Basaltic dikes, tuff-breccias,

by a. noelle

Lava flows, red-baked cinder piles, and air-fall tuffs of an extinct volcano.

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3. The red streaks and layers in these river beds were caused by oxidation of iron in the sediments. Varicolored “redbeds” sometimes contain fossils of plants and animals. Further along the hike, bands of more intense red are found at the tops and bottoms of lava flows, where iron was oxidized and reddened by baking and steam action; these bands are called “bake zones.”

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4. The bedding in the mudstones gives the appearance of drag-folding resulting from relative uplift of the lava occurring during the past 10 million years. Alternatively, the disruption of the mudstones may have occurred earlier, at the time of volcanic activity. This site was close to, or was in, the wall of the volcano, and would have been subject to slumping, sliding, and plowing.

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5. Massive basalt was removed from this major quarry pit.

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6. A sequence of basaltic tuffs lying on top of a dark lava, all tilted steeply to the east. The base of the tuffs was baked red, probably because the lava below was still hot and steaming when the ash landed. At the top of the sequence there is another lava flow that baked the underlying tuffs red.

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7. A basalt flow, massive at left, rubbley to the right. The lower parts, coming into contact with cold land surfaces, harden early and are often jumbled by turbulence and drag. The angle of the cut makes the flow structure difficult to visualize, and the picture is further complicated by a shear zone cutting from lower left to upper right in the massive basalt. The rubbley part of the flow is filled with vesicles caused by gas pockets, which were later filled with chalcedony, opal, calcite, zeolites, and sometimes green celadonite. Thin parallel bands in the soft tuff suggest the ash fell into water because bedding in ash that falls on dry land is usually disrupted by tree roots.

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Sibley Volcanic Regional Preserve

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