"Stark's Knob is a mass of volcanic rock of unknown age and relations, locally known as a volcano, which probably holds the key to the interpretation of some of the geological history of this area and western New England..."
- From a report of the State Geologist, New York State Museum (1952)

Stark's Knob is formed of volcanic rock, but is not an old volcano. The triangular "volcano shape" shape of the hill is due to early quarrying that removed the east side of the hill. The significance of the hill is that it is a submarine pillow lava called pillow basalt - one of the very few in eastern North America.

Volcanic rock is of two main types: (1) Mount St. Helen's- or east Pacific-type andesites, which result from explosive eruptions of stiff, silica-rich lava (i.e., liquid rock); and (2) the Hawaiian Island- or Icelandic-type that feature flows of very hot basalt lava from deep in the earth.

"Pillows" form when basalt lava flows up from deep cracks (vents) in the earth and erupts under water. A low ridge builds along an elongated vent, and the basalt lava spills away from the vent. The cooling basalt lava rolls downslope, and ball-like forms known as pillows pile up.

Pillow Lava
An example of Pillow Lava in Scotland

The pillows in pillow basalt have convex upper surfaces. When a pillow rolls onto and settles on older pillows, its lower surface shows concave depressions that conform to the rounded tops of underlying pillows. If you inspect Stark's Knob pillows closely, you can determine that this pillow basalt deposit has been tilted sharply to the west because the convex pillow tops are inclined west (away from the Hudson River). Small cavities, sometimes filled with the mineral calcite, are ancient steam bubbles within the pillows.

Stark's Knob and Taconic Mountain-Building

Pillow basalts from western Newfoundland in Canada, to Stark's Knob in New York; and south into New Jersey and Pennsylvania are similar in age and reflect the first orogeny (mountain-building episode) in the Appalachians. This is the ca. 460 to 440 million year old Taconic orogeny.

The Taconic orogeny featured a collision of the eastern edge of ancient North America with a volcanic island arc similar to modern-day Japan. Remnants of this explosive volcanic island arc are preserved along the modern Connecticut River valley.

As the volcanic arc was pushed west into Vermont and eastern New York and the continent buckled in front of it, cracks opened in the earth's crust. The Stark's Knob basalt lava flowed out of these cracks into the sea covering eastern North America. Fossil snails found between the basalt pillows by State Museum researchers indicate an Early or, more likely, Middle Ordovician age of the pillow basalt. These snails also show that the pillows were formed in relatively shallow sea water.

With continued thrusting of the island arc, the Stark's Knob pillow basalt was caught under the Taconic thrusts and up ended, so that it is no longer flat lying.

The site is important as many classes of geology students and professional geologists from far and near visit the site. It provides a rare opportunity to study this phenomenon, in part due to the cross-cutting rock exposure created when mining activities were concentrated here at the turn of the last century.

The reason most people refer to this feature as a "volcano" probably comes from early accounts, beginning in the early 20th century:

It has been described at length in the reports of the Geological Survey as a volcano or volcanic plug and as such is the only geological phenomenon of this kind known to occur in the State of New York.
- From the Report of the Director of the New York State Museum (1913)

The fact that this hill is not evidence of a once thundering volcano standing beside the Hudson River does not detract at all from its monumental geological interest and significance.


The New York State Geological Survey (NYSGS) is a bureau of
the New York State Museum in the New York State Education Department.