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 »
Pillow Basalt FAQ
Where does pillow basalt form?
Pillow basalt is a volcanic igneous rock that forms when lava of basaltic compositionis erupted underwater. The rapid cooling of the lava by cold water on all sides forms the pillow-shaped bodies, which can then break open and extrude more of the hot lava from inside. The rapid cooling also creates pillows that are composed of volcanic glass on the outside and that have very tiny, almost invisible crystals on the interior. Pillow basalt typically forms at volcanoes at mid-ocean ridges or at oceanic hot-spot volcanoes, such as those that formed the Hawaiian Islands. Basalt forms the crust of all the ocean basins and is therefore the most common rock in the Earth’s crust.
What gives basalt its color?
Basalt has a lower percentage of silica and a higher percentage of iron and magnesium than other volcanic rocks. These characteristics give it a very dark, almost black color. Hot, mineral-rich seawater flowed through much of the basalt that is part of the Franciscan Complex, changing some of its minerals into chlorite and other green minerals. This altered basalt is called greenstone.
What makes basalt sometimes appear speckled?
Sometimes basalt in the Franciscan Complex will have small white or pink speckles or even small round holes in it. These are the result of gas bubbles that were captured in the lava when it cooled. Sometimes the gas bubbles are empty, but other time they get filled with quartz or calcite that precipitated out of mineral-rich water circulating through the basalt, producing the speckles. Sometimes the gas bubbles in Franciscan basalt get filled with a bright orange semi-precious form of quartz called carnelian. If basalt is erupted too far under the ocean, the pressure from all the overlying water will not allow the gas bubbles to form. This is the case for most Franciscan basalt, but the basalt at Point Bonita is an exception.
Did You Know?
The international peace symbol was designed in 1958 as the logo for the Campaign for Nuclear Disarmament (CND) and has deliberately never been copyrighted.