Geologists have studied the confused mass of rocks that make up the Coast Ranges, the Great Valley, and the Sierra Nevada for a long time. However, it was not until the late 1960s that they were able to explain how the rocks of these three provinces could be so different and yet be of the same age.
Basic to an understanding of the geology of Coe Park is an appreciation of geologic events that began to occur thousands of miles out in the Pacific Ocean about 200 million years ago. Most geologists now accept the theory of plate tectonics, which is based on studies of processes taking place in the modern oceans. This theory proposes that the earth's crust is a jigsaw puzzle of plates that are in constant motion with each other.
In the depths of the earth's oceans, there are mid-ocean ridges-called rifts-along which molten lava is extruded onto the sea floor. Since the sea floor is always youngest at the mid-ocean ridge and is progressively older toward the margins of the ocean, geologists have concluded that new sea floor is forming at the ridges and slowly being carried away as more material wells up. This process is known as sea-floor spreading.
There are great currents in the earth's mantle, driven by heat released from the decay of radioactive elements. Heat slowly accumulates in the material deep within the earth, expanding and partially melting it so that it becomes less dense than the material above. Hot magma rises slowly through the earth's interior and is able to break the surface at the mid-oceanic ridges. There, the magma cools and solidifies and begins to spread slowly in both directions from the ridge, pushed along by more hot magma rising from below.
The ocean floor continues to cool and subside as it is carried away from the mid-ocean ridge. If it meets the lighter material of a continental mass riding atop another plate, the ocean floor sinks (subducts) into the earth's interior, creating a trench along the continental margin. When the ocean floor is subducted to depths of about eighty miles, heat and pressure melt it, forming more molten material that rises to form volcanoes and underlying batholiths. The giant conveyor belt created by this process moves about two inches a year.
It was just such a conveyor belt that carried what is now the continent of North America away from Pangea, the supercontinent where reptiles began their long reign over the earth. The North American plate began moving west about 200 million years ago, driven by the sea-floor spreading that continues to expand the Atlantic Ocean. As it was pushed west, it met other oceanic plates that have long ago disappeared beneath the North American plate except for scraps preserved in the Sierra foothills.
These vanished plates have added material to the continental land mass of North America. Some of the material was contributed by smaller continental masses that rode on subducting plates and became welded to our continent, but much of it came from old sea floor. The leading edge of the North American continent has scraped off thick, muddy sediments that formed on the surface of subducting ocean crust and has sometimes scraped off basaltic material from the sea floor crust itself. The sediments, which were crushed together and jammed onto and beneath the old continental edge, created the California Coast Ranges.
Over time, the growing masses of new coastal mountains built steadily westward as the seaward edge of the continent sliced more and more muddy sediments off the moving sea floor. Since each new slice of sediment came from farther out at sea, the rocks in western California tend to be younger than those in the east.
The relatively unimpeded east-to-west movement of the North American plate ceased about 30 million years ago, and the sediments that had been deposited in the trench, called the Franciscan Complex, began to rise and add more material to the coastal mountains. The trench material pierced the overlying sediments in the Diablo Range along an egg-shaped fault. Since geologists did not know at first that the eastern and western limbs of this fault were joined, the fault has two names. On the west, the fault passes just below Coe Park headquarters and is called the Madrone Springs fault. The eastern limb of the fault is two to three miles east of the eastern park boundary and is called the Tesla-Ortigalita fault. Thus, nearly all of the park is composed of Franciscan Complex rocks.