This is an Earthcache – as such, there is no physical cache. Instead after examining the basalt columns at the posted coordinates, you will answer 6 questions and send me the answers. This sculpture is in Magill's Hill Park with permission. The park grounds are open daily from dawn to dusk.

"TONKA" is a creation of local Lambertville sculptor Harry H. Gordon that was constructed from basalt columns purchased from a quarry near Moses Lake, Washington. Basalt ("ba-SALT") is an Extrusive Igneous rock that makes up 90% of the Earth’s Volcanic Rock. Most of it erupts quietly deep below the ocean along tectonic plate ridges. Lesser amounts of it erupt on volcanic ocean islands, or in occasional large outbursts elsewhere on the planet. It is formed from the rapid cooling of LAVA that is exposed at or very near the surface of the planet.

It was between 17 to 16 million years ago when the lava started flowing that created these pillared, tall rock outcroppings that rise up at a 90-degree angles from the ravines of the Columbia Plateau. This is a wide flood basalt plateau between the Cascade Range and the Rocky Mountains, cut through by the Columbia River. The lava that formed this basalt did not come cascading down from a traditional volcano, but rather from many fissures in the earth's crust. In geology, a fissure is a fracture or crack in rock along which there is a distinct separation; fissures are often filled with mineral-bearing materials. The fissures in the Columbia Plataea were elongated fractures and cracks at the Earth’s surface from where the lava erupted.

It is estimated there was about 300 individual lava flow outbreaks during that period that flooded the landscape. The lava generated from these fissures was massive and covered an area of 63,000 square miles throughout parts of Oregon and Washington, Oregon, Idaho, and Nevada.

When the lava flooded the plateau it rapidly cooled and crystalized into basalt rock. Then, as the rock slowly cooled and contracted, it cracked into long geometric columns. This process is called columnar jointing. Typically, these columns cooled in polygonal joint patterns.

Basalt is darker, denser, and finer grained when compared to the intrusive granites of the world. Just compare the Basalt Sculpture here to the light Vermont Granite rectangular base that it is displayed on. Granite is an Igneous Intrusive rock, which cooled slowly under the earth’s surface. Take a moment to compare the size of the crystals that make up both rock types.

Compared to other rocks found on Earth's surface, basalts weather relatively fast. It is a porous rock making it more exposed top the elements. Typically iron-rich minerals in the basalt oxidize rapidly in water and air. This stains the rock a brown to red color due to the rusting iron oxide. Basalt is largely composed of minerals with little resistance to weathering. Hence, basalt also tends to crack and disintegrate faster than granite and other felsic rock types.

LOGGING REQUIREMENTS:

To log this Earthcache: Read the geology lesson above. Answer all three questions posted below. Answers for 1-6 can be sent via e-mail or messenger contacts on my Geocaching profile.

QUESTION 1. TRUE or FALSE: The Basalt that formed these Columns cascaded from a volcano.

QUESTION 2. Are the crystals that make up the Intrusive Granite base larger or smaller than that of the the extrusive Basalt columns?

QUESTION 3. These columns where formed into polygonal joint patterns. The column on top has 7 sides and is a Regular Heptagon. How many sides and what polygonal shape is the vertical standing column?

QUESTION 4. Look at the polished and unpolished sections of this sculpture. What color is the polished areas of basalt columns?

QUESTION 5. Observe the stained color of the unpolished basalt. Does this indicate that iron rich minerals are present in the stone?

QUESTION 6. What other indications of weathering did you observe during your visit?

OPTIONAL PHOTO: Posting a photo that readily indicates that you (and anyone else logging the find) are at the location.

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