Lecture 5 on Geology: Antarctica's Subglacial Landscape and Geological History
Core Locations:
The SMS core (Southern McMurdo Sound) is the core currently under study.
The MIS core comes from a different location nearby but on the other side of the volcanoes.
While the two cores are expected to show similar patterns due to proximity, they will not be identical because of their positions relative to the volcanic activity.
MIS Core Utility: One of the primary observations to be made is that the dates from the MIS core fill in the chronological gaps or missing time periods found in the SMS core record.
Antarctica’s Subglacial Landscape
Visual vs. Reality: Maps Typically depict Antarctica as a smooth, white surface dominated by glaciers. However, underneath the ice lies a complex topography controlled by underlying geology.
Impact of Melting: If the ice sheets were to melt, the underlying landscape would be revealed. This landscape has been shaped by a long history of continental formation and glacial action.
East vs. West Antarctica: Comparative Basics:
East Antarctica: Characterized by a significantly thicker ice sheet and a much higher elevation of the underlying bedrock.
West Antarctica: Characterized by a thinner ice sheet and a lower-lying land surface underneath.
Topographic Reconstructions:
If the ice were simply stripped away without accounting for crustal adjustment, most of West Antarctica would be below sea level, appearing as a series of islands.
Much of East Antarctica would remain above the waves due to its inherently higher geological elevation.
Data Acquisition and Resolution
Data Density: Geophysical data is significantly more abundant near landing strips and research stations that can support aircraft.
Resolution and Bias: Data resolution is high in accessible areas but remains low in remote regions further from stations.
Technological Tools:
ICESat: This satellite imaging system provides broad coverage of the ice surface.
Radio Echo/Geophysical Tools: These are used to "see" through the ice. Sound is sent down through the ice and the time it takes to bounce back (echo) is measured.
Calculations: These measurements allow scientists to determine the thickness of the ice and the specific depth to the bedrock.
Glacial Erosion and Flow Dynamics
Ice Flow Velocity: Ice does not move uniformly; some areas flow much faster than others. Fast-flowing areas are known as ice streams.
Erosion and Canyons: High-velocity ice streams are responsible for major erosion, carving deep canyons and valleys into the subglacial bedrock. These canyons correlate directly with the fast-moving sections of the ice sheet.
Regional Differences in Erosion:
The edges of the East Antarctic Ice Sheet and a fair amount of the West Antarctic Ice Sheet show significant canyon carving.
The center of East Antarctica experiences very little erosion because the ice flow there is relatively stagnant (it serves as a high point or source area from which ice flows away).
Modeling: Space agencies and universities use animations to represent ice flow speed, which visually resembles river systems flowing from high points toward the ocean.
Isostatic Subsidence and Rebound
Glacial Weight: An ice sheet that is or thick exerts immense pressure on the Earth's crust ( to ago in North America, ice sheets reached their maximum extent).
Subsidence: This weight causes the land surface to subside or sink.
The Hudson Bay Example:
In North America, the ice sheet was centered around Hudson Bay.
Despite the ice having melted approximately ago, Hudson Bay remains a low-lying area.
The crust was bowed down so significantly that it has not yet "bounced back" or achieved full isostatic rebound.
The Great Lakes: These are largely thought to be the result of carving by ancient ice streams.
Antarctic Recovery Models: To understand what Antarctica looked like in the past or will look like in the future, models must adjust elevation based on how much the land will rise (rebound) once the weight of the ice is removed.
Climate Change and Rates of Change
Melting Speed vs. Landscape:
Rapid Melting: If ice sheets melt quickly, the land does not have time to rebound. Consequently, a large portion of the continent remains underwater initially.
Slow Melting: If melting is gradual, the land can rebound synchronously with the ice loss, resulting in the "adjusted elevation" model where more land remains above sea level.
Evidence of Rapid Transitions: Sedimentary evidence from the Ross Sea suggests that while the transition to colder climates (glacial growth) was a long, slow process, the transitions to warmer climates (melting/deglaciation) occurred very rapidly.
Broader Implications: Understanding rates of change is critical in geology and environmental management, such as deciding whether to dredge openings (e.g., Lake Ellesmere) and calculating how quickly natural processes (e.g., sand movement) might reverse those actions.
Geological History and Crustal Composition
Age Disparity:
East Antarctica: Contains the older "core" of the continent, with rocks upwards of (one billion) years old. This old crust is thick and stable.
West Antarctica: Contains much younger rocks, labeled as less than (two hundred million) years old (Jurassic age and younger).
Rock Types:
East Antarctica: Primarily metamorphic rocks.
West Antarctica: Primarily young volcanic and igneous rocks.
Tectonic History:
West Antarctica was once a long-lived subduction zone (a convergent plate boundary), similar to the Andes Mountains in South America today.
This connection traces back to when Antarctica was part of the supercontinent Gondwana. There are significant similarities between the rock types of New Zealand and West Antarctica because both were situated on that tectonic edge.
Current Tectonic State: Today, there is no subduction. Instead, Antarctica is surrounded by divergent plate boundaries.
Active Extension: West Antarctica is currently undergoing extension (rifting), where the crust is being pulled apart and thinned. This thinning brings hot magma closer to the surface.
Volcanic Activity: This thinning results in high heat flow and active volcanoes.
Ross Island is home to Mount Erebus, which is currently active and producing steam.
Numerous active volcanic centers exist throughout the Ross Sea and West Antarctic regions.
Future Geometry: If current extension and rifting continue, West Antarctica may eventually split from East Antarctica, becoming a chain of islands separate from the main continental core.
The Transantarctic Mountains
Location: These mountains mark the boundary between the thick, cold crust of East Antarctica and the thin, hot crust of West Antarctica.
Causes of Uplift:
Subduction Remnants: They were part of an ancient mountain-building event related to the former subduction zone.
Thermal Uplift: Heat from the adjacent rifting in West Antarctica causes the nearby crust to rise.
Crustal Transition: The abrupt change in crustal thickness naturally results in mountainous terrain at the suture/boundary.
Ice Weight Variance: Unlike the center of the continent, the mountains do not have the full weight of the thickest ice sheets depressing them.
Faulting: Active tectonic faults contribute to the high relief.
Future Continental Projections
Scientists use models to predict plate movement over the next (one hundred million) years.
Projected Movements:
Australia and New Zealand: Expected to rotate upward and eventually merge with Asia.
The Americas: South America is projected to move closer to North America.
New Pangaea: Over a hundred million years, most continents may merge back together into a new supercontinent.
Antarctica: The internal rifting between East and West is expected to continue and accelerate.
Questions & Discussion
Question on Mountain Elevation: A student asked why the Transantarctic Mountains have their specific shape/height.
Response: It is an accumulation of factors: remnants of subduction-zone mountain building, thermal expansion from rifting heat making the crust rise, and the location on the edge of the glacier rather than under its center.
Question on Core Sediment Colors: A student asked about the meaning of the colors used in core diagrams.
Response: The colors act as a shortcut for sediment descriptions:
Green: Represents coarser material such as conglomerates and gravels, often including fine material deposited by glaciers.
Gray: Represents muds, typically deposited in open water environments.
Tan: Represents sands, indicating high-energy environments like beaches or near-shore areas.
Question on Tutorial Procedures: A student asked about the nature of the upcoming tutorial work.
Response: Students will plot ages and types of sediment on a graph and perform basic calculations (subtraction and division) to determine rates of environmental change. It is a collaborative group-work session, not a test.