The Lost (Micro)Continent of Avalonia and the Geological Origin of Brookline
The Rock Beneath Our Feet
Earth wears a garment of soil, plants, ice and water. Underneath that outer layer is rock—everywhere—even at the bottom of the ocean. In most cases, we do not get the chance to see the rock that lies beneath our feet. But it is there. And the stories these rocks tell are often very different than the story told by the surface. Surface coverings of dirt, sand, clay or water are, from a geological perspective, ephemeral and short-lived. The earthen covering of subsurface rock is generally recent—in Brookline it dates to the end of the last glaciation about 12,000 years ago (see the “ice” section of our site). But the rock beneath—which geologists refer to as basement rock for obvious reasons—is much older, recording a geological history that stretches back into the shadowy depths of time. In the case of Brookline, this rock originated about 600 million years and thousands of miles away.
This site seeks to tells the story of Brookline's geological origins. If you are new to or unfamiliar with some of the key concepts in geology, feel free to briefly review the links below. When you are ready, click on the other links explore key aspects of Brookline's geological origins.
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The Basics (for people new to geology)
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If you don't have much background in geology or would like to know more about any of the concepts mentioned in this story, the links below will be helpful. They address concepts that are important to this story but we will not cover in depth in the narrative below.
Plate Tectonics
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Learn about the theory that has revolutionized our understanding of the earth
Different Types of Rocks
Learn the difference between igneous, methamorphic and sedimentary rocks.
Radiometric Dating
Understand the principles of using radiometric methods for establishing the dates of rocks
Deep Time and Geochronology
Understand the meaning of "deep time" and the chronology of earth's geological history.
The Geological Puzzle of Massachusetts
If you were to strip off the top layer of soil, plants and water from the state of Massachusetts, its rocky “bones” would form a jigsaw puzzle of different types of rocks of different ages. In the geological map compiled by the U.S. Geological Survey, these different types of rocks are shown in different colors and patterns. This map hints at the story about how Massachusetts—and Brookline of course—came to be. Geologists have been working on solving this puzzle for almost 200 years. A consensus view has emerged in the last 50 years, driven by the insights of plate tectonics. The strip-like zones of the map reflect pieces of crust that have been added to the eastern edge of North America through a series of collisions as well as the resulting changes these collisions made on the rocks. These fragments of crust were not part of early North America but originated in other places on the earth and were carried toward the ancient coast as a result of "continental drift". Geologists call these crustal fragments that originated somewhere else "terranes." We will use this term in the story below.
Through this series of collisions, new rock was added to the continental edge and all the rocks involved in the collision were modified. The figure at right below captures the main events in the building of Massachusetts. The Taconic-Berkshire zone was the margin of the ancient North American continent called Laurentia. The other zones—the Rowe-Hawley Zone, the Bronson Hill Zone, the Nashoba Zone, and the Nashoba zone, represent additional crustal material added onto this ancient continental margin at different times. Our story mainly concerns the larger area called the Milford-Dedham Zone, which contains rock that was “born” about 600-630 million years ago, but not added to the North American continent until about 420 million years ago.
Image from Hatch, 1991.
The Boston Basin
Zooming in on the Boston area of the Massachusetts geological map, we see that there is another story captured in the basement rocks. The subsurface geology of the Boston area shows a sedimentary basin (tan colored on the map) surrounded by volcanic rocks (Figure below). This basin area contains two main types of rocks: mixtures of mudstone, sandstone and conglomerate, mainly found in the southern side of the basin, and a much finer mudstone called argillite, mainly found in the northern half. These rocks have all been somewhat modified by pressure and temperature that occurred upon their deep burial and through subsequent intercontinental collisions (they can properly be considered metasedimentary rocks, although the degree of modification is relatively mild compared to other kinds of metamorphic rocks). The mixed rocks in the southern portion of the basin suggest deposition in either or both an on-shore and near-shore environment, while the mudstones (called argillite) in the northern portion suggest a deep water depositional environment farther from shore. In addition, igneous rocks can be found throughout the southern portion of the basin. In general, the conglomerates and associated metasedimentary rocks are older than both the argillite and the igneous rocks of the basin. In the Brookline area we find the sedimentary mix with scattered deposits of igneous rock. We will probe the origin of the Boston Basin later.
Figure: Bedrock of the Boston area. Rocks of the Boston basin are shown in tan. The mixed conglomerate/sandstone/mudstone rocks are shown by the coarse dotted areas. Igneous rocks (and metamorphic rocks of igneous origin) surrounding the basin are shown in reds, purples, pinks and yellow. adapted from Zen et al. 1983.
The Lost (Micro)continent: The Geological Origin of Brookline
The geological history of Brookline begins with the story of a “lost continent” called Avalonia. Most of us have heard of the lost continent of Atlantis, and there is an active debate about whether it ever existed. But the story of the lost continent of Avalonia is real, and it is a far stranger tale than the myth of Atlantis. Avalonia was “born of fire” about a billion years ago off the coast of ancient continental mass called West Gondwana (composed of pieces of South America and West Africa), migrated thousands of miles from its original location, smashed into two continents, was pulled apart with the birth of the modern Atlantic Ocean, then buried under the sediment shed from high mountains and finally uncovered by the scouring work of mile-deep glaciers. Remnants of lost Avalonia are found scattered across several countries in North America and Europe. The figure below shows the present geographic areas where Avalonian rocks are located. (The figure captures the time when Avalonia collided with the ancient North American and Central European continents, which at that time were joined together.) Avalonia makes up the bedrock of Eastern Massachusetts, and some of the remnants of Avalonia arise from the ground right in Brookline—if we know where to look. To understand the ancient history of Brookline we must learn the story of Avalonia.
Figure adapted from: https://en.wikipedia.org/wiki/Avalonia#/media/File:AVALONIA.svg
The story of Avalonia is recounted here as a drama in seven acts that unfolds over a billion years (if you don't have time, the short summary is here). This "drama" is my own synthesis of the work of many of scientists over the years but is particularly indebted to the research of a smaller group of scientists who have focused on the geology of Avalonia and the geology of the greater Boston area. This story is still evolving as research and analysis continues. Our understanding of the evolution of Avalonia has been greatly modified in the last 30 years through the insights of plate tectonic theory. As a result, the story you read here may differ from other narratives online that rely on older frameworks of understanding (see example here). Although there is now broad consensus on how Avalonia was born and how it came to be part of North America--and thus the geological origins of Brookline--questions still remain, some of which are hotly contested. Because the nature of scientific knowledge is partial and progressive and driven by new research, it is possible that elements of the story presented here may change over time as new data is gathered and analyzed and results in support for one hypothesis over another.