Lesson Plan on Craters

Objectives: Students will be able to

  1. explain possible origins of craters.
  2. develop a model of craters out of flour or cornstarch in a laboratory investigation and make observations from the investigation.

Grade Levels: 8-12

Background Information:

Craters are one of the most striking features on bodies in our solar system. A crater is a bowl-shaped or flat-bottomed depression on the surface of the planet or minor object. Craters are found on planets with solid surfaces, moons, and even asteroids. Two possible origins of craters were hotly debated before the 1960's, when studies of craters on earth and the first lunar missions resolved the origin of most craters.

One possible origin is the impact of meteorites. Meteorites are fragments of asteroids or comets that strike the surface of an object, traveling at speeds of tens of kilometers per second. When a meteorite strikes, the energy of its motion is released as a huge explosion, much as the energy of the speed of a crashing car is released by destroying the car. The rock on the surface being struck is fragmented, a bowl-shaped cavity is formed, and debris ejected from the cavity forms an apron around the cavity (called "ejecta"). Rock is slightly flexible and elastic, like a rubber band, so in large craters the bottom of the cavity bounces back to form a central mountain peak. Rock just outside the rim of the crater is turned upward by the force of the explosion, forming a ring-like mountain surrounding the crater cavity. Also, tremendous heat released by the explosion melts some of the rock in the crater, and it ponds and cools on the crater floor.

Another possible origin of craters is volcanic explosions. Melted rock inside a planet (called "magma") contains a few percent of dissolved gases like water and carbon dioxide. When the magma is many kilometers below the planet's surface, the pressure of the overlying rock forces these gases to stay dissolved in the magma. However magma is lighter than solid rock and tends to rise to the surface. When it reaches a shallow depth, the pressure is less and the gases can no longer stay dissolved. They are released, fragmenting the overlying rock and causing a tremendous explosion. This is much like what happens when a warm bottle of soda is opened: the capped bottle contain carbon dioxide dissolved in water, and gas under pressure. The pressure keeps the carbon dioxide dissolved. When the cap is opened the pressurized gas escapes, and the carbon dioxide is released from the water causing a bubbling froth.

Volcanic explosions form cavities resembling those formed by impacts, but they lack the central peaks of large impact craters, and the rims are usually not raised very much. However in some other ways they do look like impact craters: magma erupted to the surface (then called "lava") may fill the floor of the cavity, and the cavity is surrounded by an apron of ejected rock and lava. A diagram is provided to illustrate the possible origins.

[Possible Origins of Craters]

Activities:

Activity: Formation of Craters

No Frames Table of Contents

[LMGFP home page] Contact Karen Krupinsky (kgurley@gsfc.nasa.gov) or
Tammy Seergae (tseergae@umd.edu) for further information.