Science Opportunities - Basic Research Opportunities in Earth Science - page 62

DEEP INTERIOR
Heat from the deep interior powers convection in the Earth's liquid outer
core, generating a planetary magnetic field, and in its solid mantle, driving plate
tectonics and shaping the Earth's surface environment. Mantle convection is the
primary generator of the Earth's topography, and it determines the geographic
distribution of continents and oceans, the chemical composition of the Earth's
surface layers, and the chemical fluxes into the oceans and atmosphere. The
surface structure that results from these deep-seated processes is unlike that of
any other planet in the solar system. Venus, comparable in size and structure to
the Earth, is tectonically active, but shows no evidence of plate tectonics and is
devoid of a planetary magnetic field. Recent space missions provide hints that
Mars, which is much smaller than the Earth, has had plate tectonics and a
planetary magnetic field in the distant geological past, although neither is
currently observed. Why the Earth is so different from its planetary neighbors in
terms of these global features remains the subject of considerable mystery and
controversy.
Major Areas of Investigation
Mantle Convection and Geochemical Reservoirs
The flow pattern of the mantle determines the thermal history of the Earth's
interior and the geological evolution of the continental and ocean floor crust.
There has been no definitive means of imaging this flow pattern, however.
Geophysical observations, particularly seismic tomography, provide snapshots of
mantle structure, while geochemical measurements, particularly isotopic and
trace element analyses, offer constraints on the past evolution of the interior;
these can be related to the pattern of flow, past and present, but only indirectly.
Numerical simulation of three-dimensional convection, which can now reach the
parameter ranges relevant to the mantle, is proving to be an effective tool for
integrating these different types of data into self-consistent models of the
convection system. Laboratory studies conducted at mantle conditions provide
essential constraints on the computational models and their interpretation.
Recent developments in geophysical and geochemical methods yield
enormous improvements in the resolution of mantle features. In particular,
seismic images now reveal that the flow driving plate tectonics, as manifested in
descending lithospheric slabs, extends to the deepest regions of the mantle, at
least in some areas. At the same time, geochemical data supporting the existence
of two or more distinct, relatively unmixed zones of the mantle