The U.S. Geological Survey (USGS) is the primary source of government information in the earth sciences and has a voluminous web presence.
From the main USGS page, you can select information by state or search the site. There are also convenient links to newsworthy information on earthquakes, volcanoes, and other hazards. An enormous amount of map, graphic, and textual information is available online through their Maps, Imagery, & Publications portal.
The agency covers the following broad science areas, which each have their own website:
Geologic maps are an important resource for studying Earth Science. USGS' National Geologic Map Database is a portal to thousands of maps and related information.
You can find information on more topics by browsing the "Science Topics" page. And they have a special Geoscience Data Catalog page that provides links to data produced by the USGS, by series title and geographic location.