Overview

Plate tectonics are believed to be crucial to the development of life on Earth and elsewhere. In this lab we will learn about how the structure of the Earth was determined by examining the propagation of different types of waves through common materials.

Science Question

How do scientists know what’s inside the Earth when we can’t dig to the center?

Materials

• A smartphone with the Vibration Meter app
• Four tennis balls or similarly light, droppable weights
• Four boxes with different materials in them
  • Packing peanuts
  • Beans
  • Gravel
  • Sand

Planetary Science

About 50% of the heat in the Earth’s core is left over from its formation, and 50% is the product of radioactive decay of the heavy elements thorium, uranium, and potassium. Remember that uranium and thorium, which make up 85% of that 50%, are produced only in supernovae and neutron star mergers. These heat sources are crucial to convection (i.e. boiling) in the mantle, which fuels the movement of tectonic plates in the crust.

Plate tectonics are essential to life on Earth because they sequester, or remove and store, carbon dioxide from the atmosphere in what is called the “Slow Carbon Cycle”. The Slow Carbon Cycle takes 100-200 million years and has four steps:

1. Carbon dioxide combines with water in the atmosphere to form carbonic acid. This falls to the Earth as rain.
2. Carbonic acid erodes, or “weathers” rocks, releasing calcium, magnesium, potassium, or sodium ions. An ion is an unhappy atom which is missing electrons or has too many of them.
3. Rivers carry the ions to the ocean, where they combine with bicarbonates (a type of molecule which happens when carbon is dissolved in seawater) to form calcium carbonates. Calcium carbonate is the white stuff on your faucet from “hard water”.
4. The calcium carbonate sinks to the seafloor, where it eventually becomes a part of the oceanic crust. When the crust is subducted under other layers of crust, it ends up in the mantle, sequestering the carbon it stores. This is not always permanent, though, because volcanoes can bring CO2 to the surface.

It’s important to have a slow carbon cycle, otherwise an atmosphere could experience the runaway greenhouse effect, in which carbon dioxide or another greenhouse gas like methane raises global temperatures. Increased temperatures melt water and frozen methane, and release CO2 more effectively from rocks. These additional greenhouse gasses in turn heat the atmosphere even more. Eventually the climate is so hot that all liquid water has evaporated. The water does not hang around in the atmosphere, either: UV rays break up water molecules at high altitudes into hydrogen and oxygen, and the lightweight hydrogen escapes into space. Fun fact: this is what happened to Venus! Funner fact: this is also why, as we will learn later in the semester, you can have a planet with a high oxygen content that is not at all hospitable to life.

Activity

In your groups, go to your first assigned station. Designate one person as the tennis ball dropper. Have someone download the Vibration Meter app, which is free on Apple and Google Play.

Turn on the app and place the phone in one end of the box. Make sure it is recording, then drop the rock from 1 ft high into the other end of the box. Observe the pattern created. The blue “z” axis is the line to watch.

Create three seismic wave patterns per box. The most effective way to estimate the wavelength and amplitude is by screen-shotting what you see and zooming in to estimate the amplitude and wavelength of the waves. The amplitude is the maximum height of the peaks of the waves measured from their center and the wavelength is the distance between crests.

Box 1: Sand

Box 2: Gravel

Box 3: Beans

Box 4: Packing peanuts

1. Compare your wavelengths from each of the boxes and rank them from highest to lowest.
2. Compare your amplitudes from each of the boxes and rank them from highest to lowest.
3. The density of the materials in each box is as follows:

QUESTIONS

1. What can you infer about the relationship between density and wavelength? What about density and amplitude?
2. “Primary” waves are compression (in-and-out) waves, like if you squished the ends of a slinky together. “Secondary waves” are transverse (up and down) like if you shook the ends of a jump rope. Primary waves travel through any medium. Secondary waves can only travel through solids. Using this information, do you think that the signal created by your marker on the paper was a primary wave or a secondary wave?
3. Based on your answer above, if we filled a box with water and attempted the same experiment, what would the signal look like?
4. Draw two grids of 16 dots each, 4 by 4. Then draw curled lines like springs between the dots on the left such that there are square boxes but no triangles. This represents atoms in a solid, which are held together by strong bonds. Then draw thin, dotted lines in the same way between the dots on the right. This represents atoms in a liquid, which are held together by weak bonds.
5. Imagine you hit the set of atoms on the left with a very small hammer. They are held together with, essentially, springs. Describe the motions of the atoms.
6. Imagine you hit the set of atoms on the right with a very small hammer. They are not held together very tightly and their connections will break when struck. Describe the motion of the atoms.
7. Based on your answers above, describe again why transverse waves cannot travel in liquids.