"Evidence of earth movement is everywhere, in the hills and the valleys and the rocks," said Sonoma State University Professor Terry Wright. From the jagged coastal rocks to the slopes of the once-volcanic Sonoma Mountain, most of the terrain was created by the same thing that causes earthquakes? the movement between mas-sive rock plates beneath the earth and sea Geologically, the Sonoma County landscape is like a reckless adoles cent?oung, dramatic and full of pent-up energy that periodically erupts or breaks apart?making for ongoing changes in the rugged hills and canyons.

One way to understand the dynamic forces behind Sonoma County's geology is to picture two blocks of moving rock. Sometimes one slips under the other lifting it up. And sometimes they move in opposite directions. The movement causes shaking and rumbling?ut all that slipping and sliding also causes changes in the landscape.

Consider the following geological facts:
When people perch on Bodega Head watching for migrating whales, the ground beneath them is migrating, too. The granite peninsula that borders Bodega Bay, the Head started as part of the Southern Sierra range 320 miles south and is continuing to move north.
Landslides, like the one that destroyed homes in Rio Nido and the one undermining coastal homes, were indirectly caused by the shifting of underground plates as was the Blucher Valley crack, the rare "transverse" landslide where four acres moved in a chunk west of Sebastopol 15 years ago.
Five million years ago the Sebastopol area was covered by a shallow inland sea, until seismic action tilted the land and drained the sea.
The heat source for the Geysers sent clouds of steam rising from the hills along the eastern border of the county like the holes of a giant pressure cooker, magma from an active part of the same fault, uplifted volcanic mountain range that includes Sonoma Mountain and Mount St. Helena.

It's a challenge to try and picture the slow, complex underground action that over millions of years shaped the landscape. But the results are everywhere to see and touch "Shield volcanoes"

Terry Wright began one of his recent geology tours on Sonoma Mountain, at a point on Lichau Road, off Petaluma Hill Road, where he picked up basalt rocks mottled by odd little holes, created by gas bubbles that formed inside what was hot nowing lava 3 to 5 million years ago. While holding in his hand proof of that long-ago eruption, Wright acknowledged that most people don't think "aha! volcano" when they look at the low meandering series of hills between Petaluma and Sonoma. Wright explained that's because this is a lesser known shape than the classic volcano cone. "The Sonoma Volcanics are 'shield' volcanos, low and spreadout because the lava nowed so fast," he said.

The presence of large amounts of tuff in the volcanic sequence of Sonoma Mountain suggest that more explosive volcanic activity was present here. So this may not have been a classic shield volcano. The main uplift of these ranges seems to be along faults, compressional thrust types that have pushed the volcanics upward Mount Konocti has the more classic rugged cone shape and is still on the active list because it erupted "recently" in geologic time, that is about 10,000 years ago. Relics of the lava flow's scorch have become one of Sonoma County's tourist attractions, the "Petrified Forest". There you can see trees that were felled and preserved. "The Petrified Forest has trees 100 feet long that were once buried by volcanic ash. An earthquake fault indirectly gets credit for those petrified trees.

Wright explained that the volcanos erupted when the earth's ocean floor plate to the west dragged deep down into the mantle of the earth, where it melted and turned to magma (molten rock). Evidence that hot magma continues flowing not too far under the surface of the North Coast is erupting daily as puffs of steam billow up along the Sonoma/Lake county line. "The Geysers is the result of magma that exists under Clear Lake. The Geysers is the largest commercial geothermal field in the world. It's like a big underground teapot," said Wright.

Plate movement also explains why a vast area of western Marin and Sonoma counties are now dry instead of underwater like they were five million years ago. One plate moved pushed horizontally past another along the San Andreas fault. This fault motion has a compressional component which forces the land east of the fault upward along low angle thrust faults, lifting the earth, causing inland areas to rise, draining the inland sea. The parts of Sebastopol and Petaluma that once were underwater came to have the fich sandy loam soil, which attracted famed horticulturist Luther Burbank a century ago and later nourished the apple industry.

Other remnants of the inland sea are oddly shaped sandstone rocks near the coast, including the "elephant rock" at Dillon Beach, also made from ocean bottom sand. And if you look closely at the rocks along Fallon-Tomales Road, you can see the patterns of ancient sealife in fossil form.

The farther you travel west in Sonoma County, the bigger the chunks of above-ground earthquake evidence. Bodega Head is the prize. This peninsula that forms the northern border of Bodega Bay once was part of the Tehachapi Mountains in Southern California.

"I tell people right off the bat that Sonoma County geology is schizophrenic. There are two distinct geologies, and the San Andreas is the dividing line," said Wright. The San Andreas is the major dividing point between the North American Plate, which extends from the West Coast across the continent to the middle of the Atlantic, and the Pacific Plate, under the Pacific Ocean. Bodega Head's big chunk of 100-million-year old granite is on the Pacific Plate, on the west side of the San Andreas, moving north. It has moved up the 320 miles from the Tehachapis, earthquake by earthquake. The last big move was 15 feet in the 1906 quake, Wright said. The movement of Bodega Head demonstrates the side-by-side movement of the plates. Go north for evidence of the other kind of plate movement with one plate moving under the other, Wright explained.

At Shell Beach, rock collectors find evidence of how the Pacific Plate pushed under the North American Plate between about 120 and 40 million years ago, long before the San Andreas fault started its northward horizontal slide. Metamorphism deep in this subduction zone occurred with great pressure at low heat. The rocks here are different than at other beaches, with a lot of serpentine, blue schist and chert. The earth-movement action of one plate going under the other, called "subduction" ended about 10 million years ago when the plates started moving side-by-side along the San Andreas Fault. "The older rocks east of the San Andreas fault are completely different than the rest," said Wright.

The geologic story moves into the modern day with landslides. The houses teetering on an eroding cliff at Gleason's Beach are on soil has been pulverized by the grinding plates, which made the individual grains so sheer and fine that they absorb water and turn into something known as "melange" which is very weak like clay, Wright explained. The same kind of melange makes up the hills around the Russian River, including in the Rio Nido canyon that where a horizontal block of weathered sandstone moved along a melange zone to cause a fluid debris flow under El Nino's deluges, Wright said. The San Andreas is the big one, but there are hundreds of other faults in this region. "It's like a plate of glass that has one major break but cracks and splinters all around the edge," Wright said.

There's no crystal ball for the geologic future, but the past offers some hints about what might happen in the future. Along the North Coast, Wright says Bodega Head will continue its slow migration toward Oregon and the coastline will continue to be pushed up along the San Andreas fault and probably. "Highway 1 in a million years will be at least 600 feet higher than it is now, I guarantee you," said Wright. "But if in a million years it's not, don't come back and say 'I told you so.' "

"California has the most exciting geology in the universe because of the constant uplift and fault movement," he said. "Our wonderfully diverse landscape is because of the geology." Wright, 55, has been teaching at Sonoma State University and exploring geology in the area for 29 years hiking up active land slides to see how they're moving, finding fossils in the most unlikely places, examining the rocks to learn what happened millions of years ago under the Earth's surface. The professor also leads tours and classes for the general public through Sonoma State University, Santa Rosa Junior College's Community Education Program, and the California Academy of Sciences.