Volcanic Wonderland - Oregon Historical Markers on Waymarking.com

Marker Name: Volcanic Wonderland
Marker Text: Geologic Gem
Diamond Craters is young in geologic terms with the last volcanic activity taking place about 17,000 years ago. The area displays a variety of volcanic features to study and enjoy. They appear fresh and uneroded because of the arid climate and lack of vegetation.
Quiet flows of very fluid basalt lava from northwest-trending fissures were the first events in the formation of Diamond Craters. These pahoehoe (pa-hoy-hoy) lava flows cover a circular area roughly 6 miles in diameter and are exposed along the northern and southern edges of Diamond Craters.
A characteristic feature of lava flows in tumuli (from the Laten word for burial mound) that look like giant loaves of bread with a crack down the center. They form when the top and sides of a lava flow cool but molten lava continues to flow to the site, lifting the crusted lava into a mound.

A half dozen Domes
Before the initial basalt lava flow completely cooled, additional magma rose up underneath that lava flow in six places forming six domes. Each dome displays distinctive features.
South Dome: When magma rose to form this dome, some of the magma was sufficiently high in gas to erupt out to the surface as cinder cones. Some of the magma had less gas but was still able to reach the surface and boil from Lava Pit Crater.
North Dome: The features of this dome are mostly covered by volcanic ash from Central Dome vents in the Central Crater Complex.
Graben Dome: As updoming occurred, some of the lava flowed out the flank. As a result the top of the dome lost its support and dropped down to form a graben ( a long channel like depression). Northeast Dome: Volcanic bombs litter the slopes of this well-developedcone on the north side of Northeast Dome. Northeast Dome is an example of a tumulus (looks like a giant loaf of bread with a crack down the center). This occurred when the initial basalt lava flow cooled enough to crack when updoming occurred.
Central Dome: When this dome formed, volcanic ash erupted from many vents at the summit of the dome. As a result the top of the dome lost is support and dropped down to form a caldera (a circular depression) that is called the Central Crater Complex.
West Dome: When the magma rose to form a dome, some of it encountered groundwater, flashed to steam, and formed explosion craters.

A closer look at Lava
Driplet Spires: Blobs of lava were blown out of a small vent and fussed together, forming a tall accumalation of lava around the vent.
Volcanic Bombs: Blobs of lava cooled in flight as they were blown out of a vent. A cored bomb is one that fell back into the vent, was coated with lava, and blown out again.
Spatter Cones: Small pieces of molten lava were blown a few feet into the air out of a circular vent like a “fire fountain”. As the spatter fell around the vent, spatter rings and cones were formed.
Lava Tube: As lava flowed, a crust formed over the top and along the sides of the flow but inside the crust the lava continued to flow leaving a tube-like cave.
Pahoehoe Lava Flow: This lava is smooth and ropy in texture.
Aa Lava Flow: Note the rough broken and jagged texture. Aa and pahoehoe are Hawaiian words describing a flow surface.

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