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United States presidential line of succession
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The United States presidential line of succession is the order in which officials of the United States federal government discharge the powers and duties of the office of President of the United States if the incumbent president becomes incapacitated, dies, resigns, or is removed from office (by impeachment by the House of Representatives and subsequent conviction by the Senate). The order of succession to the presidency is referred to three times in the U.S. Constitution – in Article II, Section 1, Clause 6; the 20th Amendment; and the 25th Amendment. Since the federal government was established in 1789, Congress has adopted three presidential succession acts. The current presidential line of succession was adopted in 1947 (and last revised in 2006).[1]
The succession follows the order of vice president, Speaker of the House of Representatives, President pro tempore of the Senate, and then the heads of federal executive departments who form the Cabinet of the United States. The Cabinet currently has fifteen members, beginning with the Secretary of State, and followed by the rest in the order of their positions' creation. Those heads of department who are ineligible to act as president are also ineligible to succeed the president by succession, for example most commonly if they are not a natural-born U.S. citizen.
Several constitutional law experts have raised questions as to the constitutionality of the provisions that the Speaker of the House and the President pro tempore of the Senate succeed to the presidency,[2] and in 2003 the Continuity of Government Commission raised a number of other issues with the current line of succession.[3]
Contents
[hide]
1 Current order
2 Eligibility
2.1 Eligibility requirements
2.2 Acting officers
3 Motivation for changes to the succession in 1945
4 Constitutional provisions
5 Acting President of the United States
6 History of succession law set by Congress
6.1 Presidential Succession Act 1792
6.2 Presidential Succession Act 1886
6.3 Presidential Succession Act 1947
6.4 Postmaster General removed 1971
6.5 Secretary of Homeland Security added 2006
7 Successions beyond Vice President of the United States
8 Constitutional concerns
9 See also
10 References
11 External links
Current order[edit]
The current presidential order of succession is as follows:[4]
Key
Eligible Democrat (D)
Eligible Republican (R)
Eligible Independent (I)
Eligible unknown
Not eligible
No.
Office
Current officer
1
Vice President Mike Pence (R)
2
Speaker of the House of Representatives Paul Ryan (R)
3
President pro tempore of the Senate Orrin Hatch (R)
4
Secretary of State John J. Sullivan (R) [a]
5
Secretary of the Treasury Steven Mnuchin (R)
6
Secretary of Defense Jim Mattis (I)
7
Attorney General Jeff Sessions (R)
8
Secretary of the Interior Ryan Zinke (R)
9
Secretary of Agriculture Sonny Perdue (R)
10
Secretary of Commerce Wilbur Ross (R)
11
Secretary of Labor Alex Acosta (R)
12
Secretary of Health and Human Services Alex Azar (R)
13
Secretary of Housing and Urban Development Ben Carson (R)
–
Secretary of Transportation Elaine Chao (R)[b]
14
Secretary of Energy Rick Perry (R)
15
Secretary of Education Betsy DeVos (R)
16
Secretary of Veterans Affairs Robert Wilkie (R)[a]
17
Secretary of Homeland Security Kirstjen Nielsen (I)
^ Jump up to: a b This listing assumes that acting officers whose prior appointment required Senate confirmation are eligible for the line of succession. See Acting officers below.
Jump up ^ Not a natural-born citizen (Chao acquired U.S. citizenship by naturalization) and consequently is ineligible for the presidency.
Cabinet officers are in line according to the chronological order of their department's creation or the department of which their department is the successor (the Department of Defense being successor to the Department of War, and the Department of Health and Human Services being successor to the Department of Health, Education and Welfare).[a]
Eligibility[edit]
Eligibility requirements[edit]
To be eligible to serve as president, a person must be a natural-born U.S. citizen, at least 35 years of age, and a resident within the United States for at least 14 years. These eligibility requirements are specified both in the U.S. Constitution, Article II, Section 1, Clause 5, and in the Presidential Succession Act of 1947 (3 U.S.C. § 19(e)).
Acting officers[edit]
Acting officers may be eligible. In 2009, the Continuity of Government Commission, a private non-partisan think tank, reported,
The language in the current Presidential Succession Act is less clear than that of the 1886 Act with respect to Senate confirmation. The 1886 Act refers to "such officers as shall have been appointed by the advice and consent of the Senate to the office therein named..." The current act merely refers to "officers appointed, by and with the advice and consent of the Senate." Read literally, this means that the current act allows for acting secretaries to be in the line of succession as long as they are confirmed by the Senate for a post (even for example, the second or third in command within a department). It is common for a second in command to become acting secretary when the secretary leaves office. Though there is some dispute over this provision, the language clearly permits acting secretaries to be placed in the line of succession. (We have spoken to acting secretaries who told us they had been placed in the line of succession.)[5]
Motivation for changes to the succession in 1945[edit]
Two months after succeeding Franklin D. Roosevelt, President Harry S. Truman proposed that the Speaker of the House and the President pro tempore of the Senate be granted priority in the line of succession over the Cabinet so as to ensure the president would not be able to appoint his successor to the presidency.[6]
The Secretary of State and the other Cabinet officials are appointed by the president, while the Speaker of the House and the President pro tempore of the Senate are elected officials. The Speaker is chosen by the U.S. House of Representatives, and every Speaker has been a member of that body for the duration of their term as Speaker, though this is not technically a requirement; the President pro tempore is chosen by the U.S. Senate and customarily the Senator of the majority party with the longest record of continuous service fills this position. The Congress approved this change and inserted the Speaker and President pro tempore in line, ahead of the members of the Cabinet in the order in which their positions were established.
