Here's why:

* Cooling Rate: Intrusive rocks cool slowly beneath the Earth's surface, giving crystals more time to grow and form. This slow cooling allows for the formation of minerals with higher silica content, such as quartz and feldspar.

* Chemical Reactions: During the slow cooling process, intrusive rocks can undergo further chemical reactions, increasing their silica content.

* Volatiles: Extrusive rocks, which cool rapidly at the Earth's surface, lose their volatiles (water, carbon dioxide, etc.) more quickly. These volatiles can influence the silica content of the magma, and their loss can result in a lower silica concentration in extrusive rocks.

Examples:

* Intrusive: Granite, a highly siliceous rock, is a common intrusive rock.

* Extrusive: Basalt, a relatively low-silica rock, is a common extrusive rock.

However, it's important to note that there are exceptions:

* Some extrusive rocks, like rhyolite, can have high silica content due to the rapid cooling of very viscous magma.

* Some intrusive rocks, like gabbro, can have lower silica content than some extrusive rocks.

In general, though, intrusive rocks tend to be more silica-rich than their extrusive counterparts.