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 ... Moreearth's tectonic plates collide, the heavier one will sink under the other. As the heavier plate moves toward the hot core of the earth, minerals in it melt and become magma. This heated magma, less dense than the solid rock around it, forces its way to the surface, sometimes explosively.Less

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 p... Morelum in the breccia pudding. Anyone have any idea what this is, or why it has been attached to this rock?

This plums-in-the-pudding formation is called breccia (pronounced BREH-chuh). It's the result of lava erupting from a volcanic vent, dragging with it chunks of rock it passes on the way. Hot lava coming up through Round Top broke itself up while solidifying, and ripped pieces of lighter-colored sandstone and bluish schist off ancient rocks thousands of feet below the surface as it made its way upward.

This rounded hump of rock is a victim of weathering. Imagine a block of volcanic rock, shrinking as it cools, forming cracks. Between those cracks are roughly cubic blocks of lava. Their exposed corners weather faster than the rest of the rock, leaving behind spherical blobs with layers peeling like onion skins.

This flow once formed a lake of lava that congealed inside Round Top's vent. Erupting magma had to go up through this lake, or along side of it and erupt out of the side of the mountain.Less

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 mi... Morellions of pieces.

A chunk of basalt, the most common rock on the face of the earth. The brownish weathering rind hides the rock's gunmetal blue interior.

We were not too busy looking at rocks to stop and admire the local wildflowers. Wyethia angustifolia, commonly known as Narrow-leaved Mule Ears, were welcome oases of yellow dotting our walk.Less

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'... Mores not a volcano," he says, noting that many people think it might be because of its shape. "Here in Sibley, we're standing in the volcano."

Steve demonstrates the volcano's position in the past, and now. His hand is the volcano's caldera, the depression left in the top of a volcano after an eruption. Here, it's upright, as it was millions of years ago when Round Top was active.

The action begins; as tectonic plates in northern California slide past each other, the forces of their movements tilt the volcano.

Over the years, several mazes have been made at the bottom of the quarry, and are regular destinations for park visitors.

Watch your step and don't go beyond the fences or you may end up part of the quarry!Less

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 f... Moreormations turn blood-red. The red results from oxidation of iron.Less

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.

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 re... Morelated to micas.

The mineral-filled pockets have a characteristic almond shape that indicates which direction the lava was flowing. The thinner, pointy end leads in the direction away from the volcano. The white rock is opal or chalcedony.

This yellowish rock, called rhyolite, is also a volcanic rock. But Round Top didn't contain the material this rhyolite is made of. Four miles north of Sibley is another vent that spewed forth rhyolite, about 9.8 million years ago. At that time, it was erupting beside Round Top. Fault motion subsequently moved it north to Berkeley.Less

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 positio... Moren helps us imagine the flow as it slowly marched across the landscape. Such creeping lava would have set all the land around it on fire, sending blocks of hot lava tumbling off of it, and consuming everything in its path--including intrepid rangers and reporters.Less

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, th... Moreey're "lappili," and if those pebble-sized pieces are rounded, it's an "agglomerate."

Behold: a close-up of the rare and beautiful agglutinated lappili agglomerate. Or maybe let's stick with the name "welded cinder."Less

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."