SCIENCE HELP: Olivine crystals are sometimes found in basaltic rock, even though though olivine is denser.....?

Probably because they crystallize from components of intrusive magma, which cools more slowly than extrusive magma, and thus has time to form large crystals.

The crystals were simply not there to start with, and would have a hard time falling out of a liquid matrix of about the same density.

Olivine is a mixture of two minerals, fayalite and fosterite, that crystallized fractionally out of a basaltic magma with a predominantly Fe and Mg chemical composition. ANY crystal is going to denser than the liquid magma it crystallized from according the ideal gas law. Look at the phase diagram. That's how it happens

http://instruct.uwo.ca/earth-sci/300b-001/kd.htm

http://instruct.uwo.ca/earth-sci/300b-001/forst.jp...

http://www.see.leeds.ac.uk/see-research/igt/people...

because both r formed from mafic lava so the olivine crystallizes then the non-crystallized lava got pushed out of volcano trapping olivine in the basalt

http://en.wikipedia.org/wiki/Basalt#Petrology