1. Oceanic Crust:

* Composition: Primarily composed of basalt, a dark-colored, fine-grained volcanic rock, and gabbro, a coarse-grained, intrusive igneous rock.

* Density: Denser than continental crust.

* Thickness: Relatively thin, ranging from 5 to 10 km (3-6 miles).

* Age: Younger than continental crust, as it's constantly being created at mid-ocean ridges and subducted at trenches.

2. Continental Crust:

* Composition: More complex and variable, but primarily composed of granitic rocks (rich in silica and feldspar).

* Density: Less dense than oceanic crust.

* Thickness: Thicker than oceanic crust, ranging from 30 to 70 km (19-43 miles).

* Age: Older than oceanic crust, with some parts dating back billions of years.

Together, these layers form a rigid lithosphere, which is the outermost layer of the Earth.

Beneath the lithosphere lies the asthenosphere, a partially molten layer of the upper mantle. The asthenosphere is weak and allows the lithospheric plates to move.

Important Note: The thickness of the crust varies significantly across different regions of the Earth. For example, the continental crust is thinner under mountain ranges and thicker under stable platforms.

Overall, the structure of tectonic plates is complex and dynamic. They are constantly being created, destroyed, and moved by the forces of plate tectonics, which shape the Earth's surface and drive many of the planet's geological processes.