Investigating craters
Impact craters are found on every rocky world. In this lesson you will plan a fair investigation into crater size and connect it to the energy of real impacts.
- Describe how simple and complex craters form.
- Identify independent, dependent and control variables in a crater investigation.
- Explain how evidence such as shocked quartz identifies an impact crater.
Links to: KS3 physics, energy: energy transfers; working scientifically: planning investigations, variables, repeat readings, graphs and conclusions.
How craters form
A large impactor hits the ground at many kilometres per second. In a fraction of a second its kinetic energy is transferred to a shock wave, which crushes, melts and throws out rock. The hole that forms is far larger than the impactor, typically more than ten times its diameter, and is surrounded by a raised rim and a blanket of thrown-out rock called ejecta.
Small craters are bowl-shaped and called simple craters, like Meteor Crater in Arizona, 1.2 km across. Larger ones collapse into complex craters with a flat floor and a central peak or a ring of hills, like the Ries crater in Germany, about 24 km across, which has an inner ring.
Proving an impact
Old craters are often eroded or buried, so geologists look for evidence that only an impact can make. Shatter cones are cone-shaped fracture patterns in rock. Shocked quartz has microscopic planes produced by pressures far higher than any volcano can create. Rare high-pressure minerals such as coesite and stishovite form the same way. Finding these, together with a round structure, confirms an impact.
Planning a fair test
A student asks: how does drop height affect crater diameter? Identify the variables, and describe how to analyse these results: 20 cm drop, mean 4.1 cm; 40 cm, 5.0 cm; 60 cm, 5.6 cm; 80 cm, 6.1 cm.
- Independent variable (what you change): drop height.
- Dependent variable (what you measure): crater diameter.
- Control variables (kept the same): the ball's mass and size, the tray material and depth, and how the surface is levelled.
- Plot crater diameter (y-axis) against drop height (x-axis) and draw a smooth curve of best fit.
- Describe the pattern: diameter increases with height, but each extra 20 cm adds less: +0.9, +0.6, +0.5 cm.
Answer: Greater drop height gives a larger crater, but the increase gets smaller each time. Higher drops give the ball more energy, but crater size does not grow in simple proportion to energy, which is also true of real impacts.
Carry out the investigation. Use a tray of flour or fine sand at least 5 cm deep with a thin dusting of cocoa on top. Drop a marble from at least five heights, three times each, smoothing the surface between drops. Measure rim-to-rim diameters, calculate means and plot a graph. Then try one change at a time, such as a heavier ball or a slanting throw, and describe how the ejecta pattern changes. Wear eye protection and keep flour away from flames.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
-
Question 1Compared with the impactor, how big is an impact crater?
The shock wave excavates rock far beyond the impactor itself.
-
Question 2What is ejecta?
Ejecta is the blanket of debris thrown out around a crater.
-
Question 3In an investigation of drop height and crater diameter, what is the dependent variable?
The dependent variable is the one you measure; the independent variable, height, is the one you change.
-
Question 4Which evidence shows that a crater was made by an impact rather than a volcano?
Shocked quartz and shatter cones need pressures that only hypervelocity impacts produce.
-
Question 5What feature does a complex crater have that a simple crater does not?
Large craters collapse after forming, producing a flatter floor and a central uplift.