In his speech supporting the changes, Truman noted that the House of Representatives is more likely to be in political agreement with the president and vice president than the Senate. The succession of a Republican to a Democratic presidency would further complicate an already unstable political situation. However, the changes to the succession, when signed into law, placed Republican House Speaker Joseph W. Martin first in the line of succession after the vice president.[7]
Some of Truman's critics said that his ordering of the Speaker before the President pro tempore was motivated by his dislike of the then-current holder of the latter rank, Senator Kenneth McKellar.[8] Further motivation may have been provided by Truman's preference for House Speaker Sam Rayburn to be next in the line of succession, rather than Secretary of State Edward R. Stettinius, Jr.[7]
Constitutional provisions[edit]
The line of succession is mentioned in three places in the Constitution:
Article II, Section 1, Clause 6 makes the vice president first in the line of succession and allows the Congress to provide by law for cases in which neither the president nor vice president can serve. The current such law governing succession is the Presidential Succession Act of 1947 (3 U.S.C. § 19).
Section 3 of the 20th Amendment provides that if the president-elect dies before his term begins, the vice president-elect becomes president on Inauguration Day and serves for the full term to which the president-elect was elected. The section also provides that if, on Inauguration Day, a president has not been chosen or the president-elect does not qualify for the presidency, the vice president-elect acts as president until a president is chosen or the president-elect qualifies. Finally, Section 3 allows the Congress to provide by law for cases in which neither a president-elect nor a vice president-elect is eligible or available to serve.
The 25th Amendment, ratified in 1967, clarified Article II, Section 1: that the vice president is the direct successor of the president. He or she becomes president if the president dies, resigns or is removed from office. The amendment also provides for the situation where the president is temporarily disabled, such as if the president has a surgical procedure or becomes mentally unfit.
The amendment also provides for vice presidential succession, by requiring vice presidential vacancies to be filled by the president and confirmed by both houses of the Congress. Previously, whenever a vice president had succeeded to the presidency or had otherwise left the office empty (through death, resignation, or removal from office), the vice presidency remained vacant until the next presidential and vice presidential terms began.
Acting President of the United States[edit]
Main article: Acting President of the United States
New president Lyndon Johnson was sworn in aboard Air Force One, following the assassination of John F. Kennedy in 1963.
Article II, Section 1 of the United States Constitution provides that:
In case of the removal of the President from office, or of his death, resignation, or inability to discharge the powers and duties of the said office, the same shall devolve on the Vice President ... until the disability be removed, or a President elected.
This originally left open the question whether "the same" refers to "the said office" or only "the powers and duties of the said office." Some historians, including Edward Corwin[9] and John D. Feerick,[9] have argued that the framers' intention was that the vice president would remain vice president while executing the powers and duties of the presidency; however, there is also much evidence to the contrary, the most compelling of which is Article I, section 3, of the Constitution itself, the relevant text of which reads:
The Vice President of the United States shall be President of the Senate, but shall have no vote, unless they be equally divided.
The Senate shall chuse [sic] their other officers, and also a President pro tempore, in the absence of the Vice President, or when he shall exercise the office of President of the United States.
This text appears to answer the hypothetical question of whether the office or merely the powers of the presidency devolved upon the vice president on his succession. Thus, the 25th Amendment merely restates and reaffirms the validity of existing precedent, apart from adding new protocols for presidential disability.[original research?] Not everyone agreed with this interpretation when it was first actually tested, and it was left to Vice President John Tyler, the first presidential successor in U.S. history, to establish the precedent that was respected in the absence of the 25th Amendment.
In 1841, John Tyler became the first person to succeed to the presidency.
Upon the death of President William Henry Harrison in 1841 and after a brief hesitation, Tyler took the position that he was the President and not merely Acting President upon taking the presidential oath of office. However, some contemporaries—including John Quincy Adams,[9][10] Henry Clay[11] and other members of Congress[10][11] Whig party leaders[11] and even Tyler's own cabinet[10][11]—believed that he was only acting as president and did not have the office itself.
