what is the difference between oceanic crust and continental crust?

Continental tholeiites have higher SiO2, K2O and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts. These differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface. Although there is little doubt that most continental tholeiites have assimilated crustal material, the lcoation and mechanism of assimilation remain uncertain. Longhi (1981) has argued that magmas derived directly from the mantle should crystallize little orthopyroxene. The abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust. Since orthopyroxene is an early crystallizing phase in layered intrusions, this assimilation must occur early, before the magma enters the chamber. Assimilation can occur at the margins of the dykes which feed magma chambers, depending on the nature of the flow. If the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls. If the flow is laminar, a conductive profile develops at the margin and the flowing magma chills against the walls, protecting them from thermal erosion. The nature of flow in a dyke can be predicted from the Reynolds number, the criteria for turbulence. Reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and, if this is the case, assimilation of low melting point components in the walls of the dyke is inevitable. It is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust.

Continental crust underlies the continents and is mainly composed of granite rock. Average thickness is about 35 km.

Oceanic crust underlies the oceans and is mainly composed of Basalt rock. Average thickness is about 7 km.

Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima. It is thinner than continental crust, or sial, generally less than 10 kilometers thick, however it is denser, having a mean density of about 3.3 grams per cubic centimeter.Although a complete section of oceanic crust has not yet been drilled, there are estimations of composition based on analyses of ophiolites, comparison of seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging (especially from ridge crests and fracture zones) and drilling. Oceanic crust is significantly simpler than continental crust and generally it can be divided in three layers.
Geochemistry
The most voluminous volcanic rocks of the ocean floor are the mid-oceanic ridge basalts (MORBs), which are derived from low-potassium tholeiitic magmas. These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements (Th, U, Nb, Ta and Pb). There can be found MORBs enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands, the Azores and Iceland.

Continental crust
The continental crust is the layer of granitic, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to its granitic rock, in contrast to the oceanic crust, called sima due to its basaltic (also called mafic) rock. (Based on the change in velocity of seismic waves, it is believed that at a certain depth sial becomes close in its physical properties to sima. This line is called the Conrad discontinuity.)

Consisting mostly of granitic rock, continental crust has a density of about 2.7g/cm3 and is less dense than the material of the Earth's mantle, which consists of mafic rock. Continental crust is also less dense than oceanic crust, though it is considerably thicker; mostly 35 to 40 km versus the average oceanic thickness of around 7-10 km. About 40% of the Earth's surface is now underlain by continental crust.
Importance
Because continental crust mostly lies above sea level, its existence allowed land life to evolve from marine life. Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time. If Earth were like the other silicate planets and lacked the duality of oceanic and continental crust, our planet would be a very different place and the evolution of Homo sapiens and civilization would have been impossible.