Earth and its Place in the Cosmos
Eruption of the Karymsky volcano in Kamchatka, Russia. Karymsky is part of a chain of stratovolcanoes riding above the subducting Pacific Plate.
The collision of India with Asia is responsible for the uplift of the Himalayas and Tibetan Plateau. The rise of the Himalayas was initiated ~50 My ago and had global climatological consequences.
A simulation of gas flow around a planetesimal using the piecewise parabolic method to solve the equations of flow. The planetesimal is the white object at the center. The density of the gas is shown. The gas is assumed to be owing from the left boundary at 8 km/s at a temperature of 400 K and a density of 10-9 g/cm-3. The planetesimal is assumed to have a radius of 1000 km.
Schematic cross-section of Earth's core and lower mantle. Light elements are segregated into the outer core as the inner core grows by solidification. The increasing concentration of light elements in the outer core causes excess light elements to precipitate as a sediment. The sediment accumulates in depressions at the top of the core, which may account for the ultra-low-velocity zones (ULVZs) inferred from seismological observations.
False color image of a 3.7-3.8 Ga sediment (banded iron formation or BIF) from Nuvvuagittuq (Canada). The horizontal yellow lines are made of magnetite, an iron oxide formed by precipitation of dissolved iron in the ocean. The scale bar is 1 cm.
The largest quartz crystal in this group is about 1mm long. It is bounded by several crystal faces and two fractures. About a dozen melt (glass) inclusions occur in the crystal and one of these has a neck that extends all the way to a crystal face. It contains a bubble about 25 microns in diameter that is mostly dark, but with a pinprick of light in its center. The pinprick of light shows that the bubble is transparent, and the dark part results from refraction of light around the low-refractive-index gas in the bubble. Plausibly the bubble is a primary bubble that was accidentally captured as the quartz crystal grew. Study of such hourglass inclusions constrains the decompression history of the magma and its pre-eruptive gas content.
Study of meteorites, which are remnants of the formation of the solar system, provides constraints on how terrestrial planets formed.
Earth's life and its habitable environment are features contingent on the processes that have occurred over the full 4.5 billion year history of the solar system and that occur deep in the Earth's interior. To this end, a variety of solid earth geophysical and geochemical, geochronometric and cosmochemical and cosmophysical research activities are underway within the department and within affiliated parts of the university.
Deep Earth
The restless interior of the Earth continually reshapes our environment. Convection in the rocky mantle powers plate tectonics and drives many of the geological processes we observe at the surface. From deeper in the planet we have indirect observations of turbulent motion in the liquid iron core, which generates the Earth's magnetic field. Many aspects of these processes are poorly understood. Part of the difficulty lies in understanding the state of matter at extreme pressures and temperatures. Another challenge arises in using this information to build models and gain insights into the internal dynamics. We are actively pushing the frontier of high-pressure research using state-of-the-art facilities at Argonne National Laboratory. We are also leading the effort to develop a more comprehensive understanding of internal processes.
Early Earth
No terrestrial rocks formed before 4.03 Ga have survived subsequent reworking by impacts, erosion, and tectonic activity. The only direct witnesses of earlier times, which can appropriately be called the dark age of the Earth, are tiny zircon grains accumulated in younger detrital sediments. There is thus a large gap in the geological record which covers the formation of the oceans, the late heavy bombardment of the Earth by comets and asteroids, and the possible emergence of life. In this respect, the large exposures of sedimentary rocks dated at 3.6-3.8 Ga in Greenland and Canada are unique. We study the isotopic and chemical compositions of these rocks to try to understand, among other things, whether the Earth was habitable by 3.7-3.8 Ga, whether life had already emerged, and if yes, what kind of organisms were present.
Formation of the Elements
The chemical elements have a variety of origins. For example, of the three most abundant elements in the human body, hydrogen was made in the Big Bang that gave birth to the Universe, carbon was mostly made in red giant stars, and oxygen was mostly made in supernovae. We study the origin of the elements by analyzing samples from stars in our laboratories. This work is made possible by a remarkable discovery made nearly 20 years ago at the University of Chicago, that certain kinds of meteorites contain tiny grains of stardust. These stardust grains, which are made of diamond, silicon carbide and graphite, as well as more common silicates, condensed at high temperature from stellar winds and survived a series of potentially destructive events in the interstellar medium, during the formation of the solar system and on asteroidal meteorite parent bodies. Individual stardust grains have huge isotopic anomalies compared to solar system material and each grain provides a record of nucleosynthesis in its parent star.
Formation of the Sun and Planets
The question of the formation of the solar system can be tackled from different points of view. It can be studied by examining meteorites, which are pieces of asteroids that fall on Earth and leave a shooting star in the sky as they heat up and volatilize during atmospheric entry. Some of these meteorites formed at the same time as the Sun and their compositions and properties can therefore be used to infer the conditions that prevailed in the protosolar nebula around the nascent Sun. The science of studying meteorites is called cosmochemistry. Cosmochemistry was born at the University of Chicago in Harold Urey's laboratory and since then scientists at UofC have remained at the forefront of research in this field. Among notable achievments are the development of the theory of condensation in the solar nebula by Larry Grossman, the discovery of isotope anomalies for oxygen by Bob Clayton, and the discovery of presolar grains by Ed Anders.
The Ups and Downs of Mountains and Oceans
Plate tectonics provides a very general explanation for the origin of mountain belts. However, more detailed accounts of the timing and style of mountain building are still the subject of considerable controversy and thus remain an area of active contemporary research. Here we have been using sedimentological and geochemical methods to address particular aspects of the temporal evolution of mountain belts. We have used 40Ar/39Ar thermochronometry to determine the unroofing rate of Himalayan plutons. We have used sediment volumes as a function of age in the Bay of Bengal together with changes in the riverine input of strontium and its isotopes into the oceans to determine a spatially averaged history of denudation of Himalaya-Tibet. A more recent major effort involves using oxygen isotopes to determine how elevation has changed with time in particular parts of Tibet. The origin of mountains is a classic geologic theme, and there is good reason to believe that we are in for an exciting and productive time in addressing this theme through a judicious combination of fieldwork and modern geochemical analytical methods.
Volcanoes and Plutons
Ours is truly an active planet. Every month there are some 10 to 20 volcanic eruptions on earth, and a handful of these, like Kilauea, Hawaii have been continuously erupting for years. Most of these go unnoticed as they are in poorly populated areas remote from where we live. Volcanoes and volcanic rifts mark the locations of plate boundaries and reveal both the places where mantle rises up as well as where crust and lithosphere decend into Earth’s interior. Volcanic rocks contain large crystals that formed before eruption, and these record the evolution of the magma including the formation of bubbles that cause buoyancy and drive eruptions. Students and I study the crystals and their inclusions of glass (formerly melt) for clues to understand why some silicic magmas erupt and others crystallize at depth. I think granitic magmas that crystallize at depth are necessary for the formation of continental crust. Granitic magmas are almost invariably buoyant and should erupt if eruptibility were mainly dependent on their relatively low density. Granitic magmas that do erupt form mainly superficial fragmental rhyolites that are easily eroded. Our studies thus seek factors that either deter eruption or that enhance crystallization at depth. Field and structural relations show that erupted rhyolites and granites may form different parts of the same magma body: the most buoyant parts may erupt leaving behind a largely crystallized and deeply buried part.
Our studies include micro chemical analyses of glasses and crystals using electron and ion microprobes as well as textural analyses of crystals and groups of crystals and bubbles using X-ray tomography available at nearby Argonne National Laboratory.