Nonetheless, Tyler adhered to his position, even returning, unopened, mail addressed to the "Acting President of the United States" sent by his detractors.[12] Tyler's view ultimately prevailed when the Senate voted to accept the title "President,"[11] and this precedent was followed thereafter. The question was finally resolved by Section 1 of the 25th Amendment, which specifies that "In case of the removal of the President from office or of his death or resignation, the Vice President shall become President." The amendment does not specify whether officers other than the vice president can become president rather than acting president in the same set of circumstances. The Presidential Succession Act refers only to other officers acting as president rather than becoming president.
History of succession law set by Congress[edit]
Presidential Succession Act 1792[edit]
Main article: Presidential Succession Act
Wikisource has original text related to this article:
Presidential Succession Act 1792
The Presidential Succession Act of 1792 was the first succession law passed by Congress. The act was contentious because the Federalists did not want the then Secretary of State, Thomas Jefferson, who had become the leader of the Democratic-Republicans, to follow the vice president in the succession. There were also separation of powers concerns over including the Chief Justice of the United States in the line. The compromise they worked out established the President pro tempore of the Senate as next in line after the vice president, followed by the Speaker of the House of Representatives.
In either case, these officers were to "act as President of the United States until the disability be removed or a president be elected." The Act called for a special election to be held in November of the year in which dual vacancies occurred (unless the vacancies occurred after the first Wednesday in October, in which case the election would occur the following year; or unless the vacancies occurred within the last year of the presidential term, in which case the next election would take place as regularly scheduled). The people elected president and vice president in such a special election would have served a full four-year term beginning on March 4 of the next year, but no such election ever took place.
Presidential Succession Act 1886[edit]
Wikisource has original text related to this article:
Presidential Succession Act 1886
In 1881, after the death of President Garfield, and in 1885, after the death of Vice President Hendricks, there had been no President pro tempore in office, and as the new House of Representatives had yet to convene, no Speaker either,[13] leaving no one at all in the line of succession after the vice president. When Congress convened in December 1885, President Cleveland asked for a revision of the 1792 act, which was passed in 1886. Congress replaced the President pro tempore and Speaker with officers of the president's Cabinet, with the Secretary of State first in line. In the first 100 years of the United States, six former Secretaries of State had gone on to be elected president, while only two congressional leaders had advanced to that office. As a result, changing the order of the line of succession seemed reasonable.[citation needed]
Presidential Succession Act 1947[edit]
Wikisource has original text related to this article:
Presidential Succession Act 1947
The Presidential Succession Act of 1947, signed into law by President Harry S. Truman, added the Speaker of the House and President pro tempore back in the line, but switched the two from the 1792 order. It remains the sequence used today[update]. Since the reorganization of the military in 1947 had merged the War Department (which governed the Army) with the Department of the Navy into the Department of Defense, the Secretary of Defense took the place in the order of succession previously held by the Secretary of War. The office of Secretary of the Navy, which had existed as a Cabinet-level position since 1798, had become subordinate to the Secretary of Defense in the military reorganization, and so was dropped from the line of succession in the 1947 Succession Act.
Postmaster General removed 1971[edit]
Until 1971, the Postmaster General, the head of the Post Office Department, was a member of the Cabinet, initially the last in the presidential line of succession before new officers were added. Once the Post Office Department was re-organized into the United States Postal Service, a special agency independent of the executive branch, the Postmaster General ceased to be a member of the Cabinet and was thus removed from the line of succession.[14]
Secretary of Homeland Security added 2006[edit]
The United States Department of Homeland Security was created in 2002. On March 9, 2006, pursuant to the renewal of the Patriot Act as Pub.L. 109–177, the Secretary of Homeland Security was added to the line of succession. The order of Cabinet members in the line has always been the same as the order in which their respective departments were established. Despite custom, many in Congress had wanted the Secretary to be placed at number eight on the list – below the Attorney General, above the Secretary of the Interior, and in the position held by the Secretary of the Navy prior to the creation of the Secretary of Defense – because the Secretary, already in charge of disaster relief and security, would presumably be more prepared to take over the presidency than some of the other Cabinet secretaries.[citation needed] Despite this, the 2006 law explicitly specifies that the "Secretary of Homeland Security" follows the "Secretary of Veterans Affairs" in the succession, effectively at the end of the list.[15]
Successions beyond Vice President of the United States[edit]
David Rice Atchison's tombstone (Plattsburg, Missouri), including the claim "President of the United States for One Day"
While nine vice presidents have succeeded to the office upon the death or resignation of the president, and two vice presidents have temporarily served as acting president, no other officer has ever been called upon to act as president. On March 4, 1849, President Zachary Taylor's term began, but he declined to be sworn in on a Sunday, citing religious beliefs, and the vice president was not sworn either. As the last President pro tempore of the Senate, David Rice Atchison was thought by some to be next in line after the vice president, and his tombstone claims that he was US President for the day. However, Atchison took no oath of office to the presidency either, and his term as Senate President pro tempore had by then expired.[16][17]
In 1865, when Andrew Johnson assumed the presidency on the death of Abraham Lincoln, the office of vice president became vacant. At that time, the Senate President pro tempore was next in line to the presidency. In 1868, Johnson was impeached by the House of Representatives and subjected to trial in the Senate, and if he had been convicted and thereby removed from office, Senate President pro tempore Benjamin Wade would have become Acting President. This posed a conflict of interest, as Wade's "guilty" vote could have been decisive in removing Johnson from office and giving himself presidential powers and duties. Johnson was acquitted by a one-vote margin.
