Tag: geology

A streamlined, wind-carved ridge of rock or compacted sediment, aligned parallel to the prevailing wind direction. Yardangs form when wind-driven sand abrades softer material, sculpting it into elongated, aerodynamic shapes — narrow at the windward end, wider at the lee. Fields of yardangs can resemble fleets of stone ships sailing across the desert floor.

The slow, large-scale rising of a portion of the earth's crust — driven by tectonic compression, magma intrusion, or the removal of weight (such as the melting of an ice sheet). Uplift is the force that builds mountains, raises coastlines above the sea, and exposes marine fossils on mountaintops. It operates on timescales that make it invisible to any single human life, but its evidence is everywhere: tilted sedimentary layers, raised beaches, river terraces perched high above the current channel. The Himalayas are still rising. Scandinavia is still rebounding from the weight of ice that melted 10,000 years ago.

A stone that has been shaped, faceted, and polished by wind-driven sand — its surface planed smooth on the windward side, its edges sharpened to ridges. Ventifacts are the desert's whittled stones, each one a record of prevailing wind direction and duration. Small ventifacts can be picked up and examined; large ones are boulders sculpted in place over millennia.

The upper surface of the zone of saturated ground — the level below which every pore and fracture in the soil and rock is filled with water. Above the water table, the ground is merely damp; below it, the ground is full. The water table rises with rain and falls with drought, and its depth determines what can grow, where wells must reach, and whether basements flood. It is the hidden waterline of the landscape.

 The irregular, pitted, hummocked terrain that forms when ice-rich permafrost thaws and the ground collapses into the voids left behind. Thermokarst landscapes are full of subsidence pits, slumping banks, tilting trees ("drunken forests"), and shallow lakes that appear, expand, drain, and disappear as the ice beneath them melts. It is the landscape of permafrost coming undone.

The unsorted mixture of clay, silt, sand, gravel, and boulders deposited directly by a glacier — dropped in place as the ice melts, without the sorting that water would provide. Till is the glacier's residue: everything it picked up, ground down, and carried along, dumped in a jumble when the ice retreated. The soil of much of the American Midwest is glacial till.

A narrow ridge of sand or gravel connecting an island to the mainland, or one island to another — built by wave refraction depositing sediment in the sheltered water behind the island. A tombolo is the sea's own causeway, created by the island's interference with the waves.

A series of ocean waves generated by a sudden, large-scale displacement of the seafloor — most commonly by a submarine earthquake, but also by volcanic eruption, underwater landslide, or calving glacier. In open ocean, a tsunami is barely perceptible — a low, fast swell traveling at jet speed across the entire ocean basin. As it enters shallow water near shore, the wave slows, compresses, and rises, arriving as a wall of water that can be 100 feet tall and travel miles inland. The word is singular and plural — one tsunami, many tsunami — and it names not a single wave but a train of waves, the second or third often larger than the first.

A gap in the geological record — a surface between rock layers where time is missing. Below the line, one world; above it, another, separated by millions of years of erosion or non-deposition that left no trace. An unconformity is the earth's silence — the ages it chose not to record.

The process by which one tectonic plate descends beneath another and sinks into the earth's mantle — the recycling mechanism of the planet's surface. At subduction zones, oceanic crust dives beneath continental or other oceanic crust at rates of a few inches per year, generating the planet's most powerful earthquakes, its deepest ocean trenches, and its most explosive volcanic chains. The Pacific Ring of Fire is a ring of subduction zones. The Cascadia subduction zone runs from northern California to British Columbia, where it is overdue for a magnitude-9 earthquake that will reshape the Pacific Northwest.