Temporary Closure of Waters Around Alcatraz Island
From 7/3 to 9/22 a 500’ marine buffer zone is in effect, closing the perimeter of Alcatraz Island to all private vessels to protect nesting seabirds during America's Cup racing. Tours to Alcatraz continue as usual during the races. More »
Muir Beach (but not nearby Muir Woods) closed July 8-November 2013, but local businesses are open
Though Muir Beach is closed from July 8-November 2013 due to construction, the PELICAN INN IS OPEN. Restrooms and parking are not available at Muir Beach. Pacific Way is closed except to residents. Check back for updates or call (415)561-3054. More »
CAUTION: Post Storm Damage to Coastal Trail
The Presidio Coastal Trail segment just north of the Pacific Overlook and adjacent to Lincoln Blvd remains CLOSED indefinitely. We have posted signage to alert bicyclists and hikers and with information for safe trail alternatives. More »
Where does diabase form?
Diabase is an intrusive igneous rock with the same mineral composition as basalt. It cools under basaltic volcanoes, like those at mid-ocean ridges. Diabase cools moderately quickly when magma moves up into fractures and weak zones below a volcano. There, it forms dikes (tabular igneous rock bodies that cut across pre-existing rock layers or bodies) or sills (tabular igneous rock bodies that form parallel to pre-existing rock layers). The moderate cooling rate allows small visible crystals to form in the rock.
Why does the diabase have large and small crystals?
Igneous rock with some large crystals among the smaller crystals is called a porphyry. The different crystal sizes are the result of different rates of cooling as the magma body moved upward. The large crystals, called phenocrysts, in diabase are feldspar crystals that grew as the magma cooled slowly deep in a magma chamber. Later the magma with the large phenocrysts moved upward quickly, causing more rapid cooling of the rest of the magma and the formation of the small crystals that make up the rest of the rock.
Did You Know?
John Fremont, the explorer, and his wife Jessie Benton Fremont, lived at Fort Mason. Both were abolitionists and their home, once located at the edge of the post, became a center of San Francisco’s intellectual life.