In his book The Shadow Presidents, which he published in 1979, Michael Medved describes a situation that arose prior to the 1916 election, when the First World War was raging in Europe. In view of the contemporary international turmoil, President Woodrow Wilson thought that if he lost the election it would be better for his opponent to begin his administration straight away, instead of waiting through the lame duck period, which at that time had a duration of almost four months. President Wilson and his aides formed a plan to exploit the rule of succession so that his rival Charles Evans Hughes could take over the presidency as soon as the result of the election was clear. The plan was that Wilson would appoint Hughes to the post of Secretary of State. Wilson and his Vice President Thomas R. Marshall would then resign, and as the Secretary of State was at that time designated next in line of succession, Hughes would become president immediately. As it happened, President Wilson won re-election, so the plan was never put into action.[18]
New President Gerald Ford is sworn in following the resignation of Richard Nixon on August 9, 1974.
Since the 25th Amendment's ratification, its Second Section, which addresses vice presidential succession as noted above, has been invoked twice.
During the 1973 vice presidential vacancy, House Speaker Carl Albert was first in line. As the Watergate scandal made President Nixon's removal or resignation possible, Albert would have become acting president and—under Title 3, Section 19(c) of the U.S. Code—would have been able to "act as President until the expiration of the then current Presidential term" on January 20, 1977. Albert openly questioned whether it was appropriate for him, a Democrat, to assume the powers and duties of the presidency when there was a public mandate for the presidency to be held by a Republican. Albert announced that should he need to assume the presidential powers and duties, he would do so only as a caretaker. However, with the nomination and confirmation of Gerald Ford to the vice presidency, which marked the first time the Second Section of the Twenty-fifth Amendment was invoked, these series of events were never tested. Albert again became first-in-line during the first four months of Ford's presidency, before the confirmation of Vice President Nelson Rockefeller,[citation needed] which marked the second time Section 2 of the Twenty-fifth Amendment was invoked.
In 1981, when President Ronald Reagan was shot, Vice President George H. W. Bush was traveling in Texas. Secretary of State Alexander Haig responded to a reporter's question regarding who was running the government by stating:
Constitutionally, gentlemen, you have the President, the Vice President and the Secretary of State in that order, and should the President decide he wants to transfer the helm to the Vice President, he will do so. He has not done that. As of now, I am in control here, in the White House, pending return of the Vice President and in close touch with him. If something came up, I would check with him, of course.[19]
A bitter dispute ensued over the meaning of Haig's remarks. Most people believed that Haig was referring to the line of succession and erroneously claimed to have temporary presidential authority, due to his implied reference to the Constitution. Haig and his supporters, noting his familiarity with the line of succession from his time as White House Chief of Staff during Richard Nixon's resignation, said he only meant that he was the highest-ranking officer of the executive branch of the Federal government of the United States on-site, managing things temporarily until the U.S. Vice President returned to Washington.[citation needed]
Constitutional concerns[edit]
Several constitutional law experts have raised questions as to the constitutionality of the provisions that the Speaker of the House and the President pro tempore of the Senate succeed to the presidency.[20] James Madison, one of the authors of the Constitution, raised similar constitutional questions about the Presidential Succession Act of 1792 in a 1792 letter to Edmund Pendleton.[21] Two of these issues can be summarized:
The term "Officer" in the relevant clause of the Constitution is most plausibly interpreted to mean an "Officer of the United States", who must be a member of the Executive or Judicial Branch. The Speaker and the President pro tempore are not officers in this sense.
Under the principle of separation of powers, the Constitution specifically disallows legislative officials from also serving in the executive branch. For the Speaker or the Senate President pro tempore to become acting president, they must resign their position, at which point they are no longer in the line of succession. This is seen by some to form a constitutional paradox. However, the current Act specifies that the Speaker becomes president "upon his resignation as Speaker" and as a member of Congress,[22] allowing for a seamless transition.
In 2003 the Continuity of Government Commission suggested that the current law has "at least seven significant issues ... that warrant attention", specifically:[23]
The reality that all figures in the current line of succession work and reside in the vicinity of Washington, D.C. In the event of a nuclear, chemical, or biological attack, it is possible that everyone on the list would be killed or incapacitated.
Doubt (such as those expressed above by James Madison) that congressional leaders are eligible to act as president.
A concern about the wisdom of including the President pro tempore in the line of succession as the "largely honorific post traditionally held by the longest-serving Senator of the majority party". For example, from January 20, 2001, to June 6, 2001, the President pro tempore was then-98-year-old Strom Thurmond of South Carolina.
