Geology Reference
In-Depth Information
Such provinces are not formed by the eruption of lava from a volcanic mountain such as
Mount Saint Helens, pictured in this topic's color photo section, but rather from fissures:
elongated cracks where the magma below erupts onto the surface without explosive
force. Today such fissure eruptions are most common on the flanks of volcanic moun-
tains and in the Hawaiian Island volcanic eruptions.
Geologists conclude that the eruption of lava from giant fissures created the flood
basalts and would have also altered the global environment. Specifically, such massive
eruptions of lava, much larger than the fissures currently erupting on the Hawaiian Is-
lands, would have been accompanied by the release of huge amounts of volcanic gas in-
to the atmosphere. The result would have been rising global temperatures and associ-
ated changes in climate patterns due to the added sulfur dioxide and water vapor in the
air.
Some of these flood basalt events are thought to have lasted for hundreds of thousands
of years at a time. While the region affected by the lava itself would be confined to a par-
ticular continent, the global effects of changed atmosphere and climate would have
reached every part of the earth, both on land and in the oceans.
Living fossils
For the most part, species today are physically very different from their distant ancestors. However, a
special few organisms alive today look exactly like their ancestors from millions of years ago. These
species are called living fossils.
One example of a living fossil is the modern ginkgo tree. Fossils indicate that the earliest ginkgo trees
from 170 million years ago had the same physical appearance as ginkgoes living today. Certain internal
characteristics of the tree have changed, but compared to the dramatic changes in other trees over
the last 170 million years, it appears the ginkgo hasn't evolved much at all.
Another example of a living fossil is the coelacanth fish. Until the 1930s, the coelacanth had been seen
only in the fossil record, and scientists presumed it had gone extinct along with the dinosaurs. But in
1938, some fishermen off the east coast of South Africa caught a coelacanth. Since then, two species
of this living fossil have been identified in that region. The coelacanth is closely related to early lobe-
finned fish that evolved 400 million years ago (check out Chapter 19 for details on fish evolution) and
hasn't changed much since then.
Additional examples of living fossils include crocodiles, some species of shark, and the giant panda.
Shifting sea levels
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