Page 386 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

350 part III The Earth–Atmosphere Interface
allows scientists to determine the date a rock formed by comparing the amount of original isotope in the sample with the amount of decayed end product in the sample. Numerical ages permit scientists to refine the geologic scale and improve the accuracy of relative dating sequences.
The oldest known surface rocks on Earth formed during the Archean Eon, about 4 billion years ago, in Greenland (3.8 billion years old), northwestern Canada (about 4 billion years old), Western Australia (4.2 to 4.4 billion years old), and northern Québec, Canada (4.3 billion years old). The most recent epoch in the geologic time scale is the Holocene, consisting of the 11500 years since the last glacial period. As the impacts of humans on Earth systems increase, numerous scien- tists now agree that we are in a new epoch called the Anthropocene (discussed in Chapter 1, GeoReport 1.1).
earth’s Structure and Internal energy
Along with the other planets and the Sun, Earth is thought to have condensed and congealed from a nebula of dust, gas, and icy comets about 4.6 billion years ago (discussed in Chapter 2). As Earth solidified, gravity sorted materials by density. Heavier, denser substances such as iron gravitated slowly to its centre, and lighter, less-dense elements such as silica slowly welled upward
to the surface and became concentrated in the outer shell. Consequently, Earth’s interior consists of roughly concen- tric layers (Figure 12.2), each distinct in either composi- tion or temperature. Heat energy migrates outward from the centre by conduction, as well as by convection in the plastic, or fluid, layers.
Scientists have direct evidence of Earth’s internal structure down to about 2 km, from sediment cores drilled into Earth’s outer surface layer. Below this re- gion, scientific knowledge of Earth’s internal layers is acquired entirely through indirect evidence. In the late 19th century, scientists discovered that the shock waves created by earthquakes were useful for identifying Earth’s internal materials. Earthquakes are the surface vibrations felt when rocks near the surface suddenly fracture, or break (discussed at length in Chapter 13). These fractures generate seismic waves, or shock waves, that travel throughout the planet. The speed of the waves varies as it passes through different materials— cooler, more rigid areas transmit seismic waves at a higher velocity than do the hotter, more fluid areas. Plastic zones do not transmit some seismic waves; they absorb them. Waves may also be refracted (bent) or re- flected, depending on the density of the material. Thus, scientists are now able to identify the boundaries be- tween different layers within Earth by measuring the depths of changes in seismic wave velocity and direc- tion. This is the science of seismic tomography; for ani- mations and information, go to www.iris.edu/hq/programs/ education_and_outreach/animations/7.
Earth’s Core and Mantle
A third of Earth’s entire mass, but only a sixth of its volume, lies in its dense core. The core is differenti- ated into two regions—inner core and outer core—di- vided by a transition zone several hundred kilometres wide (see Figure 12.2b). Scientists think that the inner core formed before the outer core, shortly after Earth condensed. The inner core is solid iron that is well above the melting temperature of iron at the surface
CrITICalthinking 12.1
Thoughts about an “Anthropocene Epoch”
Take a moment to explore the idea of naming our current epoch in the geologic time scale for humans. Develop some arguments for doing so that consider landscape alteration, deforestation, and climate change. If we were to designate the late Holocene as the Anthropocene, what do you think the criteria for the beginning date should be? •
Georeport 12.1 Radioactive Elements Drive Earth’s Internal Heat
The internal heat that fuels endogenic processes beneath Earth’s surface comes from residual heat left over from the planet’s formation and from the steady decay of radioactive elements. radiometric dating of Earth materials is based on
these rates of decay. An atom contains protons and neutrons in its nucleus. Certain forms of atoms called isotopes have unstable nuclei; that is, the protons and neutrons do not remain together indefinitely. As particles break away and the nucleus disintegrates, radiation is emitted, and the atom decays into a different element—this process is radioactivity. This provides the steady time clock needed to measure the age of ancient rocks, since the decay rates for different isotopes are determined precisely—specifically, the radioactive decay of the isotopes potassium-40 (40K), uranium-238 and 235 (238U and 235U), and thorium-232 (232Th). Scientists compare the amount of original isotope in the sample with the amount of decayed end product in the sample to determine the date the rock formed. See A Quantitative Solution at the end of the chapter for more information on radioactive dating and decay.