A concern that the current line of succession can force the presidency to abruptly switch parties mid-term, as the president, Speaker, and the President pro tempore are not necessarily of the same party as each other.
A concern that the succession line is ordered by the dates of creation of the various executive departments, without regard to the skills or capacities of the persons serving as their Secretary.
The fact that, should a Cabinet member begin to act as president, the law allows the House to elect a new Speaker (or the Senate, a new President pro tempore), who could in effect remove the Cabinet member and assume the office themselves at any time.
The absence of a provision where a president is disabled and the vice presidency is vacant (for example, if an assassination attempt simultaneously wounded the president and killed the vice president).
See also[edit]
Central Locator System
Designated survivor
List of United States presidential assassination attempts and plots
United States presidential line of succession in fiction
References[edit]
Jump up ^ 61 Stat. 380, codified at 3 U.S.C. § 19
Jump up ^ "Is the Presidential Succession Law Constitutional?", Akhil Reed Amar, Stanford Law Review, November 1995.
Jump up ^ First Report p. 4, continuityofgovernment.org Retrieved December 28, 2010 via Internet Archive to repair dead link
Jump up ^ Wilson, Reid (October 20, 2013). "The Presidential order of succession". The Washington Post. ISSN 0190-8286. Retrieved November 10, 2015.
Jump up ^ "The Continuity of the Presidency," Continuity of Government Commission, June 2009, p. 34. (Archived by WebCite at [1] Accessed: 2012-05-23)
Jump up ^ "Special Message to the Congress on the Succession to the Presidency. June 19, 1945" by President Harry S. Truman; from the American Presidency Project archives
^ Jump up to: a b Wildavsky, Aaron B. (2003). Horowitz, Irving, ed. The revolt against the masses and other essays on politics and public policy (Google Books). Transaction Publishers. p. 122. ISBN 978-0-7658-0960-5. Retrieved July 24, 2009.
Jump up ^ "kipnotes.com - Nachrichten zum trading -". kipnotes.com - Nachrichten zum trading.
^ Jump up to: a b c Schlesinger, Arthur M., Jr. (Autumn 1974). "On the Presidential Succession". Political Science Quarterly. 89 (3): 475, 495–496. JSTOR 2148451.
^ Jump up to: a b c Rankin, Robert S. (Feb 1946). "Presidential Succession in the United States". The Journal of Politics. 8 (1): 44–56. doi:10.2307/2125607. JSTOR 2125607.
^ Jump up to: a b c d e Abbott, Philip (Dec 2005). "Accidental Presidents: Death, Assassination, Resignation, and Democratic Succession". Presidential Studies Quarterly. 35 (4): 627, 638. doi:10.1111/j.1741-5705.2005.00269.x. JSTOR 27552721.
Jump up ^ Crapol, Edward P. (2006). John Tyler: the accidental president. UNC Press Books. p. 10. ISBN 978-0-8078-3041-3. OCLC 469686610.
Jump up ^ Lewis, Charlton Thomas; Willsey, Joseph H. (1895). Harper's Book of Facts: a Classified History of the World; Embracing Science, Literature, and Art. New York: Harper & Brothers. p. 884. LCCN 01020386. Retrieved April 24, 2011.
Jump up ^ "U.S. Postmasters General". postalmuseum.si.edu. Retrieved November 10, 2015.
Jump up ^ US Congress (2005). USA PATRIOT Improvement and Reauthorization Act OF 2005. 109th Congress Public Law 177. Retrieved on October 29, 2015 from http://www.gpo.gov/fdsys/pkg/PLAW-109publ177/html/PLAW-109publ177.htm.
Jump up ^ "1801: President for a Day -- March 4, 1849". May 29, 2014.
Jump up ^ David Mikkelson (October 8, 2007). "President for a Day". snopes.
Jump up ^ Medved, Michael (1979). The Shadow Presidents: The Secret History of the Chief Executives and Their Top Aides. New York, U.S.A.: Times Books. p. 149. ISBN 0-8129-0816-3.
Jump up ^ "Alexander Haig", Time, April 2, 1984
Jump up ^ "Is the Presidential Succession Law Constitutional?" Archived January 24, 2005, at the Wayback Machine., Akhil Reed Amar, Stanford Law Review, November 1995.
Jump up ^ "Article 2, Section 1, Clause 6: James Madison to Edmund Pendleton".
Jump up ^ Wikisource:Presidential Succession Act 1947
Jump up ^ First Report p. 4, continuityofgovernment.org Retrieved December 28, 2010 via Internet Archive to repair dead link
External links[edit]
Ask Gleaves: Presidential Succession from the website of Grand Valley State University
"Presidential Line of Succession Examined", a September 20, 2003 article from the Milwaukee Journal Sentinel
"WI Presidential Succession Act of 1947 held unconstitutional", David Tenner, Usenet group: soc.history.what-if, January 14, 2003.
