Giant's Stairs - Stepping Down a Dike EarthCache

Introduction

Beautiful rocky ledges near the southern tip of Bailey Island in Harpswell expose several types of bedrock that record hundreds of millions of years of Maine’s geologic history. At Giant’s Stairs, views of bedrock and eastern Casco Bay are easily accessed and enjoyed along an easy half mile loop shoreline path. 

Note: I have used the "Not available in winter" attribute because snow or ice may make the terrain unsafe and the observations difficult to impossible. 

Geologic Setting

The majority of the bedrock exposed at Giant's Stairs is assigned to the Cape Elizabeth Formation, part of the Casco Bay Group. The Casco Bay Group is made up of a series of metamorphosed volcanic and sedimentary rocks that were deposited in the Ordovician, between 450 and 470 Million years ago. Rocks of the Casco Bay Group are found in a 100-mile-long northeast-trending belt extending from Portland in the south to just south of Bangor in the north.

Basaltic Dike - Giant's Stairs

The feature that gives Giant's Stairs its name is an eight-foot-wide, east-northeast-trending dark-gray to green, fine-grained igneous rock called a basaltic dike. These large, dark, blocky stones are laid on top of each other like a huge staircase that leads down into the  sea. They are an anomaly among the flaky, streaky, rust-gray metamorphic rock around them.

How Were the Stairs Formed 

Original Sedimentary Rock 

The gigantic forces that shaped this piece of coastline go back 500 million years. Layers of mud and other sediments formed the original sedimentary rock, along with deposits of minerals. Subsequent movements in the Earth’s crust pushed the rock layers upward, causing massive buckling and cracking.

Metamorphism 

Approximately 300 million years ago, the ocean closed, and the continents came together to form Pangaea, the supercontinent. The layers of mud and sediments were buried and, subject to high pressure and heat, metamorphosed into schist. The minerals crystallized into stones such as garnet and quartz. Further, pressure from colliding tectonic plates caused the sedimentary layers to buckle and fold. Over time, weathering and erosion exposed the schist.

Magma - Heating and Cooling 

When Pangaea began to break apart 200 million years ago, separating tectonic plates made cracks and fissures in the hard rock into which molten materials flowed. On what eventually became Bailey’s Island, a vertical crack opened up in the older metamorphic rock, deep within the earth. Molten material was injected into it, hardening into basalt (an igneous rock), as it quickly cooled by coming in contact with the colder rocks surrounding it. 

Jointing

As the basalt solidified and erosion removed the outer layers, regular fractures called joints formed.  The hydraulic action of ocean waves has plucked out blocks along these joints creating the appearance of stairs leading down to the ocean This vertical seam of dark basaltic rock is referred to by geologists as an intrusive volcanic dike, but is known to visitors as the  Giant’s Stairs.

Law of Cross-cutting Relationships Glossary

On a 2012 field trip, Professor of Earth & Oceanographic Science Rachel Beane of Bowdoin College noted that the  Giant’s Stairs, “… doesn’t follow the foliation of these rocks,”  pointing to the majority of rock around her. “It went counter to the layers of metamorphic rock. So to a geologist, this suggests that this rock (the Giant’s Stairs) came after these rocks (metamorphic rocks).”

This is a good example of the Law of Crosscutting Relationships. First described by James Hutton in the 18^th century, the law states that if a fault or other body of rock cuts through another body of rock then it must be younger in age than the rock through which it cuts and displaces.

Logging Requirements

Task 1: PHOTO - include a photo with yourself (face not required), your GPSr, or a personal item, that includes the Giant's Stairs.

Task 2; Observe the rocks of the basaltic dike or the Giant's Stairs.  How do they differ in color and texture from surrounding rock?

Task 3: Igneous, metamorphic, and sedimentary rock all had a role to play in the formation of the Giant's Stairs.  In what sequence did these three play a role.

Task 4: Which is older the Giant's Stairs or the surrounding metamorphic rock?  How do you know?

Glossary References

Aphanitic [Petrolgy] Referring to the texture of an igneous rock in which the crystalline components are not distinguishable by the unaided eye.

Bedrock [Geology] General term applied to the solid rock underlying soil or any otherunconsolidated surficial cover.

Basalt [Petrology] An aphanitic crystalline rock of volcanic origin, composed largely of plagioclase feldspar (labradorite or bytownite) and dark minerals such as pyroxene and olivine; the extrusive equivalent of gabbro. 

Basaltic magma [Geology] Mobile rock material of basaltic composition. 

Dike [Geology] A tabular body of igneous rock that cuts across adjacent rocks or cuts massive rocks.

Feldspar [Mineralogy] A group of silicate minerals that make up about 60% of the outer 9 miles (15 kilometers) of the earth’s crust; they are silicates of aluminum with the metals potassium, sodium, and calcium, and rarely, barium. 

Garnet [Minerology] A generic name for a group of mineral silicates that are isometric in crystallization.  Used as a gemstone and as an abrasive.  

Igneous [Petrol0gy] Pertaining to rocks which have congealed from a molten mass.

Intrusive [Petrology] Pertaining to material forced while still in a fluid state into cracks orbetween layers of rock.

Joint [Geology] A fracture that traverses a rock and does not show any discernible displacement of one side of the fracture relative to the other. 

Magma [Geology] The molten rock material from which igneous rocks are formed

Metamorphic rock [Petrology] A rock formed from preexisting solid rocks by mineralogical,structural, and chemical changes, in response to extreme changes in temperature,pressure, and shearing stress

Ordovician [Geology] The second period of the Paleozoic era, above the Cambrian and below the Silurian, from approximately 500 million to 440 million years ago.

Plagioclase [Minerology] A type of triclinic (a crystal system arrangement having three unequal oblique axes) feldspar; they are common rock-forming minerals.

Quartz [Minerology]  A colorless, transparent rock-forming mineral with vitreous luster, crystallizing in the trigonal trapezohedral class of the rhombohedral subsystem;hardness is 7 on Mohs scale, and specific gravity is 2.65; the most abundant andwidespread of all minerals. 

Schist [Geology] A large group of coarse-grained metamorphic rocks which readily split into thin plates or slabs as a result of the alignment of lamellar or prismatic minerals.

Sedimentary rock [Petrology] A rock formed by consolidated sediment deposited in layers.  Also known as derivative rock, neptunic rock, stratifed rock. 

Tectonic plate [Geology] Any one of the internally rigid crustal blocks of the lithosphere which move horizontally across the earth’s surface relative to one another. Also known as crustal plate.

Volcanic rock lGeology] Fine crystalline or glassy igneous rock resulting from volcanicactivity at or near the surface of the earth. Also known as extrusive rock. 

References

Cook, D. 2012 Bowdoin Expert Explains Giant Stairs to Rock Enthusiasts. https://dailysun.bowdoin.edu/2012/07/bowdoin-experts-explain-giant-stairs-to-rock-enthusiasts/