Welcome to Eagle Beach State Recreation Area, this earthcache is located just feet from a parking area, but can serve as a starting point for a walk along a well maintained trail system. Please enjoy the views, take some notes and learn a little bit about the geological forces acting beneath your feet! Note that as this is an earthcache there is no container to be found at the posted coordinates. In order to log this cache as "found" you must send the cache owner answers to the logging requirements at the end of the description, this is most easily completed with the message center feature. If you have any questions or concerns, please reach out to me!
Background
The earth's crust can raise in several different ways. Most people are generally familiar with the formation of the crust's most famous upheavals: mountains. While the jutting peaks of the nearby Chilkat Range may be majestic, in this Earth Science lesson we will be focusing on a less noticeable form of geologic uplifting: Isostatic Rebound.
Isostasy
Isostasy, from the Latin word "Iso" meaning 'equal' and the Greek word "stasis" meaning 'stationary,' is a geological phenomena defined by the interaction of two physical forces: buoyancy and gravity. The theory of plate tectonics explains that the earth's crust is floating on a thick layer of mostly solid rock known as the mantle. On the geologic scale of time however, the mantle behaves as a viscous fluid. For the purposes of this lesson it may be helpful to imagine the mantle as a liquid. For more information on how an apparent solid can act as a liquid, I encourage you to read up on the Pitch Drop Experiment.
Gravity pulls downward on the crust and is counteracted by the mantle exerting a buoyant force on it. When the force of buoyancy and the force of gravity are equal the crust reaches an equilibrium. At this point there will be no more vertical movement. This is isostasy or the earth in balance. As you factor in more and more geological features and their forces, such as the weight of mountains, ice-fields and glaciers, the equation changes. At its core however, isostasy is simple: the forces of buoyancy and the forces of gravity want to reach equilibrium.
Removing a Great Weight
20,000 years ago the earth was gripped in an Ice Age. Where you're currently standing was once under thousands of feet of ice. As recently as 400 years ago, in the midst of what is colloquially referred to as "The Little Ice Age" by scientists, much of the area was again under ice. This ice weighed hundreds of thousands of pounds and as such it affected the isostatic equilibrium of the region. As more and more weight was added to the crust, it overpowered the force of buoyancy and caused the land to sink below the normal point of equilibrium.
Around 250 years ago glaciers began retreating from the area at an astounding rate of more than a quarter mile a year. As the weight of the ice was removed the crust once again sought to return to equilibrium. Over several decades scientists observed the land slowly rising upward. In other parts of Juneau this uprising formed the Mendenhall Wetlands. This return of the crust to its natural equilibrium is referred to as isostatic rebounding.
Isostatic Rebounding
Isostatic Rebounding (sometimes referred to as post-glacial rebounding) is the process by which the earth's
crust, having sunk beneath the weight of glaciers from a preceding ice age, bounces up as the ice sheets melt and the water runs back into the sea. Picture a trampoline with a bowling ball in the center. The trampoline is indented. As you remove the ball the surface snaps back and regains its original shape. Much like the trampoline (though on a far slower geologic scale of time) the earth's crust is in the process of snapping back into shape.
Juneau is one of the premier places on earth to observe this phenomena. According to Roman Motyka (Ph. D.), a galciologist from the University of Alaska, the isostatic rebounding rates in Southeast Alaska are "the fastest rates in the world."
Rising Seas and Receding Tide-lines
As glaciers shrink and melt sea levels world wide continue to rise. According to the most recent NOAA data, the world's oceans are rising at a rate of 1/8th of an inch per year. While this may seem an insignificant amount even a rise of a few inches can be devastating to low lying countries. Ironically however, in many areas where continental glaciers are melting and exposing land, the local sea levels are dropping. This apparent paradox can occur because the land may actually rebound from the weight of melting glaciers faster than the sea level water is rising.
Logging Requirements
Please answer the following questions, to your best ability, using the information above and your observations at the posted coordinates. Any "Found It" logs that fail to submit the answers to the CO in a timely manner are subject to deletion. I will attempt to reach you to resolve any discrepancies prior to taking this course of action.
- Locate the sign at the coordinates. Determine the current observed rate of rebounding. (Note that the rate in Juneau isn't the same as the rate at Eagle Beach. Estimation may be required)
- Take an elevation reading off your GPSr and report it here. (I will allow for some variation based on GPSr used)
- Based off the current rate of rebounding, what would you expect the elevation to be at GZ in 100 years? 1000 years? (Assume the rate of uplift will not change).
- Based off your observations here, do you expect the local sea level to rise or drop. Take into account the average increase in sea level worldwide. Explain why.
Sources
- “ISOSTATIC REBOUND: Glacial Depression.” Earth How, 26 Feb. 2019, earthhow.com/isostatic-rebound-glacial-depression/.
- Jenkins, Elizabeth, and Alaska's Energy Desk. “As Lands Uplift, a Wetland Refuge in Juneau Is Losing Ground.” KTOO, 26 July 2018, www.ktoo.org/2018/07/25/as-lands-uplift-a-wetland-refuge-in-juneau-is-losing-ground/.
- Joseph, Leah H. “Isostatic Rebound.” Chicana Feminism - Credits, www.umich.edu/~gs265/isost.html.
- McKinnon, Mika. “Why Are Sea Levels Dropping In Places Closest To The Melting Glaciers?” io9, io9.Gizmodo.com, 16 Dec. 2015, io9.gizmodo.com/why-are-sea-levels-dropping-in-places-closest-to-the-me-1684599241.
- US Department of Commerce, and National Oceanic and Atmospheric Administration. “Is Sea Level Rising?” NOAA's National Ocean Service, 27 Oct. 2008, oceanservice.noaa.gov/facts/sealevel.html.
This cache has been approved for placement and is authorized under permit #15-KA-1994.