Fools, Drunkards, & Presidential Succession from the Federalist Society website
Testimony of M. Miller Baker from the U.S. Senate website
The Presidency: Preserving Our Institutions Second report of the Continuity of Government Commission, June 2009 (includes the text of the Presidential Succession Acts of 1792, 1886 and 1947)
[show]
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Presidential line of succession in the United States of America
Vice President (Mike Pence)
Speaker of the House of Representatives (Paul Ryan)
President pro tempore of the Senate (Orrin Hatch)
Secretary of State (John Sullivan[b])
Secretary of the Treasury (Steven Mnuchin)
Secretary of Defense (Jim Mattis)
Attorney General (Jeff Sessions)
Secretary of the Interior (Ryan Zinke)
Secretary of Agriculture (Sonny Perdue)
Secretary of Commerce (Wilbur Ross)
Secretary of Labor (Alex Acosta)
Secretary of Health and Human Services (Alex Azar)
Secretary of Housing and Urban Development (Ben Carson)
Secretary of Transportation (Elaine Chao[c])
Secretary of Energy (Rick Perry)
Secretary of Education (Betsy DeVos)
Secretary of Veterans Affairs (Robert Wilkie)
Secretary of Homeland Security (Kirstjen Nielsen)
Jump up ^ Never confirmed by and with the advice and consent of the Senate and thus ineligible for the presidential line of succession.
Jump up ^ Sullivan serves as the Acting Secretary of State, and is thus ineligible for the presidency. Mike Pompeo has been formally nominated for the position by Donald Trump.
Jump up ^ Although Elaine Chao is the current Secretary of Transportation, she is not a natural-born citizen (acquired U.S. citizenship by naturalization) and thus ineligible for the presidency.
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Seafloor spreading
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Age of oceanic lithosphere; youngest (red) is along spreading centers.
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
Contents
[hide]
1 Significance
2 Spreading center
3 Incipient spreading
4 Continued spreading and subduction
5 Debate and search for mechanism
6 Sea floor global topography: half-space model
7 See also
8 References
9 External links
Significance[edit]
Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure, although there is typically significant magma activity at spreading ridges.[1] At a spreading center basaltic magma rises up the fractures and cools on the ocean floor to form new seabed. Hydrothermal vents are common at spreading centers. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone. Additionally spreading rates determine if the ridge is a fast, intermediate, or slow. As a general rule, fast ridges have spreading (opening) rates of more than 9 cm/year. Intermediate ridges have a spreading rate of 5–9 cm/year while slow spreading ridges have a rate less than 5 cm/year.[2][3]:2
Earlier theories (e.g. by Alfred Wegener and Alexander du Toit) of continental drift postulated that continents "ploughed" through the sea. The idea that the seafloor itself moves (and also carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s.[4] The theory is well accepted now, and the phenomenon is known to be caused by convection currents in the asthenosphere, which is ductile, or plastic, and the brittle lithosphere (crust and upper mantle).[5][clarification needed]
Spreading center[edit]
Seafloor spreading occurs at spreading centers, distributed along the crests of mid-ocean ridges. Spreading centers end in transform faults or in overlapping spreading center offsets. A spreading center includes a seismically active plate boundary zone a few kilometers to tens of kilometers wide, a crustal accretion zone within the boundary zone where the ocean crust is youngest, and an instantaneous plate boundary - a line within the crustal accretion zone demarcating the two separating plates.[6] Within the crustal accretion zone is a 1-2 km-wide neovolcanic zone where active volcanism occurs.[7][8]
Incipient spreading[edit]
This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (April 2015) (Learn how and when to remove this template message)
Plates in the crust of the earth, according to the plate tectonics theory
In the general case, sea floor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift System today.[9] The process starts by heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome (see isostasy). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120 degree angles. These areas are named triple junctions and can be found in several places across the world today. The separated margins of the continents evolve to form passive margins. Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge.
If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift'. As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The Red Sea is an example of a new arm of the sea. The East African rift was thought to be a "failed" arm that was opening somewhat more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment[10] reported that in the Afar region, September 2005, a 60 km fissure opened as wide as eight meters.[11] During this period of initial flooding the new sea is sensitive to changes in climate and eustasy. As a result, the new sea will evaporate (partially or completely) several times before the elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists.
Sea floor spreading can stop during the process, but if it continues to the point that the continent is completely severed, then a new ocean basin is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the Niger Delta. The Niger River has formed in the failed rift arm of the triple junction.
Continued spreading and subduction[edit]
Spreading at a mid-ocean ridge
As new seafloor forms and spreads apart from the mid-ocean ridge it slowly cools over time. Older seafloor is therefore colder than new seafloor, and older oceanic basins deeper than new oceanic basins due to isostasy. If the diameter of the earth remains relatively constant despite the production of new crust, a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust is forced under either continental crust or oceanic crust. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge. Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Ring of Fire of the Pacific Ocean. The Pacific is also home to one of the world's most active spreading centers (the East Pacific Rise) with spreading rates of up to 13 cm/yr. The Mid-Atlantic Ridge is a "textbook" slow-spreading center, while the East Pacific Rise is used as an example of fast spreading. Spreading centers at slow and intermediate rates exhibit a rift valley while at fast rates an axial high is found within the crustal accretion zone.[2] The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced.[12]
Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean.
