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The Rippling Rock of the 28th State

Texas is the 28th of the United States. Here’s what it was busy doing before it became a state.

Texas is the southernmost part of the Great Plains, which ends in the south against the folded Sierra Madre Occidental of Mexico. The continental crust forms a stable Mesoproterozoic craton which changes across a broad continental margin and transitional crust into true oceanic crust of the Gulf of Mexico. The oldest rocks in Texas date from the Mesoproterozoic and are about 1,600 million years old.

El Paso MountainsPhoto credit: World Best Spot

Franklin Mountains
Photo credit: Center for Inter-American and Border Studies

These Precambrian igneous and metamorphic rocks underlie most of the state, and are exposed in three places: Llano uplift, Van Horn, and the Franklin Mountains, near El Paso. Sedimentary rocks overlay most of these ancient rocks. The oldest sediments were deposited on the flanks of a rifted continental margin, or passive margin that developed during Cambrian time.

Enchanted Rock (part of the Llano uplift)Photo credit: A Landing A Day

Enchanted Rock (part of the Llano uplift)
Photo credit: A Landing A Day

This margin existed until Laurasia and Gondwana collided in the Pennsylvanian subperiod to form Pangea. This is the buried crest of the Appalachian Mountains–Ouachita Mountains zone of Pennsylvanian continental collision.

The late Paleozoic mountains collapsed as rifting in the Jurassic period began to open the Gulf of Mexico. Pangea began to break up in the Triassic, but seafloor spreading to form the Gulf of Mexico occurred only in the mid and late Jurassic. The shoreline shifted again to the eastern margin of the state and the Gulf of Mexico passive margin began to form.

Gulf of Mexico sea floorPhoto credit: Port Publishing

Gulf of Mexico sea floor
Photo credit: Port Publishing

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