The Cascades and Modoc Plateau Region
The Cascades and Modoc Plateau encompass the northeast corner of California. The region is dominated by volcanic landforms that are more closely related to features in the Pacific Northwest than the rest of California. In fact, both physically and culturally the region has more in common with the states that abut it — Oregon, Idaho, and Nevada — that with other parts of California. It is not on any of the major transportation routes and is sparsely populated, so, except for the major volcanoes that dominate the skyline for many miles along I-5, it is relatively unfamiliar territory to most Californians.
II. Geology and Landforms
A significant shift in the type of interaction between tectonic plates occurs at the Mendocino Triple Junction, which extends offshore from Cape Mendocino. South of this point, the North American and Pacific plates are moving tangentially relative to each other, as reflected in the transverse motion along the San Andreas Fault zone. North of the Triple Junction, the Juan de Fuca plate is moving eastward, and the denser oceanic crust is being subducted beneath the North American plate. This subducted material melts and bubbles to the surface in the chain of large volcanoes that constitute the Cascade Range. The two southernmost mountains in the chain, Mount Shasta and Mount Lassen, are in California, and the chain extends north through Oregon and Washington to include such well known peaks as Mount St. Helens, Mount Hood, Mount Rainier, and Mount Baker.
The Cascades volcanoes have the classic cone shape of stratovolcanoes that have been built up by sequences of lava flows and explosive eruptions. Mount Shasta, the largest of the Cascades by volume and the second highest, was formed over the last 100,000 years, with much of the activity taking place in the last 10,000 years. The most recent eruption was about 200 years ago, possibly in 1786 when the explorer LaPerouse, who was sailing off the northern California coast, reported sighting a large cloud of smoke and ash. Mount Lassen erupted periodically between 1914 and 1921, making it the site of the most recent volcanic eruption in California. There are many features in Lassen Volcanic National Park such as hot springs and fumaroles (holes with boiling steam and hot gases) that indicate continued high levels of volcanic activity in the area.
East of the Cascades is the Modoc Plateau, which is a southern extension of the Columbia River plateaus of eastern Oregon and Washington. Like the Cascades, the area is dominated by volcanic activity, but the eruptions that produced lava flows in the Modoc Plateau were generally less explosive than those in the Cascades. Repeated eruptions from cracks have produced a more subdued terrain of shield volcanoes (similar to the Hawaiian volcanoes) and broad lava plateaus. The most recent eruption in this area occurred around 200-300 years ago. On the eastern edge of the Modoc Plateau, the Warner Range, an up-lifted, tilted fault block range, and the down-faulted Surprise Valley to its east signal a transition to the distinctive fault block topography characteristic of the Basin and Range province.
Precipitation varies widely over the region, starting with high totals on the west slopes of the Cascades and decreasing eastward to near-desert values in the Modoc Plateau, which lies in the rain shadow of the Cascades and northern Sierra Nevada. For example, on the wet side a station on the Pit River averages 197cm (78″) and Mineral, near the west entrance to Lassen Volcanic National Park, averages 140cm (55″). Moving 50km-75km eastward precipitation totals drop sharply to 38cm (15″) at Lava Beds National Monument, 32cm (13″) at Alturas, and 28cm (11″) at Tulelake. The seasonal distribution of precipitation also changes somewhat moving from the west to the east side of the Cascades. On the wetter western side, precipitation is concentrated in the winter season, with more that 80% of the annual average coming during the November-April period. Winter precipitation still constitutes the majority of the annual total in the east, but the November-April percentages drop to 60%-65% as late spring and early summer precipitation become relatively more important.
The temperature regime in the Cascades and Modoc Plateau is typical of interior continental areas, with fairly large daily and seasonal ranges. Temperatures also decrease at higher elevations, but elevation ranges are not large (with the exception of Mount Shasta and Mount Lassen) so most stations show similar averages. Summers are generally warm, with temperatures ranging from daytime highs of around 30oC (85oF) to nighttime lows of around 8oC (47oF). Winters are rather cold, with daytime highs averaging about 5oC (42oF) and nighttime lows averaging about -6oC (20oF).
Vegetation patterns on the western slopes of the Cascades show strong zonation by elevation. Oak woodlands dominate in the foothills at elevations below about 1,000m (3,300′). Valley oaks and blue oaks are widespread, as are gray or foothill pines and California buckeyes. Understory plant communities include various chaparral shrub species and grasslands. Yellow pine forest becomes dominant in the 1,000m-2,000m (3,300′-6,500′) elevation range, where annual precipitation rises above about 75cm (30″) and comes as a mix of rain and snow. The primary tree species are ponderosa pine, sugar pine, white fir, and incense cedar. Black oaks add to the mix, taking over from blue oaks above the foothill zone, and chaparral shrubs add to the understory. Above about 2,000m (6,500′), in an elevation range where snowfall is usually heaviest, the composition of the montane coniferous forest changes as the lodgepole-red fir forest replaces the yellow pine forest. Lodgepole pine, red fir, hemlock, and aspen are the dominant trees. Vegetation patterns in the dry Modoc Plateau area are quite similar to those in the Basin and Range region, with Great Basin sage scrub at lower elevations and pinyon-juniper woodland at higher elevations. The higher elevations of the Warner Range support a version of yellow pine forest with Washoe pine, a unique species found only in a few of the higher elevation Great Basin ranges, replacing ponderosa pine.