Debate and search for mechanism[edit]
At the Mid-Atlantic Ridge (and in other areas), material from the upper mantle rises through the faults between oceanic plates to form new crust as the plates move away from each other, a phenomenon first observed as continental drift. When Alfred Wegener first presented a hypothesis of continental drift in 1912, he suggested that continents ploughed through the ocean crust. This was impossible: oceanic crust is both more dense and more rigid than continental crust. Accordingly, Wegener's theory wasn't taken very seriously, especially in the United States.
Since then, it has been shown that the motion of the continents is linked to seafloor spreading by the theory of plate tectonics. In the 1960s, the past record of geomagnetic reversals was noticed by observing the magnetic stripe "anomalies" on the ocean floor.[13] This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from simply towing a magnetometer on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. The seafloor must have originated on the Earth's great fiery welts, like the Mid-Atlantic Ridge and the East Pacific Rise.
The driver for seafloor spreading in plates with active margins is the weight of the cool, dense, subducting slabs that pull them along. The magmatism at the ridge is considered to be "passive upswelling", which is caused by the plates being pulled apart under the weight of their own slabs.[14][15] This can be thought of as analogous to a rug on a table with little friction: when part of the rug is off of the table, its weight pulls the rest of the rug down with it.
Sea floor global topography: half-space model[edit]
The depth of the seafloor (or the height of a location on a mid-ocean ridge above a baselevel) is closely correlated with its age (age of the lithosphere where depth is measured). The age-depth relation can be modeled by the cooling of a lithosphere plate[16][17] or mantle half-space in areas without significant subduction.[18]
In the half-space model,[18] the seabed height is determined by the oceanic lithosphere temperature, due to thermal expansion. Oceanic lithosphere is continuously formed at a constant rate at the mid-ocean ridges. The source of the lithosphere has a half-plane shape (x = 0, z < 0) and a constant temperature T1. Due to its continuous creation, the lithosphere at x > 0 is moving away from the ridge at a constant velocity v, which is assumed large compared to other typical scales in the problem. The temperature at the upper boundary of the lithosphere (z=0) is a constant T0 = 0. Thus at x = 0 the temperature is the Heaviside step function T 1 ⋅ Θ ( − z ) {\displaystyle T_{1}\cdot \Theta (-z)} . Finally, we assume the system is at a quasi-steady state, so that the temperature distribution is constant in time, i.e. T=T(x,z).
By calculating in the frame of reference of the moving lithosphere (velocity v), which have spatial coordinate x' = x-vt, we may write T = T(x',z,t) and use the heat equation: ∂ T ∂ t = κ ∇ 2 T = κ ∂ 2 T ∂ 2 z + κ ∂ 2 T ∂ 2 x ′ {\displaystyle {\frac {\partial T}{\partial t}}=\kappa \nabla ^{2}T=\kappa {\frac {\partial ^{2}T}{\partial ^{2}z}}+\kappa {\frac {\partial ^{2}T}{\partial ^{2}x'}}} where κ {\displaystyle \kappa } is the thermal diffusivity of the mantle lithosphere.
Since T depends on x' and t only through the combination x = x ′ + v t {\displaystyle x=x'+vt} , we have: ∂ T ∂ x ′ = 1 v ⋅ ∂ T ∂ t {\displaystyle {\frac {\partial T}{\partial x'}}={\frac {1}{v}}\cdot {\frac {\partial T}{\partial t}}}
Thus: ∂ T ∂ t = κ ∇ 2 T = κ ∂ 2 T ∂ 2 z + κ v 2 ∂ 2 T ∂ 2 t {\displaystyle {\frac {\partial T}{\partial t}}=\kappa \nabla ^{2}T=\kappa {\frac {\partial ^{2}T}{\partial ^{2}z}}+{\frac {\kappa }{v^{2}}}{\frac {\partial ^{2}T}{\partial ^{2}t}}}
We now use the assumption that v {\displaystyle v} is large compared to other scales in the problem; we therefore neglect the last term in the equation, and get a 1-dimensional diffusion equation: ∂ T ∂ t = κ ∂ 2 T ∂ 2 z {\displaystyle {\frac {\partial T}{\partial t}}=\kappa {\frac {\partial ^{2}T}{\partial ^{2}z}}} with the initial conditions T ( t = 0 ) = T 1 ⋅ Θ ( − z ) {\displaystyle T(t=0)=T_{1}\cdot \Theta (-z)} .
The solution for z ≤ 0 {\displaystyle z\leq 0} is given by the error function erf {\displaystyle \operatorname {erf} } :
T ( x ′ , z , t ) = T 1 ⋅ erf  ( z 2 κ t ) {\displaystyle T(x',z,t)=T_{1}\cdot \operatorname {erf} \left({\frac {z}{2{\sqrt {\kappa t}}}}\right)} .
