Presentation on theme: "Earthquakes and Volcanoes Section 1 Earthquakes. Earthquakes—large vibrations that move through rock or other Earth materials Elastic rebound—when rocks."— Presentation transcript:
Earthquakes and Volcanoes Section 1 Earthquakes
Earthquakes—large vibrations that move through rock or other Earth materials Elastic rebound—when rocks strain and then break, the broken pieces snap back. Rocks deform slowly over long periods of time Potential energy builds up in them Energy is released suddenly when rocks break and move. The movement causes vibrations that move through Earth.
Fault—the surface of a break in rock Normal fault—caused by tension forces, rock above the fault moves down compared to rock below the fault Reverse fault—caused by compression forces, rock above the fault moves upward compared to rock below the fault Strike-slip fault—caused by shear forces, rock on either side of the fault moves past one another in opposite directions
Seismic waves—when strained rock’s potential energy is released, it moves outward from the fault in seismic waves Focus—the point inside Earth where movement along a fault first occurs and energy is released Epicenter—the point on Earth’s surface located directly above the focus
Seismic waves start at the focus and travel away in all directions Primary waves—cause rock to move back and forth in the same direction the waves are moving Secondary waves—cause rock to vibrate at right angles to the direction the waves are moving Surface waves—slowest, largest, most destructive waves
Measuring earthquakes Seismograph—instrument that records and earthquake’s vibrations If seismic wave arrival times are recorded from three stations, the epicenter can be determined Richter scale—measures an earthquake’s size, or magnitude, based on the heights of lines representing the amount of energy released through seismic waves recorded on a seismograph
Earthquake damage Modified Mercalli intensity scale—measures an earthquake’s intensity based on the amount of structural and geologic damage Most earthquake damage is caused by surface waves Tsunamis—when an earthquake occurs on the ocean floor, the sudden movement pushes against the water and creates powerful waves that can travel thousands of kilometers
Seismic-safe structures are able to stand up against an earthquake’s vibrations. Many high-rise buildings stand on huge steel and rubber springs Underground water and gas pipes are replaced with pipes that will bend. Highways have cement pillars with spiral reinforcing rods around them Predicting earthquakes Long-range forecasts predict whether an earthquake is likely to occur in a given area within 30 to 100 years.
Earthquakes and Volcanoes Section 2 Volcanoes
Volcanoes—cone shaped hills or mountains formed by magma eruptions When magma flows onto Earth’s surface through a vent, it is called lava Tephra—bits of rock or solidified lava dropped from the air after an explosive eruption Some volcanoes form where Earth’s plates collide One plate subducts, or is forced underneath, the other Part of the plate that is forced underneath melts, forming magma chambers
Avalanches of hot, glowing molten rock that flow in cushions of hot gases down a side of a volcano are called pyroclastic flows How forceful an eruption is depends on the composition of the magma More silica makes magma thicker and more resistant to flow Water vapor trapped in the magma becomes steam and creates pressure The type of lava and gases contained in the lava determine the type of volcano that forms
Four types of volcanoes Shield volcanoes—basaltic lava, which flows easily Forms a broad volcano with gently sloping sides Largest type of volcano Form where Earth’s plates are separating and magma is forced upward between plates Cinder cone volcanoes—high gas content in the magma Explosive, but short-lived, eruptions Form a small cone of volcanic material from tephra
Composite volcanoes—made of alternating layers of lava and tephra Steep-sided mountains Form where Earth’s plates are colliding and being forced underneath each other, or subduction zones Fissure eruptions—magma that is very fluid Oozes from cracks in Earth’s surface Magma flows freely across the land, as flood basalts Most of Earth’s crust beneath the ocean is flood basalts
Earthquakes and Volcanoes Section 3 Earthquakes, Volcanoes, and Plate Tectonics
Earth’s crust is broken into plates that move around. Most volcanoes form where plates are colliding or moving apart. Divergent plate boundaries—where move apart Long cracks called rifts form between them Fissure eruptions are common where plates separate Convergent plate boundaries— where plates collide and denser plates subduct, or are forced underneath less dense plates Most earthquakes occur where plates are colliding or moving apart.