Paleozoic Geologic History of Mt. Washington, White Mountains New Hampshire

Mt. Washington, in the White Mountains of New Hampshire have always been near and dear to my heart. As a kid, I took numerous excursions there with my dad and brother and was amazed at the geology and landscapes. It's actually because of these weekend hiking adventures that I became so interested in the natural world around me, eventually leading me to geology.

A more recent trip to the mountain rekindled my childhood scientific curiosities...though several years of geology experience has allowed me to look at the same rocks with a fresh set of eyes. 

View of Mt. Washington Summit from Lake of the Clouds Hut on the
Ammonoosuc Ravine Trail

The White Mountains are actually just a tiny part of the greater Appalachian Mountains, which extend from Alabama to Newfoundland, and are some of the oldest mountains in North America. The Appalachians story began shortly after the breakup of the supercontinent Rodinia, around 750 Ma (million years ago). Rifting of Rodinia created numerous smaller continents, or fragments of landmasses that moved away from the Gondwana landmass. Three of the larger remnant continents include Laurentia (the core of North America), Baltica (making up most of present day Europe), and Avalonia. 

Three continents and the proto-Atlantic Ocean in the early Paleozoic.

Source: (5)

In between these three was another product of the rifting: the Iapetus Ocean. The Iapetus was essentially the proto-Atlantic Ocean, situated between Europe and the Americas today. All in all, the area would have looked something like SE Asia looks like today: a collection of island arcs and with a vast ocean in between. Interestingly, the ocean was so named after the titan "Iapetus" in Greek mythology, who was the father of the "Atlas", after whom the Atlantic Ocean is named.

Snapshot in time showing Laurentia (proto-North America) in the Early Devonian,
400 Ma. Mt. Washington is noted by the red dot - right in the middle of the
newly formed mountain chain from the Acadian Orogeny.
 Source: Modified from (1), (5), and (6). 

Some nicely folded schists near Lake of the Clouds Hut

Just as is done today, sediments were deposited off the shores of these ancient micro-continents into the Iapetus. Most of the rocks that make up Mt. Washington were once part of these sediments that were deposited off the eastern coast of Laurentia in the Late Silurian and Early Devonian periods (around 410 Ma; Eusden et al., 1996). The sediments were predominantly marine turbidites, part of the Littleton Formation, which today comprise most of the Presidential Range (Billings, 1946; Eusden et al., 1996).

Andalusite crystal

Shortly after their deposition (well....geologically shortly that is of course - around 408 Ma), the Littleton sediments got lucky enough to see some tectonic action. Caught between the Avalonia and Luarentian continents, the sediments were subsequently buried and heated to the point of undergoing weak to moderate metamorphism. As the two continents moved closer together, they eventually collided, becoming physically sutured together during the Acadian Orogeny (Eusden et al., 1996) , which affected most of New England, and up through eastern Canada. The Appalachian Mountains that formed from this orogeny were nearly as high as the Himalayas or the Rocky Mountains today, and would have dominated the eastern Laurentian coastline. Further succumbing to the tectonic forces, the Littleton sediments were further metamorphosed into schist grade. Today, most of the rocks that make up Mt. Washington are observed as folded grey biotite-staurolite-garnet schists with interlayered quartzite layers (think of really cooked sand...). The schists were probably formed during the Acadian Orogeny's hayday, between about 490 and 360 Ma (Eusden et al., 1996)

Sillmanite crystals at the summit

Two of the minerals that make up the Mt. Washington rocks though, are quite important for unraveling it's geologic history - andalusite (above left) and sillmanite (above right). Both minerals are polymorphs of each other, meaning they are chemically identical (Al2SiO5), but form at different pressures and temperatures. Geologists often use these polymorphs to track a rocks progression through time, as is shown in the figure below. The path outlined is the path through pressure and temperature the Littleton sediments took on their journey from the red start (410 Ma) to the blue star (today). Because we see andalusite and sillmanite in the Mt. Washington rocks, we know the sediments must have gone through temperature and depths at the yellow and purple starts respectively.

Pressure-Temperature-Time diagram showing the path in time of the
Littleton Sediments and subsequent metamorphic rocks that make up the
Mt. Washington rocks. Modified from: (2) and (3).

The last bits of the Acadian Orogeny were marked by thrust faults and granitic intrusions. The Littleton schists made their slow ascent back up to the surface through uplift and erosion from above. Later magmatism in the Jurassic, and even later glacial processes further shaped the landscape...but we'll end our story here for now.

A successful climb to the summit! Conquering 400 million years
 of  history in only several hours!

Akiel taking in the view....I wonder if he's also marveling at the geology??

Sources:

(1) Ron Blakey (https://www2.nau.edu/rcb7/geology_illustrated.html)
(2) Buchwaldt and Dudas, 2013, DEAPS 2013: Geology of the Mt. Washington area, New Hampshire, a glimpse into the evolution of the Appalachian Mountains.
(3) Eusden, 2010
(4) Eusden et al., 1996
(5)Colorado Plateau Geosystems. Inc., 
(6) http://written-in-stone-seen-through-my-lens.blogspot.ca/2014/10/a-visit-to-miocene-sea-at-marylands.html