6th Grade Science – Earthquakes
Students learn that earthquakes are sudden motions along breaks in the Earth’s crust called faults. They also learn that major geologic events such as earthquakes, volcanoes and mountains result from plate movement.
Have students compare what they know about plate tectonics with what they learned from the plate tectonics video. Then ask them to jot down questions they still have.
Plate tectonics is the theory that Earth’s outer layer, called the lithosphere, is broken into large and small rigid plates that move in relation to each other and over an underlying layer of partially molten asthenosphere. This motion gives rise to fundamental geological phenomena such as continents drift, earthquakes and volcanoes.
The plates move toward each other, away from each other or grind past each other at tectonic plate boundaries. When oceanic plates collide, the denser oceanic lithosphere is forced downward into the hotter, less dense continental lithosphere. This process is known as subduction.
When a plate moves over a stationary hot spot, magma forms at the surface of the plate and forms deep-sea trenches. Volcanic islands and seamounts form at these plates’ edges as the molten magma cools. Plates also break apart along mid-ocean ridges and at transform faults. Plate movement is evidenced by the Ring of Fire, a global network of active volcanic and seismic zones.
A fault is a break in the Earth’s crust where rocks move suddenly. Before an earthquake occurs, tectonic plates build up tension along the fault. Friction keeps the rocks from moving immediately, but when the stress builds up to a critical point, the rocks slip along the fault. This releases the accumulated stress and causes the earthquake. There are three main types of faults: normal, reverse, and strike-slip. Each type of fault is associated with specific types of plate boundaries:
To illustrate the different forces that cause rocks to move, give each student a piece of Silly Putty and ask them to stretch it by pulling on opposite sides. Students can compare the results to how an earthquake works in the Earth by explaining that while the material can withstand some amount of stress, eventually it will break. Lesson 3 introduces shear stress that creates a reverse fault and shear stress that creates a normal fault or transform fault (also known as a strike-slip fault on continents). This is explored through a hands-on model of a land formation created by pushing Earth materials.
Earthquakes are caused when elastic energy builds up slowly within the Earth’s crust and then is released suddenly along cracks in the surface called faults. This release of energy creates waves known as seismic waves.
As seismic waves travel through the Earth, they encounter different types of rock. When a wave encounters a change in rock type, part of the energy of the wave is reflected and the rest passes through. This process is called refraction. You can demonstrate this with a coin dropped in water. The coin will not sink to the bottom vertically because it has been refracted.
After describing how an earthquake generates seismic waves, students will work in pairs to answer questions about the nature of these waves. Then they will use the information about seismic waves to solve problems related to the way seismographs record an earthquake. They will also determine how the process of finding an epicenter works and what effect tsunamis have on human populations.
Whether a region feels an earthquake depends on its proximity to the epicenter. The severity of damage an earthquake causes also depends on local geology and construction.
Using the research station students will become seismologists and input data about S and P waves to determine where an earthquake is located. They will use a process of triangulation to locate the epicenter and then calculate the magnitude of an earthquake.
This station lab has four “input” stations and four “output” stations. It is a great way to challenge students by providing them with different ways to learn and demonstrate their understanding of the same material.
Before student pairs begin to work on this station, have them complete the Engage section of the Finding Epicenters and Measuring Magnitude Worksheet. Then, have them evaluate their answers to assess their understanding of the topic. If possible, arrange for each student pair to have a computer that can access the Earthquakes Living Lab.