Eastern
U.S. Travels – Diabase and basalt
Kurt C. Friehauf
The first stop on my first fieldtrip with the northeast section of the Geological Society of America visited a diabase outcrop in jungle of a different kind than I’m used to. The fieldtrip mission focused on visiting contacts between sedimentary rocks and the diabases[1] and basalts of New York City and northern New Jersey. The debate between geologists on the fieldtrip centered on the origins of some unusual pale rocks composed mostly of either sodium-rich plagioclase (rocks called “trondjhemites”) and potassium-rich feldspar (rocks called “syenites”). Discussion never actually came to blows – we aren’t that nerdy - rather we all had a great time analyzing the rocks.
The researchers who had spent a lot of time on the outcrops and had actually studied the rocks chemically, microscopically, etc. – the guys who really knew what they were talking about – interpreted these rocks to have formed when the hot magma heated up the surrounding sedimentary rocks enough to partially melt the sedimentary rocks. The melted sedimentary rocks formed new molten magmas, some of which were sodium-rich, and others which were potassium-rich.
My personal feeling, upon seeing the rocks in outcrop only for a few hours without the support of chemical or petrographic data (i.e., just me spouting off) differed pretty dramatically, but that’s part of the fun of science. To me, the rocks looked like the product of hydrothermal activity. Hydrothermal activity is what happens when you have hot water circulating through cracks in the rocks deep within the earth. We see such hot waters jetting out of the ground in the form of geysers at Yellowstone National Park. Hot water can react chemically with the rocks through which it passes, changing the mineral compositions. My impression of the rocks on this fieldtrip was that they resulted from just such water-rock interaction.
Palisades sill – the cliffs of New Jersey/New York City formed by cooling of basaltic magma emplaced during the initial stages of the formation of the Atlantic Ocean (as the Africa-Europe split from North America approximately 200 million years ago). Basaltic magma is a type of gooey, iron- and magnesium-rich molten rock like the stuff that makes up the lava flows in Hawaii and Iceland. It freezes at about 950ºC (that’s 1750ºF) to form a black igneous rock. When basaltic magma freezes quickly, the crystals are very tiny and the rock is called “basalt.” When the same basaltic magma cools slowly, such as when the magma cools in fractures within the Earth, the mineral crystals have time to grow larger and the rock that forms is called “diabase.”
Basalt flows of Hoboken New Jersey – part of the same suite of igneous rocks that formed during opening of the Atlantic Ocean. These are similar to the diabase in composition, but because the magma oozed out onto the earth’s surface as lava flows, it cooled quickly.
Freshly-broken basalt is dark black in color. These outcrops in New Jersey, as you can see, are light tan colored. The tan coloring is a surface feature that is caused by the interaction of rainwater with the rock – a process called chemical weathering. Rainwater reacts with the feldspar minerals in the rock to form a thin veneer of light-colored clay. A good geologist looks at both the fresh surface and the weathered surface because each tells us something different about the rock. The fresh surface tells us the “condition” of the original minerals and highlights some subtle features such as “cleavage,” “twinning,” and “perthitic intergrowths,” which are important ways of distinguishing between some look-alike minerals. The weathered surface, though, highlights the feldspar minerals and the iron-magnesium-rich minerals, allowing for easier distinction from other glassy minerals. Many rock textures can be highlighted, too, by weathering surfaces.
Much of our focus on this trip was along the contact between the igneous rocks and the sedimentary rocks – where the melting would have occurred. Here, we are looking at the contact between basalt and the underlying Lockatong argillite. The Lockatong argillite is a rock that deposited as mud many millions of years before the basaltic lava flow poured out over the top of it. The mud was buried beneath other sediment and solidified into hard rock called argillite by a process called lithification. Erosion then exhumed the argillite, exposing it on the surface where, millions of years later, the basaltic lava flows covered it back up.