Earth Responds to Stress
(Cover image by CF Lovelace & WM House; ArcheanArt ©2021)
I recently wrote about isostatic rebound, a long-term process causing continents to rise after their ice covering melts. The focus was on melting ice sheets over both Greenland and Antarctica. But there is more to the story about how Earth handles stress, or in this case, stress relief. Unfortunately, the process involves earthquakes and volcanos, neither of which are particularly helpful to local residents.
The issue of stress is unavoidable on a dynamic planet, with continents constantly shifting over a liquid mantle, giving rise to plate tectonics. When the plates meet, mountains rise, and ocean trenches appear as deep furrows along the sea bottom. It is a never-ending story of stress and strain. Stress, created by the forces at work moving continents to and fro, leads to strain, where rocks deform to relieve the stress.
Earthquakes are a form of catastrophic stress relief. We refer to it as catastrophic since it happens almost instantaneously. Think of bending a strip of wood versus a piece of copper wire. The copper wire will bend and deform in a slow process to accommodate the force your hands are applying. The wood strip will also start by bending under pressure, but eventually, it will catastrophically fail by breaking in two. There is no noise when the copper wire bends, but the snapping of the wood creates a loud noise as pent-up energy is released.
The same process occurs deep in the Earth, where hot rocks under high-pressure experience plastic deformation like the copper wire. But cooler brittle rock is more like the wood, and it eventually snaps when too much stress accumulates. When the rock snaps, it creates a fault and releases the energy we hear and feel in an earthquake.
What Does Ice Have to do With It?
Earthquakes are simply the rock’s response to stress imbalance. If stresses balance perfectly around a rock, slight increases or decreases in the total stress field usually won’t affect the rock. But if stress in only one direction increases, the stress imbalance transmits to the rock, and when the imbalance is large enough, something must give.
I researched the structural geology of a granite quarry many years ago and examined large sheets of granite that had detached from the rock mass below. The process is common and referred to as exfoliation or spalling. The process occurs almost instantaneously, and quarry workers report it sounds like dynamite going off.
Understanding the origins of granite is critical to deciphering the exfoliation process. Originally the granite was emplaced deep in the Earth’s bowels when molten magma from below forced its way upward and accumulated in the crust as pluton or batholith. The magma cooled and solidified into granite rock under extremely high pressures. Eventually, the granite pluton was uplifted and exposed to the atmosphere by erosion of the overlying rock.
But at the ground surface, the rock is out of balance. Some of the high stress from its formation remains in the crystalline matrix of the rock. A force imbalance occurs when the pressure at the granite’s top surface disappears, and the rock responds by exfoliating to relieve the imbalance.
When thousands of feet of ice melts, a tremendous downward force on the underlying rock disappears, Earth’s crust feels the new stress imbalance. It may respond by faulting, fracturing, and creating earthquakes.
Stress and Strain
Massive ice sheets covered much of the northern hemisphere during the last ice age, and their weight caused the continental crust to sink. Now, many of those ice sheets have melted, and others, like those in Greenland and Antarctica, are still melting. Removal of the ice weight creates stress imbalances, and Earth has no choice but to respond. The primary response is an isostatic rebound, where the land immediately below the former ice sheet slowly rises over many thousands of years.
Scientists expect that part of the rebound process creates earthquakes when brittle sections of crust snap instead of slowly deforming. Smaller earthquakes may have been building for millennia but were suppressed by the weight of the ice sheets. When the downward force of the ice disappears, the faults reactivate.
Another pathway for triggering earthquakes may be renewed volcanic activity. One of the largest U.S earthquakes in modern history, the New Madrid Earthquake, occurred in the middle of the continent. Some scientists speculate that this quake represented a delayed stress response to deep magma attempting to push up through the crust. The extra weight of the ice sheets suppressed the magma bubble, and once the ice melted, the rising bubble set off the New Madrid earthquake.
It is safe to say we don’t fully understand all of the connections between melting ice sheets and increased earthquake and volcanic activity. But we do know that Mother Earth always responds to the stress and strain of daily life, and this response may include increases in earthquake activity.
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