Glossary

40Ar/39Ar dating
A method of dating, where the age is determined from the difference between argon-40 and
argon-39 isotope abundances. Dating is possible since the abundance of 39Ar is constant,
whilst that of 40Ar increases with time.

Anhydrite

A mineral found in evaporite deposits. I.e. precipitated from water. It contains the sulphate group, SO42-.

Biostratigraphic

Term describing variations in biotic zones through the sedimentary rock strata.

Biotic zones

Periods of time that are defined by the presence, or dominance, of certain species.

Carbonate

A compound consisting of carbon and oxygen (CO3)2-, often bonded to calcium to
form limestone.

Cenotes

Often water-filled holes found in limestone areas. They form where weakly acidic
waters dissolve the carbonate rock, often leading to underground networks of caverns
forming.

Chron

A period of time over which the direction of the Earth's magnetism is switched
through 180 degrees. It currently points close to the North Pole.

CO, CO2

Carbon monoxide and carbon dioxide - gases that accumulate in the upper atmosphere,
breaking down the ozone layer and allowing an increase in the amount of the sun's
radiation that reaches the Earth's surface; global warming occurs.

Continental crust

Low density rocks that float high on the molten mantle below. In general, this
underlies the land whilst the oceanic crust underlies the sea.

Cretaceous

The period of time between 144 and 65 million years ago.

Cretaceous/Tertiary boundary

The strata in the geologic record that mark the transfer from the Cretaceous to the
Tertiary time periods. These boundary deposits were formed 65 million years ago, and
are found all around the world.

Volatilisation

Formation of volatiles (gases). In this case, they are released from the terget rocks during impact.

Dolomite

A carbonate rock containing calcium and magnesium.

Evaporite deposit

A deposit formed by precipitation from water, during evaporation of that water.

Feldspar

A group of rock-forming silicate minerals (containing the SiO4 tetrahedra).

Fern spike

Refers to the graphical representation of a rapid increase in fern abundance following
an event detrimental to the environment. Ferns grow quickly in environments where
there is little competition for survival, but then decline as more advanced species take
over.

Geophysical techniques

Techniques used to map subsurface geology, using fundamental laws of maths and
physics. These include gravity and magnetic surveying.

Global warming

Increase in mean global temperatures due to gases in the atmosphere insulating the
Earth. Icebergs melt and sea levels rise.

Gravity survey

A survey to measure differences in density between an area and its surrounding rocks.
The anomalies recorded may also represent variation in the size of the object being
surveyed - therefore, interpretation of results is a trade-off between shape and density
of the object being identified beneath the surface.

Impact breccia

A rock consisting of angular fragments thought to have been broken up and ejected
from the crater during impact.

Impact glasses

Glass-like rocks formed by a meteorite impact. They are found within the K/T
boundary deposits.

Impact-wave deposit

Strata formed from deposition from large waves that are formed when a meteorite
lands in the sea. The deposit may be indirectly linked to an impact, since the force of
an impact may trigger other events that will cause these deposits to form e.g.
Submarine slumping or tectonic readjustments.

Impact winter

The cold period thought to follow a large meteorite impact. This may be caused by
impact debris in the atmosphere preventing the sun's radiation from reaching the
Earth.

Iridium levels

Iridium (Ir) is an element present in the Earth's core. Concentrations at the Earth's
surface are thought to be deposited on Earth by meteorite impacts. There is a high
amount of iridium in the K/T boundary deposits.

Isotopic data

An isotope is a particular structural form of an element. Isotopic data shows the
variations in quantity of one isotope of an element relative to another of the same
element.

K/T boundary

An abbreviation for the boundary between the Cretaceous and Tertiary time periods. K
is for 'kreta', meaning chalk, and T is for Tertiary.

Limestone

A sedimentary rock composed of carbonates. Often formed by secretion of calcium
carbonate from plants and animals.

Maastrichtian

The last stage of time before the K/T boundary is reached. I.e. the last 5 million years
of the Cretaceous period.

Magnetic survey

A survey to measure the differences in magnetic intensity and direction that is
preserved in rocks when they are formed. Magnetic intensity refers to the abundance
of magnetic minerals, such as magnetite (Fe3O4). The direction of magnetism is
determined by the position of the Earth's magnetic pole at the time of formation of the
rocks.

Mantle

The molten rocks beneath the Earth's crust.

Marl

A calcium rich mudstone.

Mass extinction

A large extinction of species, often plants and animals, occurring on a global scale.
Analysis of fossil abundance suggests a mass extinction to have occurred at the end of
the Cretaceous period.

Quartz

A relatively unreactive mineral composed of silicon dioxide (SiO2).

Sandstone

A sedimentary rock consisting of quartz, feldspar and/or fragments of previously
formed rock material (lithics).

Shale

A sedimentary rock consisting of clay minerals that often align themselves into stiff
sheets.

Shocked quartz

Quartz grains, exposed to intense pressures, that exhibit multiple sets of planar
elements (stress fractures). Found in areas of shock metamorphism; i.e. impact sites.

SO2, SO3

Sulphur dioxide and sulphur trioxide - atmosphere polluting gases that combine with
water to form sulphuric acid. This is the main cause of acid rain.

Strata

Layers of rocks that are distinguishable from each other.

Stratigraphy

The term used to the describe a unit of strata. I.e. The geology of a vertical section.

Tektites

2-3mm diameter glassy objects formed from molten rock in the impact crater, and
expelled in the ejecta curtain.

Tertiary

The period of time from 65 million years ago to the present. Chicxulub Home Topics