The Geology of Salt Island
Lindley Hanson, Professor Emerita, Salem State University
Salt Island is the product of tectonic and geologic processes spanning over 600 million years. The Island is a rocky ledge, connected to the mainland at Good Harbor Beach by a sandy tombolo. Although the tombolo may be less than a few thousand years old, the island itself is composed of rock dating a little over 420 million years. Its bedrock is composed of a medium to coarse-grained alkali granite typical of the Cape Ann Granite, a complex of granitic plutons intruded during the late-Silurian. The Cape Ann Granite and the more mafic Salem Gabbro-Diorite, exposed farther south, together form the mid-Paleozoic plutonic belt that bisects the ~600-million-year Avalon Terrane that collided with North America during the Devonian Acadian orogeny, approximately 400 million years ago.
The Avalon terrane is an exotic terrane originating from the low-latitude Proterozoic supercontinent we call Gondwana. Between 600 and 480 MA ago this continental fragment rifted from Gondwana and started its long journey northward towards North America. During this time the margin of northeastern North America lay along western New York and faced a southerly direction along the equator. Collision of Avalon with North America was the final tectonic event responsible for the formation of the Northern Appalachians and creation of the New England states. However, prior to its collision with North America Avalon began to internally split apart for reasons that are yet unknown. This rifting event formed a series of igneous intrusions that comprise the Mid-Paleozoic pluton belt that extends from Gloucester southward through eastern Massachusetts. The geochemistry and mineralogy of the Cape Ann granite and related Salem Gabbro-Diorite are typical of igneous plutons formed in a such a rift setting. Both have been radiometrically dated, so we know the age of this event. Salt Island is part of this complex. So, although Salt Island lies within a terrane that is over 600 million years old, its bedrock is only 420 million years of age.
Although the granite was intruded 3 to 5 km beneath the surface, uplift and erosion over the last 420 million years have uncovered these rocks and shaped the landscape we see today.
The Cape Ann Granite exposed on Salt Island is composed of quartz, alkali feldspar, and hornblende. The granite is also traversed by factures formed by subsequent deformation events, notwithstanding Mesozoic rifting accompanying the formation of the Atlantic Ocean. The shape of Salt Island and surrounding ledges are no doubt the result of differential erosion, and scouring of exposed rocks during more recent glaciations.
When exposed to weathering, granite undergoes a style of breakdown know as granular disintegration whereby individual mineral grains are released from the rock and broken down. For this reason, granites are notable producers of sand. Once released from the granite, grains are rework by glaciers, rivers, and waves and redeposited. Weathering and surficial processes working in concert over the millennia are responsible for the extensive sandy beaches that attract so many to this area. By comparing the sand grains of Good Harbor beach with minerals in the granite exposed on Salt Island one might possibly grasp the depth of geologic time and the extensive evolution required to produce the local beaches enjoyed by all.