Making Sense of the Conclusions

Use your observations to formulate a conclusion about the crystals formed from the two drops of saltwater.

How is this information related to igneous rocks?

Igneous came from the Latin word ignis, meaning fire. Similar to the hot saltwater solution, as the molten rock cools, crystals begin to form. The magma under the earth's surface cools over long periods as intrusive igneous rocks. Lava on top of the Earth's surface cools more quickly in the open air, forming extrusive igneous rocks.

What differences would you expect to see in the crystal structures of igneous and extrusive igneous rocks?

Intrusive rocks form as magma cools under the Earth's surface. The long cooling process of intrusive rocks creates large crystals that are easily visible to the unaided eye. Granite is a good example of an intrusive igneous rock with crystals that are easy to see. 

Extrusive rocks form as lava cools on the Earth's surface. Extrusive rocks experience a short cooling period so that only small crystals form. Basalt is an example of extrusive igneous rocks with small crystals that are more difficult to see.

Modifications

If time is limited, time-lapse clips can be completed ahead of time and provided to students. You could also prepare in advance a classroom set of slides of crystals formed at fast and slow evaporation rates for students to examine with the microscope.

Complete this activity as a demonstration by preparing time-lapse clips prior to the class or use the clips we have filmed [Fast Evaporation Video and Slow Evaporation Video]. Show the video clips to the entire class and discuss the differences between the salt crystals formed at a fast rate of evaporation versus a slow rate of evaporation.

After your students have completed the crystal structure analysis, they can take snapshots of rock samples and then create a slide show using the QX3 software in which they have grouped rocks into intrusive and extrusive groups.




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