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.

A mineral found in evaporite deposits. I.e. precipitated from water. It contains the sulphate group, SO₄^2-.

Biostratigraphic

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

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

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

form limestone.

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

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.

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.

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.

A carbonate rock containing calcium and magnesium.

Evaporite deposit

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

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

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

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

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

An abbreviation for the boundary between the Cretaceous and Tertiary time periods. K

is for 'kreta', meaning chalk, and T is for Tertiary.

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

The molten rocks beneath the Earth's crust.

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.

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

A sedimentary rock consisting of quartz, feldspar and/or fragments of previously

formed rock material (lithics).

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

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.

Sulphur dioxide and sulphur trioxide - atmosphere polluting gases that combine with

water to form sulphuric acid. This is the main cause of acid rain.

Layers of rocks that are distinguishable from each other.

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

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

expelled in the ejecta curtain.

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