page_title: Geology of the Lassen volcanic area
section_title: Non-eruption hazards
hierarchical_section_title: Geology of the Lassen volcanic area / Volcanic hazards / Non-eruption hazards
context_page_description: The geology of the Lassen volcanic area presents a record of sedimentation and volcanic activity in the area in and around Lassen Volcanic National Park in Northern California, U.S. The park is located in the southernmost part of the Cascade Mountain Range in the Pacific Northwest region of the United States. Pacific Oceanic tectonic plates have plunged below the North American Plate in this part of North America for hundreds of millions of years. Heat from these subducting plates has fed scores of volcanoes in California, Oregon, Washington and British Columbia over at least the past 30 million years and is also responsible for activities in the Lassen volcanic area.
  Between 3 and 4 million years ago, volcanic-derived mud flows called lahars streamed down several major mountains that included nearby but now extinct Mount Yana and Mount Maidu to become the Tuscan Formation. Basaltic and later andesitic to dacitic flows of lava covered increasingly larger areas of this formation to eventually form the lava plateau that the park sits on.
context_section_description: Additional volcano hazards at Lassen are rockfalls and landslides not directly related to eruptions. Recently erupted volcanic domes are unstable and can collapse, generating small to large rockfalls. Approximately 350 years ago, collapse of one of the Chaos Crags domes generated huge rockfalls, creating an area now called the Chaos Jumbles. The first and largest of these traveled 4 mi (6.4 km) downslope and was able to climb 400 feet (120 m) up the side of Table Mountain. The trigger for the rockfall is unknown, but it was most likely a large earthquake. Normal weathering also weakens fractured volcanic rock and contributes to small rockfalls. In the summer of 1994, a rockfall of 13,000 cu yd (9.9×10⁻⁶ km³) occurred on the northeastern flank of Lassen Peak. During periods of extreme rainfall or snow melt, mudflows are sometimes generated by mobilization of loose volcanic debris and soil on the slopes of volcanoes.
  The only current visible activity in the Lassen volcanic area is from the various geothermal areas in Lassen Volcanic National Park; boiling hot springs, bubbling mud pots and fuming fumaroles. Most of these features lie in or are closely adjacent to Mount Tehama's caldera. In each thermal area, the highest temperature of water generally is close to the boiling temperature at the altitude of the particular spring or fumarole 198 °F (92 °C) at Bumpass Hell and 191 °F (88 °C) on the northwest flanks of Lassen Peak.
  The hottest and most vigorous hydrothermal features in the Lassen volcanic area are at Bumpass Hell, which marks the principal area of upflow and steam discharge from the Lassen hydrothermal system. A prominent steam plume marks the site of Big Boiler, the largest fumarole (steam and volcanic-gas vent) in the park. The temperature of the high-velocity steam jetting from it has been measured as high as 322 °F (161 °C). A thin crust of material often covers these boiling hot features, making them a serious burn hazard to anyone walking off trail. The steam-heated waters of the features are typically acidic and, even if cool enough, are not safe for bathing.
caption: Fear of rockslides from Chaos Crags prompted the closure of the Visitor Center at nearby Manzanita Lake
exif: {"Image ImageWidth": "1966", "Image ImageLength": "814", "Image BitsPerSample": "[8, 8, 8]", "Image PhotometricInterpretation": "2", "Image Make": "Canon", "Image Model": "Canon PowerShot S110", "Image Orientation": "Horizontal (normal)", "Image SamplesPerPixel": "3", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop Elements 11.0 Macintosh", "Image DateTime": "2013:08:10 08:21:13", "Image YCbCrPositioning": "Centered", "Image ExifOffset": "304", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "626", "Thumbnail JPEGInterchangeFormatLength": "4620", "EXIF ExifVersion": "0230", "EXIF DateTimeOriginal": "2005:10:25 16:35:33", "EXIF DateTimeDigitized": "2005:10:25 16:35:33", "EXIF ComponentsConfiguration": "YCbCr", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "1966", "EXIF ExifImageLength": "814", "EXIF CameraOwnerName": ""}