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Mica
Modified Mercalli Intensity scale
I. People do not feel any Earth movement.
II. A few people might notice movement if
they are at rest or on the upper floors of
tall buildings.
III. Many people indoors feel movement.
Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A
few people outdoors may feel movement. Parked cars rock.
V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.
VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off the walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.
VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.
VIII. Drivers have trouble steering. Houses that are not bolted down might shift on
their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack
if the ground is wet. Water levels in wells
might change.
IX. Well-builtbuildingssufferconsiderable
damage. Houses that are not bolted
down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.
X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, and lakes. The ground cracks in large areas. Railroad tracks are bent slightly.
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in
the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.
XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
The Mercalli scale does not use any instrumental measurements. Thus seismologists can use newspaper accounts, diaries, and other historical records to make intensity ratings of past earthquakes. This is helpful for estimating
future hazards.
movement can be compared to the way that water churns over when heated in a saucepan. Where hot rock rises, plumes of molten rock cause the crust to bulge up. They are the spreading boundaries that occur, for the most part, in the center of the oceans and create long lines of volcanic mountains often called midocean ridges, such as the Mid-Atlantic Ridge. Smaller plumes of molten rock rise under the continents and form more isolated volcanoes.
(See also: Mohorovicic discontinuity and Plate.)
Midocean ridge
A long mountain chain on the ocean floor where basalt periodically erupts, forming new oceanic crust. These ridges are connected to plumes of magma rising from the mantle. The largest of these is the Mid-Atlantic Ridge. It is the longest mountain system in the world.
Mineral
From a geologist’s point of view a mineral is any naturally occurring nonliving substance of definite chemical composition— for example, calcite (calcium carbonate).
Core Mantle
Crust
Modified Mercalli scale
A scale for measuring the impact of an earthquake on people
and places (see: Table right).
It is composed of 12 levels
of increasing intensity ranging from imperceptible, designated by Roman numeral I, to catastrophic destruction, designated by XII. It is not a scientific scale because it is based on how severe the earthquake effects are on a place, and that might be influenced by such things as how well the buildings are constructed. It is used in part because, until the last century, when the seismograph was developed, there was no scientific way of comparing earthquakes,
and people had to rely only
on descriptions of the effects.
The Mercalli scale allows rough comparisons between the earthquakes in the past and those
of modern times.
 Mantle—The mantle makes up the largest part of the Earth. The upper mantle is probably molten at least in places. It is the material that experiences convection currents and is responsible for the movement of the Earth’s crustal plates.
Mica
Any sheetlike mineral that breaks up into flakes. There are many micas, but the most common
are brown, white, or colorless muscovite and black biotite. Micas are soft, but quite resistant to weathering. They are flat, platelike crystals that normally form small, shiny specks in a rock. They are very common in igneous rocks, for example, granite.
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