8.8: Summary
8.1 The Global Perspective
Earth is the prototype terrestrial planet. Its interior composition and structure are probed using seismic waves. Such studies reveal that Earth has a metal core and a silicate mantle. The outer layer, or crust, consists primarily of oceanic basalt and continental granite. A global magnetic field, generated in the core, produces Earth’s magnetosphere, which can trap charged atomic particles.
8.2 Earth’s Crust
Terrestrial rocks can be classified as igneous, sedimentary, or metamorphic. A fourth type, primitive rock, is not found on Earth. Our planet’s geology is dominated by plate tectonics, in which crustal plates move slowly in response to mantle convection. The surface expression of plate tectonics includes continental drift, recycling of the ocean floor, mountain building, rift zones, subduction zones, faults, earthquakes, and volcanic eruptions of lava from the interior.
8.3 Earth’s Atmosphere
The atmosphere has a surface pressure of 1 bar and is composed primarily of N 2 and O 2 , plus such important trace gases as H 2 O, CO 2 , and O 3 . Its structure consists of the troposphere, stratosphere, mesosphere, and ionosphere. Changing the composition of the atmosphere also influences the temperature. Atmospheric circulation (weather) is driven by seasonally changing deposition of sunlight. Many longer term climate variations, such as the ice ages, are related to changes in the planet’s orbit and axial tilt.
8.4 Life, Chemical Evolution, and Climate Change
Life originated on Earth at a time when the atmosphere lacked O 2 and consisted mostly of CO 2 . Later, photosynthesis gave rise to free oxygen and ozone. Modern genomic analysis lets us see how the wide diversity of species on the planet are related to each other. CO 2 and methane in the atmosphere heat the surface through the greenhouse effect. Increasing amounts of atmospheric CO 2 from burning of fossils fuels and release of frozen methane are causing the global warming of our planet.
8.5 Cosmic Influences on the Evolution of Earth
Earth, like the Moon and other planets, has been influenced by the impacts of cosmic debris, including such recent examples as Meteor Crater and the Tunguska explosion. Larger past impacts are implicated in some mass extinctions, including the large impact 65 million years ago at the end of the Cretaceous period that wiped out the dinosaurs and many other species. Today, astronomers are working to predict the next impact in advance, while other scientists are coming to grips with the effect of impacts on the evolution and diversity of life on Earth.