Planet Earth and Earth systems - Physical Processes in Earth and Environmental Sciences - page 13

1.5.5 Further notes on some key interfaces

boundary. It may vary widely in the concentration of dis-
solved gases and also chemically in ways that ultimately
control the style of volcanic eruption. The sudden or
gradual juxtaposition of silicate melt, temperature
600-1,200

Ocean seawater : atmosphere This is a complex interface,
circulating on a rotating spherical Earth, strongly modified
by the polar cryosphere and global sea level dependent upon
polar exchanges. Tides and waves raised on the ocean surface
generate kinetic energy that is dissipated on shallow conti-
nental shelves and coastlines, causing sediment transport,
deposition, and erosion. Atmospheric winds act upon the
ocean's surface and gases mix into the ocean surface. Water
vapor from the ocean surface is transported, the resulting
rainfall washing out continent-derived silicate dusts. Heat
energy is exchanged, but ocean water seldom reaches tem-
peratures

C, containing variable amounts of dissolved
gases under very great pressure, with atmosphere or sur-
face water at c .20 C and 1 bar pressure results in various

possible combinations of slow and fast lava eruption,
explosive fragmentation, and vertical and lateral volcanic
blasts. Volcanic hazard prediction involves second-guessing
the resulting eruption style from basic principles.
Mantle : crust This is the famous MOHO interface
between the mantle and the crust, detectable by either
refracted or reflected seismic waves, the wave speeds
increasing by 25 percent across it. This variation of trans-
missibility matches that predicted between silicate crustal
rocks rich in feldspar and quartz, and silicate upper mantle
rich in the mineral olivine. To their great delight, geolo-
gists also directly recognize the MOHO (a very sharp
interface) within mountain ranges where gigantic faults have
thrust it up toward the surface during past plate collisions.
Fragments of mantle may also turn up in volcanic vents.
Lithospheric plate : asthenospheric mantle This is the
fundamental dynamic interface that enables plate tectonics
to operate. Rigid lithospheric plates slide around the sur-
face of the Earth (velocities of a few centimeters per year)
on a lubricating layer containing a tiny proportion of par-
tially molten rock. The partially molten zone is termed the
low velocity zone (LVZ), since the melt slightly slows
down by (

C. Solar radiation penetrates 10s of meters
into the ocean surface waters causing massive photosynthetic
reactions in the presence of seawater nutrients - biological
gases are added and organic carbon produced.
Continental crust : atmosphere/hydrosphere Humans
in their billions live on, modify, and pollute this delicate
interface. Silicate and other minerals are weathered by
aqueous chemical and biochemical reactions involving
atmosphere and biosphere. The weathered regolith is
mixed with organic breakdown products to form soil that
acts as both valve and filter, regulating the two-way flow
of materials to surface and near-surface ecosystems.
Surface water runs off, over and through rock and soil,
sometimes eroding it. Lakes collect on the interface and
rivers run through it, sometimes cutting down deeply as
tectonics locally uplifts the surface. The rivers flood their
channels in response to extreme rainfall. Atmosphere
flows over the interface, sometimes eroding it, transport-
ing soil particles as sand and silt, the latter lifted into the
atmospheric boundary layer as a dusty aerosol. Effects of
climate change and war wreak havoc at some interface
margins.
Mid-ocean ridge crust : ocean seawater A new plate forms
from the cooling of molten magma rising from the mantle.
Cool seawater penetrates the cracked hot rocks of the ridge
flanks and is redistributed below the surface, some reemerg-
ing as jets of superheated water, the famous “black smokers”
noted in Section 1.3.6, containing dissolved ions scavenged
from the hot rock. Far from the penetrating influence of
solar radiation, entire specialized ecosystems live around the
smokers and in the hot rocks as chemoautotrophs, obtaining
their energy from bacterially induced reactions taking place
between the hot fluid and cool ambient seawater.
Magma/atmosphere/hydrosphere This most spectacular
interface occurs during volcanic eruptions when molten
magma from Earth's interior crust or mantle approaches
the surface. The magma is generated at lithospheric plate
boundaries or by hot plumes rising from the core-mantle

30

1 percent) the passage of certain kinds of
seismic waves across it.
Outer core : mantle This interface is between mantle
silicate rocks transmitting all kinds of seismic waves at
predictable velocities and a metallic liquid core, the latter
demonstrated by widespread wave refraction and disap-
pearance of certain key wave types. The interface is the
ultimate site of submerged lithosphere plate, the so-called
“slab graveyard.” According to some, the interface layer
periodically melts the slabs and erupts molten silicate
material upward like a lava lamp. These plumes rise to the
surface of the Earth to form voluminous volcanic erup-
tions (oceanic islands like Hawaii and Iceland are thought
to overlie such plumes).

1.5.6 Summary of Earth's interfaces

We can distinguish interfaces between compositionally
distinct layers (e.g. the crust-mantle interface at the
MOHO), layers of distinct states of different materials
(e.g. the ocean-atmosphere boundary), and layers of