The Earth Story

Historical footage of the Apollo 13 astronauts training on the slopes of Kilauea during an active eruption in 1970. 

Geology of New England, 1826 #tbt

I was so excited when I found this image that I wasted time I should have been working on an abstract. Oops. This was just too neat.

This is a gigantic geologic cross section of New England drawn about 130 years before the acceptance of plate tectonics. It starts on the left in Boston Harbor and ends in the right at Lake Erie. Some noteable places like Boston, Syracuse, and Utica give context for the cross section.

There’s so much content on here that gives insight to how early geologists viewed this continent – rocks that were gradually folded, single units of shale or greywacke running across half the section, missing faults, but mostly granite/gneiss bodies rising upwards inexplicably to make up the cores of mountain ranges. The Old Red Sandstone, one of the most famous units from Great Britain, even makes an appearance. Today we understand that plate tectonic processes have often caused those igneous cores to be thrust upwards by faults and that some of the igneous rocks west of Boston actually are an accreted island arc terrane.

This file was published last week and made available under a creative commons license from the collections of the New York Public Library. Look closer and see what else you can find.

-JBB

Image credit: The Miriam and Ira D. Wallach Division of Art, Prints and Photographs: Print Collection, The New York Public Library. "Geological profile extending from the Atlantic to Lake Erie." New York Public Library Digital Collections. http://digitalcollections.nypl.org/items/510d47d9-7f0e-a3d9-e040-e00a18064a99

Pulpit terrace print #TBT

Rock fabric #TBT

This picture comes from Pogibshi Island near Kodiak, Alaska and was taken during a geological survey in 1899. Even in this classic image, geologists can recognize two different fabrics showing the start of metamorphism. First you can see the obvious fabric, pointing diagonally up and to the left. There are layers of varying thickness, all parallel. Those layers represent original bedding; these are sedimentary rocks that vary from quartz sand-rich to clay-rich.

If you squint a bit though, there’s a separate pattern, one closer to vertical. This pattern is the beginnings of a metamorphic fabric. It’s not strongly developed in every layer, the clay-rich layers have developed the metamorphic fabric more easily because clay layers grow new minerals at less intense pressure and temperature conditions than quartz-rich layers, but it’s definitely there.

This vertical pattern is forming from the rocks being folded and compressed. The pressure is causing mineral grains within the rocks to grow and they’re forming in such a way that their flat sides are perpendicular to the most powerful stress direction. Once the flat sides of these newly grown minerals mostly line up, there will be easy planes for the rock to break or “cleave” on, leading to the development of a slate or phyllite texture.

-JBB

Image credit: USGS http://library.usgs.gov/photo/#/item/51dc5c7ce4b097e4d3836992

Neat History #TBT

The Yellowstone area was declared America’s first National Park in 1872. For decades, no one really knew what a national park meant other than it was something of an interesting area – the U.S. Army even occupied the territory for years to provide some amount of protection for its resources against outsiders who wanted to make money off of the park’s features.

Even though no one really knew what a national park was, within 6 years of the park being declared, geologists had a basic idea of what was going on there, as shown by this 1878 vintage geologic map. The largest unit on the map is labeled as “rhyolite”. The mappers didn’t know the age of the rhyolite so it isn’t included in the stratigraphic column, but they recognized that most of the park was filled with igneous units of rhyolitic composition. Today we break those units apart based on their ages and eruption characteristics, but for a preliminary survey that isn’t’ bad at all. The darker red splotches are labeled as “basalt”, so the mappers already did recognize there were distinct rock types erupting within the park. Many of the features we’re familiar with in the park today, such as the Grand Canyon of the Yellowstone and the geyser basins, even have the same names as they do today.

The Park is surrounded by mountain ranges filled with Mesozoic-aged sedimentary rocks; those mountain ranges appear on the north, south, and east sides of the map and are marked with their ages (Jurassic, Cretaceous, etc.) with decent accuracy.

Within a few years of the Park’s founding, it was clear to geological surveyors that it was a large area of igneous rocks that had blasted through surrounding sedimentary rocks and mountain ranges. Found that really neat.

-JBB

Image credit: US Library of Congress https://wikimediafoundation.org/wiki/File:1878GeologicMapofYellowstone.jpg