The Earth's crust is mostly composed of oxygen and silicon. These two elements combine to form silicates, which make up about 90% of the crust's minerals.

Here's a breakdown of the main elements and their proportions:

* Oxygen (O): 46.6%

* Silicon (Si): 27.7%

* Aluminum (Al): 8.1%

* Iron (Fe): 5.0%

* Calcium (Ca): 3.6%

* Sodium (Na): 2.8%

* Potassium (K): 2.6%

* Magnesium (Mg): 2.1%

Other elements: These make up a small percentage of the crust but are important for various minerals and processes.

Important Note: The composition of the Earth's crust varies between continental crust and oceanic crust. Continental crust is thicker and has a higher proportion of granite and less dense minerals, while oceanic crust is thinner and mostly composed of basalt, which is denser.