Lassen Crossroads Igneous Rock Garden - California
Posted by: 
Volcanoguy
N 40° 32.726 W 121° 34.581
10T E 620551 N 4489271
Local rock type display near northwest entrance to Lassen Volcanic National Park.
Waymark Code: WMJF7Z
Location: California, United States
Date Posted: 11/10/2013
Views: 1
The Lassen Crossroad information site is located at the junction of Calif. Hwys. 44 & 89 at the Northwest Entrance to Lassen Volcanic National Park in California. The site is a cooperative development by the Forest Service, National Park Service, and Lassen Park Foundation. The site features rest rooms, four kiosk structures with history, geology, and travel information and a rock garden displaying local rock types.
Text of display sign: Lassen Crossroads - Igneous Rock Garden
Matching Game - Become a Geologist
Intrusive igneous rocks form when magma cools and solidifies beneath the Earth’s surface. Granite is an example of an intrusive igneous rock.
Extrusive igneous rocks or volcanic rocks are formed when magma is erupted onto the Earth’s surface where it cools and solidifies rapidly. Examples of extrusive igneous rock types found in this rock garden are basalt, andesite, dacite, rhyodacite, and pumice.
Become a geologist, look closely and discover the many types of igneous rocks found around the Lassen area. To discover the secrets of these rocks:
Read the rock descriptions.
Walk into the garden to get a closer look.
Match the descriptions to the rocks in the garden.
1915 LASSEN PEAK DACITE - This rock was pushed up and emplaced inside the crater of Lassen Peak between May 14 and May 19, 1915, forming a new lava dome. It is a dark black colored rock and has a slightly shinny surface with pea sized white feldspar mineral crystals. (9)
ANDESITE - This piece of andesite rock is dark gray with many small white feldspar crystals imbedded within the matrix of the rock. Also note the orange and brownish stains on top of the rock. (11)
BANDED PUMICE - This rock was formed in the eruption of Lassen Peak on May 22, 1915. It is made up of banded layers of gray and black lava. Mineral crystals of white feldspar are scattered throughout the rock. There are also small holes (vesicles) that formed when trapped gas bubbles expanded within the rock as it cooled and escaped into the atmosphere leaving behind the small holes or cavities. (6)
BASALT - This lava rock is recognized by its dark grey color and has very small mineral crystals within it. There are also yellowish stains caused by oxidation. The numerous holes called vesicles are cavities left behind when expanding gas bubbles trapped in the solidifying lava bursts. (3)
BRECCIA - Breccia is a mixture composed of cinder, ash, and broken fragments imbedded in andesitic lava. This rock has a reddish color and a chunky rough surface. It was formed during the violent eruptions of Brokeoff Volcano (Mt. Tehama). (1)
DACITE - This rock is light to medium gray in color and has a very smooth texture. Typical of most Dacite lava rocks there are many visible mineral crystals imbedded within the rock. (13)
GRANITE - This is the best known of the intrusive igneous rocks. This rock is light colored with many mineral crystals giving it a salt and pepper like appearance. Granite is a common rock that makes up the Sierra Nevada Mountain Range in Lassen National Forest. (4)
LASSEN PEAK DACITE #1 - This is the original rock that makes up what we know as Lassen Peak. It has a pink like appearance with white mineral crystals of feldspar found throughout the rock. Note the large inclusions of fine-grained rock within the mass of this dacite rock. (7)
LASSEN PEAK DACITE #2 - This pink colored rock is also one of the rocks from the eruptions that formed Lassen Peak. Note the white mineral crystals foud throughout this rock. It is very similar to rhyodacite #1 & #2. (10)
PUMICE - These pieces of pumice are white to light gray in color. Small mineral crystals can be seen throughout the rock. Pumice is lighter in weight than most igneous rocks. Some pieces of pumice may float in water due to their low density. (5)
RHYODACITE #1 - Rhyodacite is recognized by its light pink color and the abundance of many different mineral crystals. Rhyodacite may also be gray or black in color. (8)
RHYODACITE #2 - Note the pink color and large mineral crystals of white feldspar. This rock shows signs of oxidation and staining from the venting gases possibly at the time of eruption. (12)
RHYODACITE OF KINGS CREEK - This dark colored rock has a shinny appearance and darker colored bands on one side of the rock. It has a smooth like texture and is unique in its appearance compared to the other rhyodacite rocks found in this rock garden. (2)
Type of Display: Geological
|
Visit Instructions:
When posting a visit, please provide at least one photo - including yourself in one of of the photos, your GPSr, or a photo different than the default photo.
Also, please provide your impression of the specimen or display.