Siletz River Volcanics facts for kids
Quick facts for kids Siletz River VolcanicsStratigraphic range: Selandian-Ypresian (Tiffanian-Bridgerian) 59.6–47.6Ma |
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Exposed pillow lava in the Northern range
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| Type | Formation |
| Underlies | Yamhill Formation |
| Lithology | |
| Primary | Sills of tholeitic to alkalic basalts |
| Other | Tuff-breccia, siltstone, sandstone |
| Location | |
| Coordinates | 44°54′N 123°24′W / 44.9°N 123.4°W |
| Approximate paleocoordinates | 47°00′N 107°30′W / 47.0°N 107.5°W |
| Region | Benton, Coos, Douglas, Lane, Lincoln, Polk, Tillamook, Washington & Yamhill Counties, Oregon |
| Country |  United States |
| Type section | |
| Named for | Siletz River |
The Siletz River Volcanics are a group of ancient volcanic rocks found in the Oregon Coast Range in the United States. These rocks are mostly made of basaltic pillow lava. Imagine lava erupting underwater and cooling quickly, forming rounded, pillow-like shapes! These pillow lavas originally came from submarine volcanoes, which are volcanoes under the ocean. They formed a long time ago, during a period called the Eocene Epoch. These rocks are also part of a larger piece of Earth's crust called Siletzia.
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What are the Siletz River Volcanics?
The Siletz River Volcanics formed between about 58 and 50 million years ago. This time period is known as the Paleocene and Eocene epochs. These rocks include flows of lava and sills. Sills are like flat sheets of hardened lava that squeezed into cracks between older rock layers.
The lava that formed these rocks was a type of basalt. Basalt is a common volcanic rock. Sometimes, the lava also mixed with other materials like tuff-breccia, which is made of volcanic ash and broken rock pieces. You can also find siltstone and sandstone mixed in. Siltstone is a rock made from fine mud, and sandstone is made from sand.
Many of these lava flows have tiny holes, like sponges. These holes are called vesicles. Over time, these holes filled up with minerals like zeolite. When the holes are filled, they are called amygdules.
How Did These Rocks Form?
The Siletz River Volcanics started out as oceanic crust and seamounts. Oceanic crust is the part of Earth's crust that makes up the ocean floor. Seamounts are underwater mountains, often formed by volcanoes.
Scientists can figure out how old these rocks are using a method called potassium-argon dating. This method measures tiny amounts of certain elements in the rocks to tell how long ago they formed. Using this method, scientists found that the Siletz River Volcanics are between 58.1 and 50.7 million years old. This time frame covers parts of the Selandian and Ypresian ages.
The Siletz River Volcanics are divided into two main parts. The lower part is made of pillow-shaped basalt. The upper part is also basalt, but it has a different texture. It's called "porphyritic," which means it has larger mineral crystals mixed in with smaller ones.
You can find these volcanic rocks in many counties in western Oregon. These include Benton, Coos, Douglas, Lane, Lincoln, Polk, Tillamook, Washington, and Yamhill counties.
Fossils Found in the Siletz River Volcanics
Scientists have found interesting fossils in the sedimentary layers within the Siletz River Volcanics. These layers are like ancient mud and sand that got trapped between the lava flows. At places like the Ellendale Basalt and Portland Cement Company Quarries, fossils of ancient sea snails have been discovered. These snails are a type of archaeogastropod. Some of the specific types found include Pleurotomaria (Entemnotrochus) baldwini, P. (E.) schencki, and P. (E.) siletzensis. Finding these fossils helps scientists understand what life was like in the oceans millions of years ago when these volcanoes were active.
Images for kids
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Exposed pillow lava in the Northern range
