Book of discoveries · 154
Geodetective · Petal ejecta
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In the upper image, a smaller crater lies within the petal-shaped ejecta blanket of a large impact crater. Which crater formed first?
On the right side of the upper image, the ejecta of two craters meet. Which lies on top, and is therefore younger?
Younger layers are deposited on top of older layers. This is Nicolaus Steno’s law of superposition.
The lower image shows the edge of a lava plain. Beside it is an ancient, rugged, eroded area that rises above the lava plain and was therefore not flooded by lava.
When it fell, the rocky material in the ejecta blankets around the craters mixed with groundwater and moved across the surface as a mudflow, giving the blanket a lobate edge. It may consist of several layers if the impact penetrated several water-bearing layers. The thin ejecta blanket is eroded or buried relatively quickly, so craters where it is still visible are younger.
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Principle from the book
Younger layers are deposited on top of older layers. This is Nicolaus Steno’s law of superposition.