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Sibley Volcanic Regional Preserve Exploration

As sure as the earth moves in Berkeley, there's a volcano just off Skyline Boulevard.

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Rating: 5 out of 5 by EveryTrail members
Difficulty: Moderate
Length: 2.3 miles
Duration: 1-3 hours

Overview:  As sure as the earth moves in Oakland, there's a volcano just off Skyline Boulevard. Not just any volcano, mind you. This one's lying ... more »

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Tips:  6800 Skyline Blvd, Oakland, CA 94611
Phone: 888-EBPARKS (888-327-2757), option 3, extension 4554

Open between 5 a.m. and 10 p.m.... more »

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Points of Interest

1. Soil layers in the hillsides

Bands of green and brown on the hillsides near Sibley reveal layers of soils created by different geologic processes. Greener grass means clay-like soils that hold moisture. Water drains out faster from gravely soils, leaving the grass brown.

Our guide, Steve Edwards, demonstrates how California's volcanoes came to be. When two of the earth's... More

2. Lava feeder

Sibley's central geologic feature is its volcano, Round Top. Over millions of years, the movements of tectonic plates have pushed Round Top over onto its side. In this spot, you are standing in what was the interior of the volcano. This black lava flowed up through a crack in the earth's crust, to fill Round Top's crater.

A man-made plum in the ... More

3. Basalt cobblestones

The path out to the quarry resembles basalt cobblestone. The rocks here are the same as those that made up the lava lake at Round Top. Geologists think they might have broken into small pieces after an eruption emptied the lake, but left its hardened surface intact. The hard lava collapsed into the void left behind by the eruption, breaking into... More

4. The Quarry

Sibley's volcano, pushed over on its side, has had a second life as a quarry dug by Kaiser Sand and Gravel. The quarry, which cut through the interior of the top part of the volcano, provided basalt used in making roads.

Ranger Steve Edwards points across Round Top's crater, now tipped on its side, to Mt. Diablo in the distance. "That's not a... More

5. Bake zone

The red part of this basalt outcrop is called a bake zone. The basalt lava on the left may have still been hot and steaming when the lighter volcanic ash on its right landed on it. The steam coming from the basalt may have turned the lighter volcanic ash red.

Bake zones can contain some dramatic features. After a rain, these red rock formations... More

6. North quarry pit

You're looking northerly at another quarry pit left by Kaiser, exposing layered basalt lava.

In a close up of the wall of the quarry, we see layers of basalt lava, nearly perpendicular to the original flow. These layers were caused by jointing as the lava cooled and shrank.

7. Steamy lava holes

Holes, or vesicles, lend a swiss-cheese texture to some of the lava. These pockets were formed when water in the lava escaped as steam while lava was cooling.

Over time, many of the pockets in the rock were filled with minerals precipitated from ground water or volcanic fluids. The green fillings shown here are celadonite, a mineral related to... More

8. Basalt flow

This basalt flow, about 150 feet thick, stretches all the way to Highway 24 east of the Caldicott Tunnel on Highway 24. Unlike the lavas in Hawaii, Round Top's flows were thick and slow-moving, advancing about as fast as leisurely walk.

Like the volcano it came from, this lava flow has been tilted onto its side, and its current position helps us ... More

9. Welded cinder

These textured red blobs are a rare formation called welded cinder. They were formed when basalt exploded out of Round Top in small hot, fragments that landed on each other and stuck together. Their "real" name also offers a good example of geologist jargon: rocks stuck together are "agglutinated," if the pieces stuck together are pebble sized, ... More

10. Petroglyph?

Ranger Steve points to a spiral pecked into a stone that juts out from a ridge top.

Alas, our guide tells us, upon closer inspection, it's clearly been chipped out with a steel chisel. So, unlike a petroglyph, which is sacred, this carving is declared a "profanoglyph."