Transcript G19-1pow
Inside Earth Chapter 2 Section 1 Forces Within Earth Section 19.1 Forces Within Earth Faults form when the forces acting on rock exceed the rock’s strength. Review Vocabulary fracture: the texture or general appearance of the freshly broken surface of a mineral Stress and Strain Along the boundaries between two tectonic plates, rocks in the crust often resist movement. Over time, stress builds up. I. Types of Stress • Forces that act on rock per unit volume (changes its shape) Effects of Stress (Taiwan) A. Compression 1. 2. 3. 4. Squeezes rock Folds or breaks rock Convergent boundary Results in mountains B. Tension 1. 2. 3. 4. Pulls on crust Stretches rock Divergent boundary Results in valley C. Shearing 1. Rocks slip past each other 2. Slip apart or break 3. Transform boundary Section 19.1 Stress and Strain Forces Within Earth •Compression causes a material to shorten. •Tension causes a material to lengthen •Shear causes distortion of a material. The deformation of materials in response to stress is called strain. Even though rocks can be twisted, squeezed, and stretched, they fracture when stress and strain reach a critical point. At these breaks, rocks can move, releasing the energy built up as a result of stress. Earthquakes are the result of this movement and release of energy. II. Types of Strain A. Elastic deformation 1. Low stress 2. material is compressed, bent, or stretched 3. When the stress is removed, material returns to its original shape. B. Plastic deformation 1. When stress builds up past a certain point a. elastic limit 2. permanent deformation a. Failure of rock Most materials exhibit both elastic and plastic behavior.* As pressure increases, rocks require greater stress to reach the elastic limit. At high enough temperatures, solid rock can also deform, causing it to flow in a fluidlike manner. This flow reduces stress. * pg. 529 III. Kinds of faults Crustal rocks fail when stresses exceed the strength of the rocks. The resulting movement occurs along a weak region in the crustal rock called a fault, which is any fracture or system of fractures along which Earth moves. A. Reverse fault 1. Where rocks converge 2. Horizontal and vertical compression 3. Shortening of the crust 4. One side pushes up relative to other side B. Normal Fault 1. Partly horizontal, partly vertical movement 2. Pulls rock apart 3. One side moves downward 4. Occurs where plates diverge C. Strike-Slip fault 1. Very little up & down motion 2. Rocks move past each other (horizontal shear) Section 19.1 Forces Within Earth Visualizing Fault Movement Types of Faults Table 19.1 Page 531 IV. Earthquake Waves Irregular surfaces in rocks can snag and lock along faults when movement occurs. As stress continues to build in these rocks, they undergo elastic deformation. Beyond the elastic limit, they bend or stretch. Before that limit, an earthquake occurs when they slip or crumble. A. Seismic waves vibrations of the ground produced during an earthquake 1. P waves a. Primary waves b. Compress & expand c. Moves through solids & liquids d. Fastest B. S waves 1. Secondary waves 2. Move side to side and/or up and down 3. Moves through solids only 4. Slower than P-waves Primary and Secondary waves travel through Earth’s interior and are, therefore, known as body waves 3. Surface waves a. Ground moves like ocean waves b. Slowest B. Generation of seismic waves 1. Focus a. point of initial fault rupture (failure) b. usually several kilometers below Earth’s surface 2. Epicenter – the point on Earth’s surface directly above the focus Faults form when the forces acting on rock exceed the rock’s strength. Stress is force per unit of area that acts on a material and strain is the deformation of a material in response to stress. Reverse, normal, and strike-slip are the major types of faults. The three types of seismic waves are Pwaves, S-waves, and surface waves.