Urban Basalt
You are currently standing in Freeway Park. Dedicated on the Bicentennial anniversary of the nation's founding on 4 July 1976, this park is relatively unique in that it recovers pedestrian space previously lost when Interstate 5 was installed. In fact, as you stand at GZ, cars are zipping around (or parked in gridlock depending on the time of day) directly beneath your feet.
As you enjoy this carefully crafted park, take some time to enjoy the Urban Basalt placed here in tribute to James R. and Mary Lou Ellis who were instrumental in the founding of the Freeway Park. As this is an Earthcache there is no container to find here, rather, after making your observations at Ground Zero, please send your answers to the associated questions to either my email or messenger and then log your find!
EARTHCACHE LESSON
Basalt is a type of igneous rock formed as molten magma cools and solidifies in the upper levels of the mantle. This positioning is important as it means that the magma cools incredibly fast which results in small nearly invisible crystalline structure. Another fascinating component of basalt is its mineral structure – compared with other rocks basalt consists of a significant amount of iron as a primary component. This creates an opportunity for a unique type of chemical weathering to occur that we don’t typically see in rocks: rusting.
Rusting is an oxidation process where iron or iron components of a structure lose an atom loses one or more electrons. As rust occurs, iron ions change from one form to another, and they lose an electron along the way. As basalt is exposed to oxygen and water, this form of oxidation occurs which will result in an orange-brown coloration. As we look at the chemical reaction, we’ll note that the chemical reaction can be simplified into the below formula:
4Fe + 3O2 + 6H2O → 4Fe(OH)3
The rusting process requires both the elements of oxygen and water.
Basalt also has a relatively unique method of physical weathering. As magma cools, if the composition is at least 90% basalt, the fine crystalline structure causes the rock to fracture in geometric patterns which are referred to as columnar jointing. One of the most famous cases of columnar jointing occurs in the Giant’s Causeway. So how does this columnar joiniting occur?
Imagine a huge flow of hot, liquid magma that is settling. The outer layer is starting to cool and darken in color from orange to black. As it cools, it needs to shrink a bit, as hot materials usually takes up more space than cooler ones. Because of this shrinking, the surface of the lava starts to crack. But this cracking isn’t always random.
In this case, the lava starts to crack into regular shapes. Those shapes are forming because of how the lava cools. It starts at different spots called “centers.” If those centers are evenly spaced, the forces that pull inward toward the centers end up creating different chunks of cooling lava that are hexagonal (6-sided), or close to it. The more uniform the material of the lava is (or basically how smooth and well mixed it is), the more evenly those centers pull. That means it is more likely it will cool into hexagonal chunks. Scientists also think that faster cooling, like when lava is exposed to water, may also help with the formation of these columns.
These chunks begin to form at the top layer of the lava, which is cooling the fastest. As lower levels of the lava start to cool, they are also pulled into the shape under each center. In this way, you get lava cooling and contracting down into these cylinders. Even when the material is a bit irregular, these cylinders can still form. They just might not be hexagons… instead of 6, they might have 5 or 7 sides, or a more irregular shape.
Columns formed in this way can be straight or bent into interesting shapes. When they form long, straight columns, these are called colonnade. When they have more irregular shapes that look like they’ve been pushed or morphed, it’s called entablature.
QUESTIONS:
Please try your best to answer the questions completely and accurately based on your observations at GZ and your reading of the Earthcache Lesson. Effort is more important than accuracy – if your answers are wrong, then we’ll discuss how you came to your conclusions. Please do not log this cache if you do not intend to send in answers. Logs without answers will be subject to deletion after conferring with the cacher.
- Are there signs of chemical weathering at ground zero. Does this support or contradict the assumption that the rocks here are basalt?
- Examine the rocks physical shape, provided they are naturally forming, characterize the columns as either colonnades or entablature. Does this mean that the rock before you is closer to pure basalt or is it mixed with other rocks?
- Take a photo in the courtyard but do NOT include the basalt rocks in your image. This image should include either yourself or a personal item.
Earthcaches are meant to be a fun exploration of the world around us! Take time to note your answers and I look forward to reading your logs!
REFERENCES