Why is it that continental crust is less dense than oceanic, where in fact continental crust is thicker than oceanic crust?
Thickness has nothing to do with density. The density is how much a given volume weighs. If a block of 1m × 1m × 1m weighs 60kg, and another block the same size weighs 100kg, then the second block is more dense than the other.
So if the material that makes up the continental crust is less dense, then the continental crust will be less dense.
This almost sounds like a homework question, so I will leave references and such for now, but finding densities and compositions of these varying crusts is not too difficult.
The other answers are only partially correct. Yes the density is the key but the fundamental question is why are the continents less dense than the oceans. This is what makes the continents float higher - less dense.
The reason for the density differences is due to plate tectonics. The continents are derived from two process mostly. 1) subduction zone arc volcanics and 2) accretion of land masses as subduction occurs.
It is controversial but I believe that the continental crust formed by direct melting of the oceanic crust in subduction zones starting at least 3 billion years ago. The older parts of the continental crusts consist of a rock called trondhjemite which has the geochemical characteristics of slab melting in subduction zones.
In contrast, the ocean floor is created at mid ocean ridges in rift zones by direct partial melting of the mantle to form basalt. Basalt is much more dense than trondhjemite.
As mtb-za correclty pointed out, density has nothing to do with thickness. Oceanic crust is made up from basalts coming from mid ocean ridges while continental crusts are made up of all different sorts of materials such as for instance sedimentary rocks.
Both continental and oceanic crust "float" on the mantle beneath. Because continental crust is less dense it "floats" higher then oceanic crust. This is called isostacy http://en.wikipedia.org/wiki/Isostasy
The continental crust is continental crust because of its density, not the other way around.
The most dense is the material, the deepest it goes.
The continental crust is made up of lighter granitic rock, while deep-sea drilling reveals that the oceanic crust is basaltic in composition.
Rock composition of typical oceanic crust consists of mineral assemblages that are significantly denser than typical continental crust. Thickness of crust has really nothing to do with its density. Plate tectonics, sedimentation, and volcanic activity will play a significant the crust thickness.
Ability of upper crust to float on top of the lower crust/mantle is due to isostacy. Similar to about a helium balloon in the atmosphere.