Page 397 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 361
(a) The day after a record rainfall, several square kilometres are covered by water only a few centimetres deep.
▲Figure 12.11 Death Valley evaporites after a rain. [robert Christopherson. Inset by Andrea Paggiaro/Shutterstock.]
Metamorphic rock may form from igneous rocks as the lithospheric plates shift, especially when one plate is thrust beneath another (discussed with plate tectonics, just ahead). Contact metamorphism occurs when magma rising within the crust “cooks” adjacent rock; this type of metamorphism occurs adjacent to igneous intrusions and results from heat alone. Regional metamorphism occurs when a large areal extent of rock is subject to metamor- phism. This can occur when sediments collect in broad depressions in Earth’s crust and, because of their own weight, create enough pressure in the bottommost layers to transform the sediments into metamorphic rock. Regional
(b) A month later, the water has evaporated and the valley floor is coated with evaporites (inset photo shows close-up view of crystallized salt deposits).
metamorphism also occurs as lithospheric plates collide and mountain building occurs (discussed in Chapter 13).
Metamorphic rocks have textures that are foliated or nonfoliated, depending on the arrangement of miner- als after metamorphism (Table 12.4). Foliated rock has a banded or layered appearance, demonstrating the align- ment of minerals, which may appear as wavy striations (streaks or lines) in the new rock. Nonfoliated rocks do not exhibit this alignment.
On the Isle of Lewis in the island group known as the Outer Hebrides northwest of the Scottish coast, people constructed the Standing Stones of Calanais (Callanish) beginning approximately 5000 years ago (Table 12.4, far-right photo). These ancient people used the 3.1-billion-year-old metamorphic Lewisian gneiss for their monument, arranging the stones so that the foliations aligned vertically. The standing stone in the photo is about 3.5 m tall.
The Rock Cycle
Although rocks appear stable and unchanging, they are not. The rock cycle is the name for the continuous alteration of Earth materials from one rock type to an- other (Figure 12.13). For example, igneous rock formed from magma may break down into sediment by weather- ing and erosion and then lithify into sedimentary rock. This rock may subsequently become buried and exposed to pressure and heat deep within Earth, forming meta- morphic rock. This may, in turn, break down and become sedimentary rock. Igneous rock may also take a shortcut through that cycle by directly becoming metamorphic rock. As the arrows indicate, there are many pathways through the rock cycle.
▼Figure 12.12 Metamorphic rocks. A meta- morphic rock outcrop in greenland, the Amitsoq gneiss, at 3.8 billion years old, one of the oldest rock formations on Earth. [Kevin Schafer/Documentary Value/Corbis.]
Animation
Foliation (Metamorphic Rock)