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Glacial Striations on a Basalt Outcrop

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Hidden : 7/16/2012
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Geocache Description:

Short description:  Striations on a basalt outcrop mark the path of thick glaciers that advanced on Isle Royale over 10,000 years ago.  Hard rocks, frozen into the bottom of these moving rivers of ice scratched out their pathways as they slowly glided over the exposed basalt surfaces

Key Earth Science/Geological Vocabulary Words-

Basalt:  a dark-colored fine-grained extrusive igneous rock.

Flow:  a stream of molten or solidified lava.

Outcrop:  an exposed rock formation, on the surface of the Earth.

Glacier:  a large mass of moving ice in a very cold region often called, “rivers of ice”.

Gravity:  a force that pulls objects toward the center of the Earth.

Glacial striations:  line patterns scraped into bedrock by hard rocks frozen into the bottom of moving glaciers.

Advancing glacier:  A glacier sliding and creeping slowly across the land, gouging and carrying material as it moves. 

Receding glacier:  The loss of snow and ice as the glacier melts, leaving behind sand and rocks that were frozen into it.

Content Explanation and Images-

Billions of years ago, a huge crack in the earth flooded the area with basalt lava.  Around 400 different times, hot liquid rock spread out forming layers 10,000 feet thick.    Over time, the enormous weight of the lava caused it to sag in the center, tilting up the edges.   The middle sank down becoming the bottom of Lake Superior.  The edges of some flows are now visible as ridges. Isle Royale and the Keweenaw Peninsula are formed by these ridges.  The ridges and valleys we see on Isle Royale are the tilting lava flow layers.

Flood Basalt Eruption

Figure 1:  Source:  Huber, N.King, The Geologic Story of Isle Royale National Park p.9


Millions of years later, Michigan and the Great Lakes Region were in the middle of an area that was covered by the biggest and widest glaciers in the nation.  (241 Mi Geog and Geol) Advancing glaciers slid over Isle Royale during all four major glaciations of a time called the Pleistocene Epoch.  We only see signs from the latest event called the Wisconsin Glaciation, which receded about 11,000 years ago.  Hard rocks (like granite) got frozen into the bottom of the glaciers and when they passed over the basalt flows they left marks (striations) showing us which way they were going.  These marks show that the direction of ice movement on the east half of the island was toward the southwest, parallel to the ridges and valleys made by the tilting lava flows.  In some places the glacier left behind lots of sand and rocks as it was receding, creating some new landforms on the island.  (Glacial and Postglacial Geologic History of IRNP, Mi.  by N.King Huber. 


Pleistocene Ice Sheets

 Figure 2: Source:  Huber, N.King, The Geologic Story of Isle Royale National Park,  p.42



Basalt outcrop with glacial striations:  Scoville Trail, Isle Royale, Mi.

Picture taken by:  Tim Elyea at Earth cache site.

Logging Questions:

1.     Compare and contrast the direction of the striations on the basalt outcrop to the direction of the main trail.

A.     Perpendicular.

B.      Nearly parallel.

C.     Other…please explain.

2.     Compare and contrast the direction that the glaciers advanced to the pattern of ridges and valleys on Isle Royale. 

A.     Perpendicular.

B.      Nearly parallel.

C.     Other…please explain.

3.     What is the approximate height of the basalt outcrop at this location?

A.     3-10 ft.

B.      11-18 ft.

              C.     19-30 ft.

Reference and Citations-

Blobaum, Cindy.  (1999).  Geology Rocks.  Nashville, Tennessee.  Williamson Books.

Dixon, Dougal., Bernor, Raymond L.  (1992).  The Practical Geologist.  New York, N.Y.  Simon and Schuster Inc.

Huber, N.King, (1973).  Glacial and Postglacial Geologic History of Isle Royale National Park, Michigan.  Washington D.C., U.S. Geological Survey Professional Paper 754-A.

Huber, N.King, (1983).  The Geologic Story of Isle Royale National Park.  Marquette, Michigan,  Geological Survey Bulletin 1309.

Sands, Stella.  (2007, July).  Kids Discover Glaciers.  New York, NY.  Kids Discover Publications.

Schaetzl, R.J. Darden, J.T., & Brandt, D.S.  (2009).  Michigan Geography and Geology.  New York.  Pearson Custom Publishing.

Webster, Christine.  (2006).  Science Matters:  Glaciers.  New York.  Weigl Publishers Inc. 



Connections to Content:

Essential Question: How do natural forces alter the geosphere of the Earth?

Earth Science Literacy Principles:

Big Idea 4.  Earth is continuously changing.

4.1    Earth’s geosphere changes through geological, hydrological, physical, chemical and biological processes that are explained by universal laws.  These changes can be small or large, continuous or sporadic, and gradual or catastrophic. 

4.7    Landscapes result from the dynamic interplay between processes that form and uplift new crust and processes that destroy and depress the crust.

4.8    Weathered and unstable rock materials erode from some parts of Earth’s surface and are deposited in others. 


An advancing glacier could never leave scratches in rock outcrops. 

The Earth hasn’t changed all that much over time.

The glaciers occurred shortly after the lava flows cooled off. 

There has never been any volcanic or glacial activity around Michigan because there isn’t any here now.

Earth Science Standards:

E3.3C- Describe the motion history of geologic features using equations relating rate, time and distance.

E5.3C- Relate major events in the history of the Earth to the geologic time scale, including formation of the Earth, formation of an oxygen atmosphere, rise of life, Cretaceous-Tertiary and Permian extinctions and Pleistocene ice age.









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