of coral formation. It is the opinion of Dana,
who has made a careful study of coral formations, especially in the
Pacific, that in all probability even the low islands of the Pacific
were originally volcanic, and that the deposits of coral had been made
along their shores after their volcanoes had become extinct.
The islands of the Pacific, like the shores of the continents and
most of their mountain ranges, extend in two great lines of trend, or
general direction, which intersect each other nearly at right angles.
These lines extend from the southeast to the northwest, and from the
northeast to the southwest respectively, those extending in a general
direction from southeast to northwest being the most common in the
Pacific.
Now, perhaps, the greatest number of the earth's volcanoes are
arranged along fissures, or cracks in the earth's crust. The craters
are situated along the cracks, the openings being kept clear at the
crater, and gradually closing elsewhere, probably by pressure. In
other words, most of the volcanoes follow one another along more or
less straight lines. For example, in the western part of South America
they follow the Andes Mountains. A similar arrangement exists in the
volcanoes of Central America, Mexico, and the United States. Now, this
is especially true of mid-ocean volcanoes of the Pacific which lie
along lines extending from southeast to northwest, or from northeast
to southwest, though mainly along the former.
Some of the volcanic islands of the Pacific have already been
described or referred to, as, for example, the Aleutian Islands,
which stretch in a curved line from the southwestern extremity of
the peninsula of Alaska to Kamtschatka on the coast of Asia. We have
already described the island of Hawaii, the great volcanoes of the
Sandwich Islands chain, and besides these there are in the North
Pacific the Ladrone Islands, lying east of the Philippines.
Some of the principal remaining islands are: the Fejee Islands, which
are volcanic, with numerous hot springs and craters. The Friendly
Islands, with the peak of Tafua, 2,138 feet high, an active volcano
with a large crater always burning, and two other volcanoes, Apia,
and Upala. Tahiti, to the east, is at present extinct. One of its
mountains, Orobena, said to be 10,000 feet high, has a crater on its
summit. The Marquesas, still further to the east, are also volcanic.
All of these islands lie generally in the lines of the northeast
trend.
The Tongan or New Zealand Island chain extends in the direction of
the northeast trend. This, as you will see, is the direction in which
the two islands of New Zealand extend. The Tongan Island chain is
continued to the south through Auckland and the Macquaire Islands to
58° S. Towards the north, in almost the same line, are the Kermadec
Islands near 30° S.
There are several active volcanoes in New Zealand. An explosive
eruption of Tarawera, in New Zealand, in 1883, continued for several
days, and was followed, three days afterwards, by an outburst in an
active volcano in the Bay of Plenty, and two months afterwards, by a
violent outburst in a volcano on the island of Ninafou in the Tongan
Islands.
Coming now to the Atlantic Ocean we find a number of volcanic
mountains in the deep waters near mid-ocean. The principal of these,
besides Iceland, are the Azores, the Canaries, Cape Verde Islands,
Ascension Island, St. Helena Island, and Tristan d'Acunha. The Peak
of Pico, in the Azores, rises to a height of 7,016 feet. The Peak
of Teneriffe, in the Canaries, reaches the height of 12,225 feet.
Teneriffe is a snow-capped mountain. It has a cone on its summit with
precipitous walls like Vesuvius. Sulphurous vapors are continually
formed at its summit, but no flames can be seen.
In the Cape Verde Islands is to be found the active volcanic mountain
of Fuego, rising 7,000 feet above the sea. It has a central cone that
has been broken down on one side like that of Somma on Vesuvius. Fuego
was in eruption in 1785, and also in 1799.
Ascension Island, south of the equator, is formed entirely of volcanic
materials. This island rises from an apparently granite floor on the
bed of the ocean, in water 12,000 feet deep.
St. Helena lies further to the south. It is an extinct volcano, and
has the remains of a crater on its summit with lava dikes in various
parts of the island.
Tristan d'Acunha is an isolated mountain that lies in the South
Atlantic, south of St. Helena, 1,500 miles from Africa, the nearest
land. It is an extinct volcano that rises from a depth of 12,150 feet
to a height of 7,000 feet above the sea. It has a truncated cone on
its summit and a lake of pure water in its old crater.
There are only a few volcanic islands in the Indian Ocean. Kerguelen
Island lies in the southern waters. St. Paul and Amsterdam to the
north, lying near 40° S. lat., as well as the Crozet Islands, are
extinct volcanoes.
In the Arctic Ocean is the volcanic island of Jan Mayen. In the
Antarctic Ocean, as far as is known, there are only two volcanoes,
Mt. Erebus and Mt. Terror. Mt. Erebus, 12,400 feet high, is an active
volcano. Mt. Terror, 10,990 feet high, is an extinct volcano.