Million Years Ago: Terminology and Standards in Science
When discussing the vast history of the universe or the evolution of Earth, scientists require precise language to describe immense spans of time. The term million years ago, often abbreviated as Mya, Myr, or Ma, serves as a fundamental unit of measurement equal to 1,000,000 years (1 × 106 years). While it may seem straightforward, the usage of these abbreviations varies significantly across different scientific disciplines.
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Key Facts
- Definition: One million years ago equals 1,000,000 years.
- Common Abbreviations: Mya (Million years ago), Myr (megayear), and Ma (megaannum).
- Disciplinary Use: Myr is standard in astronomy; Ma is preferred in geology.
- SI Compliance: The abbreviation "mya" is not compliant with the International System of Units (SI).
- Standardization: ISO 80000-3 and NIST 811 provide recommended practices for these measurements.
Usage Across Scientific Fields
The application of these terms depends largely on whether a scientist is referring to a quantity of time (duration) or a specific point in time (age).
Earth Science and Cosmology
In cosmology and Earth science, Myr is frequently used. When used alongside Mya or Ma, they create a reference system: one denotes the length of a period, while the other denotes a point in time relative to the present.
Geology vs. Astronomy
In the field of astronomy, Myr is the standard abbreviation for a million years. In geology, however, Myr is considered deprecated. Geologists typically use Ma, conforming to the standards set by ISO 31-1 (now ISO 80000-3) and NIST 811.
The Debate Over Geological Notation
There is an ongoing debate in geology regarding the most accurate way to express time. One traditional approach uses a combination of Ma for points in time and Myr for durations. For example: "The Cretaceous started 145 Ma and ended 66 Ma, lasting for 79 Myr." In this context, "ago" is implied.
Conversely, some argue that mixing Myr and Mya creates inconsistency and leads to capitalization errors. Because "mega" must be capitalized while "million" does not have to be, some prefer to use Ma for both duration and age, explicitly adding the word "ago" or the abbreviation BP (Before Present) to clarify the meaning. In this alternative style, a phrase like "lasting for 79 Ma" refers strictly to the quantity of time.
Conflicts with the International System of Units (SI)
The common abbreviation "mya" fails to comply with the International System of Units (SI) for three primary reasons:
- The Prefix: In SI, a lowercase "m" stands for milli (one thousandth). Therefore, "mya" technically suggests a thousandth of a year. To align closer to SI, some use kya (kilo years ago) or Gya (giga years ago).
- The Unit: The letter "y" or "yr" is not a coherent SI unit. The accepted derived unit is "a" (from the Latin annus), which is a multiple of the SI base unit, the second.
- Point vs. Period: SI units typically measure periods of time, whereas "mya" refers to a specific point in the past.
To resolve these conflicts, scientists often replace "mya" with "Ma." If the context does not make it clear that the date is in the past, "bp" (before present) is added. However, it is noted that "Ma bp" may not mean exactly the same thing as "mya."
Summary of Time Unit Abbreviations
| Abbreviation | Full Term | SI Compliant? | Primary Meaning |
|---|---|---|---|
| Mya | Million years ago | No | Point in time (Past) |
| Myr | Megayear | Partial | Duration/Quantity |
| Ma | Megaannum | Yes | Point in time or Duration |
| Ga | Gigaannum | Yes | Billion years |
Frequently Asked Questions
What is the difference between Ma and Myr?
Generally, Ma (megaannum) is used to denote a specific point in time (e.g., 66 Ma refers to 66 million years ago), while Myr (megayear) is used to denote a duration or quantity of time (e.g., a period lasting 79 Myr).
Why is "mya" considered non-compliant with SI standards?
It is non-compliant because the lowercase "m" is the SI prefix for "milli" (1/1,000) rather than "million," and "y" is not a recognized SI unit for time; the correct symbol is "a" for annus.
Which unit is preferred in astronomy?
In astronomy, Myr is the standard abbreviation used to represent a million years.
What does BP stand for in geological dating?
BP stands for "Before Present." It is often added to time measurements to explicitly clarify that the date refers to a point in the past rather than a duration of time.
How is a billion years represented in SI-compliant notation?
A billion years is represented as Ga, which stands for Gigaannum (one billion years).