Facts of the Matter
Rocks record conditions of extinctions
THERE is no single location on the planet that contains a complete record of Earth's history in layers of rock.
Over the past 200 years, geologists correlated the layers, mostly using fossils that have global range, and pieced together the geologic column.
The geologic column is a graph of geologic time that uses the relative ages of rock layers to show a continuous history from 4 billion years ago to the present.
Four eras mark the principal divisions of the geologic column:
» Precambrian (4,600 to 545 million years ago).
» Paleozoic (545 to 245 million years ago).
» Mesozoic (245 to 65 million years ago).
» Cenozoic (65 million years ago to present).
Each era is subdivided into periods, such as the Cretaceous, Jurassic and Triassic periods of the Mesozoic Era, sometimes referred to as the age of dinosaurs.
Each division of the column is defined by rock layers that may contain different types of rocks in different parts of the world, but contain the same assemblage of fossils.
The thickness of layered rock formations, each a succession of layers that share common properties, varies from a few inches to hundreds of feet. The rock formations tell local stories of ecosystem changes, but it is the boundaries between the eras and periods that are the chapters and subheadings in the book of Earth.
Eras and periods are of special interest because across their boundaries, many species and whole genera disappear in "extinction events."
A mass extinction is an event in which 50 percent or more of life forms disappear from the fossil record. These are of special interest to geologic historians because they represent a major global ecosystem shock of great magnitude.
Since the beginning of the Paleozoic 550 million years ago, there have been five mass extinctions, several minor extinctions and many less severe but still notable extinction events.
Four of the five mass extinctions mark the boundaries between the Precambrian, Paleozoic, Mesozoic and Cenozoic, each representing a catastrophic change in the global environment.
The most famous of the mass extinctions is the K-T event, which wiped dinosaurs from the face of the earth and which marks the boundary between the Mesozoic and Cenozoic eras.
Below the K-T boundary, there are dinosaur fossils; above it, none have been found.
Along with the dinosaurs, 85 percent of all species and 60 percent of all genera appear as fossils below the boundary but not above it.
As dramatic as the K-T event was, the greatest mass extinction of all occurred at the end of the Permian period, between the Paleozoic and Mesozoic eras, 245 million years ago.
Several things were going on around that time, any or all of which could have contributed to the Permian extinction that wiped out 95 percent of all life on Earth.
One was a period of global warming and severe climatic fluctuations in temperate climates produced by concurrent glaciation events on the north and south poles.
Another was the formation of Pangaea as continental fragments drifted together to form the massive supercontinent.
But among the five mass extinctions, there are two factors common to all: impacts by large asteroids or comets and volcanic eruptions.
THE SOLAR SYSTEM contains many objects of all sizes, from dust grains to continent-size rocks, whose orbits intersect the orbit of Earth, increasing the probability of a collision.
The bigger the object, the fewer of them there are.
According to NASA, large meteorites with the potential to cause major global environmental change strike the earth on average every 100,000 years.
It was a thin layer of iridium-rich sediment at the K-T boundary that led to the theory that an impact by an asteroid a few miles across caused the great K-T extinction. Iridium is rare on earth but more abundant in meteorites.
Impacts alone might not be sufficient to explain an event on the scale of the K-T mass extinction.
Volcanic eruptions add particles, aerosols and greenhouse gases to the atmosphere, all of which affect the absorption, reflectance or transmittal of the sun's energy and thus affect climate, which affects ecosystems.
Explosive eruptions like Mount St. Helens generate much excitement but are relatively small and relatively rare.
Effusive eruptions occur when a mantle plume arrives in the lithosphere. They are richer in lighter elements and hotter than the surrounding mantle where they form for reasons that are not entirely clear, and they rise slowly through the mantle.
As they rise, magma forms by partial melting of the plume material. The liquid magma injects into the lithosphere and erupts onto Earth's surface, where it forms basalt lava flows.
When a plume occurs under oceanic crust, it forms volcanic islands or the fiery midocean ridges.
When a plume rises beneath a continent, it can erupt effusively as a "flood basalt," a strange type of volcano.
Flood basalts cover thousands of square miles with lava flows that are 150 or more feet thick, extend for hundreds of miles and contain tremendous volumes of lava.
The three largest are the Central Atlantic Magmatic Province; the Deccan Traps, which has a volume of 240,000 square miles; and the even larger Siberian Traps (trap is a Sanskrit word meaning "step"). In the Unites States, there is the Columbia River plateau, but it is minuscule compared with its Siberian and Deccan counterparts.
Flood basalts are generating more interest as earth scientists learn more about the timing of mass extinctions and flood eruptions.
There are strong correlations between them. The three largest extinctions correlate with the three largest flood basalt eruptions and impact events, while the other two correlate with smaller flood eruptions.
The timing of massive flood eruptions with major extinctions is too close to be pure chance, but there is no verified causal link between them. Some flood basalts do not match with any indicators of past ecosystem stress, and some smaller extinctions do not appear to correspond with any flood basalts.
This suggests to researchers that it is unlikely that there is a single cause for all mass extinctions.
There is growing suspicion that a large impact and a flood basalt eruption that occur around the same time might be necessary to cause mass extinctions, although there are additional ecosystem stressors such as climate, continental drift and rifting, as well as volcanic eruptions and impacts on a smaller scale.
Although we cannot predict events such as flood eruptions and most likely could not prevent the impact of a meteorite the size of the K-T "monster," one thing is certain. Earth is undergoing a mass extinction, and not because of some external force. It is happening because of us, Homo sapiens, the "exterminator species."
Richard Brill, professor of science at Honolulu Community College (honolulu.hawaii.edu/~rickb
), teaches earth and physical science and investigates life and the universe. His column is published on the first and third Sundays of every month. E-mail questions and comments to email@example.com
picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at firstname.lastname@example.org