Mapping commingled magmas, Eastern Head of Isle Au Haut, Maine

Dykstra Eusden
Bates College
Author Profile
Initial Publication Date: December 12, 2011 | Reviewed: January 17, 2015


This is a week-long bedrock mapping project on Isle Au Haut, in Penobscot Bay, Maine, with three to four field days followed by 1-2 lab days. The bedrock is Silurian gabbro and granite that commingled in a magma chamber yielding hybrid magmas and spectacular examples of magmatic pillows and pipes that can be used to evaluate the fluid kinematics of the magma chamber. Students mapped intrusive contacts, collected GPS-located data on contact strike and dips and pipe trend and plunges, and finally made a map in ArcGIS.

Used this activity? Share your experiences and modifications



Part of an undergraduate field methods course in geology that has one required introductory geology class as a pre-req.

Skills and concepts that students must have mastered

Basic understanding of rock types and minerals, some familiarity with a compass and field methods.

How the activity is situated in the course

This is one of three mapping projects in a 5-week field methods course. Students only take one course during this short semester, which is offered in May.


Content/concepts goals for this activity

  • Mapping contacts of igneous rocks.
  • Measuring strike and dip and trend and plunge of planar and linear features.
  • Identifying common igneous minerals and rock types.
  • Geologic map and cross section compilation and construction.

Higher order thinking skills goals for this activity

  • Understanding fluid to fluid contacts between magmatic rocks.
  • Interpreting magmatic kinematics from data collected in the field.
  • Inferring contacts and map patterns through the woods from areas of well exposed bedrock on the coast.
  • Understanding the differences in physical properties of chemically distinct magmas (e.g. viscosity, melting temperature, density).

Other skills goals for this activity

  • Learning to use the Brunton, GPS, and "base map with Mylar" method in the field.
  • Using ArcGIS to make a geologic map.
  • Oral presentation of research articles relevant to the project.

Description of the activity/assignment

The Silurian coastal Maine magmatic province is characterized by commingled gabbroic and granitic magmas that have resulted in magmatic structures such as pillows and pipes that can be used to determine the kinematics of the magma chamber. On the Eastern Head of Isle Au Haut, students map and collect structural data on the contacts between commingled gabbroic and granitic/dioritic sheets and the pipes and pillows that form along the base of these. A bedrock geologic map and cross section are then made in ArcGIS showing the geometry of the magmatic sheets. We kayak out to the island to map it, but there is also a ferry there. Eastern Head is situated within Acadia National Park and, as a result, no hammers can be used.

Determining whether students have met the goals

Students give oral presentations in the field about the island geology as gleaned from research articles. Students submit a digital map, with their field partner. I examine their field notebook to see how their data collection and rock identification in the field are going.

More information about assessment tools and techniques.

Teaching materials and tips

Other Materials

Supporting references/URLs