A lot of people think that Mount Diablo is a volcano. Various legends from the local native peoples have mentioned that Mount Diablo is a volcanoes. In fact, Mount Diablo is not a volcano at all. The mountain was formed by geologic forces pushing the mountain upwards. These pressures started millions of years ago when the land in this area was still under the ocean. The pressures are still working at pushing the mountain higher.
You can still see remnants of the sea creatures that existed then in fossils
as you walk the trails of Mount Diablo and it's surrounding foothills. When you hike along the trails, take a look at the rocks and
boulders such as at Shell Ridge. You can see fossils in the rocks. There
is also a paleontology dig going on in the foothills near Blackhawk and Danville.
Over the years, coal, sand, mercury and other materials were mined from Mount Diablo and surrounding area's. On the northeast side of Mount Diablo, in Antioch, there is the Black Diamond Regional Preserve. Going back to the 1860's there were several towns, including Nortonville and Somersville, that were founded when there was mining of coal and sand. There isn't much left of these towns except for the cemetery (which some say is haunted by Sarah Norton) and miles of tunnels used for mining. Some of these tunnels are open for tours on a first come first serve basis. The park system has been working on finding and closing up tunnels that are unsafe. Please do not enter any of the tunnels unless on the tour since carbon monoxide gases can build up in dangerous levels.
There are many trails and picnic areas on Mount Diablo for people to enjoy. There are also camping area's for those who are so inclined. In Rock City, there are rock formations and wind caves that people can look at. Near Castle Rock Park, there is a nature preserve where endangered birds nest.
If you wish to drive up Mount Diablo, the only roads that go towards the summit is from Northgate Road in Walnut Creek or the Southgate Road in Diablo/Danville. These two roads join part way up the mountain to form a single road that goes up the upper part of the mountain. There is parking and a museum at the summit of Mount Diablo. At the summit, you can see most of California on a clear day. You can even see Half Dome in Yosemite on a clear day.
There is a variety of wild life on Mount Diablo as well. There are bob cats, mule deer, a variety of hawks and eagles, and even wild boar. In the spring time, you can enjoy California Poppies and Lupine along the roads and trails. There even several types of flowers only found on Mount Diablo.
The links below go to articles that explain more about the geology and nature of Mount Diablo and info about picnic, hiking and camping area's.
Mount Diablo site by sfgate.com with info about geology and links to history and things to do