The Dynamic Crust Chapter 4
Crust • The crust is the solid outer rock zone of Earth. • The crust is undergoing constant change. • Weathering and erosion • Volcanoes • Earthquakes • Tectonics is the study of the movement of the Earth’s crust.
Solid and Liquid Zones • The lithosphere is the crust and uppermost part of the mantle. • The asthenosphere is the upper part of the mantle that behaves like a thick, plastic fluid. • The interface between the crust and the mantle is called the Mohorovicic discontinuity or “Moho” Interface.
Earthquakes An earthquake is the vibration or shaking of the Earth’s crust caused by rapid movement of rocks in the lithosphere. The sudden release of energy in the rocks as they move and break causes waves.
Earthquake Waves Earthquakes generate three types of waves. Compressional waves called primary waves or P-waves. Shear waves called Secondary waves or S-waves. Long waves or L-waves.
Compressional WavesP-waves Compressional waves are like sound waves. P-waves cause particles in the rock to vibrate in a back and forth motion in the same direction the wave is traveling.
Shear Waves Shear waves are similar to the waves in rope when it is shaken. S-waves vibrate rock particles at right angles to the direction the wave is traveling.
Long WavesL-waves Long waves travel along the Earth’s surface at relatively slow speeds.
Seismic Waves • When an earthquake occurs, it generates energy waves, called seismic waves, that travel outward from the point in the crust where the earthquake originates. • This point of origin is called the focus of the earthquake.
Locating the Epicenter of an Earthquake • The epicenter of an earthquake is the point on Earth’s surface directly above the earthquake focus.
Earthquake Velocity • The different earthquake waves have different properties as they travel through the layers of the Earth. • Waves are bent, or refracted, as they move through materials of different densities. • Shadow zones are zones on the Earth’s surface where no seismic waves are received.
Earthquake Strength • Earthquake intensity and energy are measured on two different scales. • Magnitude is the amount of energy released during an earthquake and is measured using the Richter scale. • Scale is 0 to a high of any number • Each step in magnitude represents and increase of 10 times the next lower number. • Intensity is a measure of the earthquakes effect on people and buildings and is measured using the modified Mercalli scale.
Evidence for Crustal Movement • Continental Drift theory in 1910 by Alfred Wegener suggested that the continents were once fit together as one and have since drifted apart to their present locations.
More Evidence • Rock, mineral and fossil correlation also provides evidence that the continents were once joined together.
Plate Tectonics • This theory proposes that the Earth’s crust is divided into a number of large “plates”. • The plates are moving across the surface in a manner that some are separating, colliding, or sliding past one another.
What Causes Movement? • Convection cells in the mantle cause the plates above them to move.
Plate Boundaries There are three basic types of plate interactions. • Divergent • Convergent • Transform
Divergent Boundaries Diverging boundaries occur where two plates move apart from each other. • Examples are mid-ocean ridges. • Mid-ocean ridges develop valleys called rift valleys.
Convergent Boundaries Convergent boundaries occur where plates move toward each other. There are many types of convergent boundaries. • Collision boundary = two continents converge and mountains form. • Subduction boundary = ocean plate and continental plate converge and denser ocean plate is pushed under continental plate and back into mantle; volcanoes and mountains form.
Transform Boundaries Transform boundaries occur when one plate slides horizontally past another along a single fault or group of parallel faults. • Ex: San Andreas fault in California.
Geosyncline Theory River sediment causes land to sink and remain wetlands until enough land is formed that uplifting “builds” coastline.
Evidence • Crustal composition and magnetic poles/reversal. • Ocean floor spreading. • Age of igneous rocks. • Minor crustal changes – deformed rock strata; syncline/anticline. • Displaced rock strata and fossils. • Vertical crustal movements.
Crustal Thickness The Earth’s crust is divided into two parts: • Continental crusthas ~ 20 – 40 km average thickness; composed mostly of low density rocks. • Oceanic crusthas ~ 10 km average thickness; composed mostly of high density rocks.
Moving Magnetic Poles The rock “record” shows that the magnetic poles have shifted as the tectonic plates have moved. • This evidence supports the theory of plate tectonics.