Due to the large velocity, the temperature dependence on the horizontal direction is negligible, and the height at time t (i.e. of sea floor of age t) can be calculated by integrating the thermal expansion over z:
h ( t ) = h 0 + α e f f ∫ 0 ∞ [ T ( z ) − T 1 ] d z = h 0 − 2 π α e f f T 1 κ t {\displaystyle h(t)=h_{0}+\alpha _{\mathrm {eff} }\int _{0}^{\infty }[T(z)-T_{1}]dz=h_{0}-{\frac {2}{\sqrt {\pi }}}\alpha _{\mathrm {eff} }T_{1}{\sqrt {\kappa t}}}
where α e f f {\displaystyle \alpha _{\mathrm {eff} }} is the effective volumetric thermal expansion coefficient, and h0 is the mid-ocean ridge height (compared to some reference).
Note that the assumption the v is relatively large is equivalently to the assumption that the thermal diffusivity κ {\displaystyle \kappa } is small compared to L 2 / T {\displaystyle L^{2}/T} , where L is the ocean width (from mid-ocean ridges to continental shelf) and T is its age.
The effective thermal expansion coefficient α e f f {\displaystyle \alpha _{\mathrm {eff} }} is different from the usual thermal expansion coefficient α {\displaystyle \alpha } due to isostasic effect of the change in water column height above the lithosphere as it expands or retracts. Both coefficients are related by:
α e f f = α ⋅ ρ ρ − ρ w {\displaystyle \alpha _{\mathrm {eff} }=\alpha \cdot {\frac {\rho }{\rho -\rho _{w}}}}
where ρ ∼ 3.3 g / c m 3 {\displaystyle \rho \sim 3.3g/cm^{3}} is the rock density and ρ 0 = 1 g / c m 3 {\displaystyle \rho _{0}=1g/cm^{3}} is the density of water.
By substituting the parameters by their rough estimates: κ ∼ 8 ⋅ 10 − 7 {\displaystyle \kappa \sim 8\cdot 10^{-7}} m2/s, α ∼ 4 ⋅ 10 − 5 {\displaystyle \alpha \sim 4\cdot 10^{-5}} °C−1 and T1 ~1220 °C (for the Atlantic and Indian oceans) or ~1120 °C (for the eastern Pacific), we have:
h ( t ) ∼ h 0 − 350 t {\displaystyle h(t)\sim h_{0}-350{\sqrt {t}}}
for the eastern Pacific Ocean, and:
h ( t ) ∼ h 0 − 390 t {\displaystyle h(t)\sim h_{0}-390{\sqrt {t}}}
for the Atlantic and the Indian Ocean, where the height is in meters and time is in millions of years. To get the dependence on x, one must substitute t = x/v ~ Tx/L, where L is the distance between the ridge to the continental shelf (roughly half the ocean width), and T is the ocean age.
See also[edit]
Wikimedia Commons has media related to Seafloor spreading.
Divergent boundary
Morley-Vine-Matthews hypothesis
References[edit]
Jump up ^ Tan, Yen Joe; Tolstoy, Maya; Waldhauser, Felix; Wilcock, William S. D. "Dynamics of a seafloor-spreading episode at the East Pacific Rise". Nature. 540 (7632): 261–265. doi:10.1038/nature20116.
^ Jump up to: a b Macdonald, K. C. (1982). "Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone". Annual Review of Earth and Planetary Sciences. 10 (1): 155–190. doi:10.1146/annurev.ea.10.050182.001103.
Jump up ^ Searle, Roger (2013). Mid-ocean ridges. New York: Cambridge. ISBN 9781107017528. OCLC 842323181.
Jump up ^ Hess, H. H. (November 1962). "History of Ocean Basins" (PDF). In A. E. J. Engel; Harold L. James; B. F. Leonard. Petrologic studies: a volume to honor A. F. Buddington. Boulder, CO: Geological Society of America. pp. 599–620. Retrieved 8 September 2010.
Jump up ^ Elsasser, Walter M. (1971). "Sea-Floor Spreading as Thermal Convection". Journal of Geophysical Research. 76: 1101. Bibcode:1971JGR....76.1101E. doi:10.1029/JB076i005p01101.
Jump up ^ Luyendyk, Bruce P.; Macdonald, Ken C. (1976-06-01). "Spreading center terms and concepts". Geology. 4 (6). doi:10.1130/0091-7613(1976)4%3C369:sctac%3E2.0.co;2. ISSN 0091-7613.
Jump up ^ Daignieres, Marc; Courtillot, Vincent; Bayer, Roger; Tapponnier, Paul (1975). "A model for the evolution of the axial zone of mid-ocean ridges as suggested by icelandic tectonics". Earth and Planetary Science Letters. 26 (2): 222–232. doi:10.1016/0012-821x(75)90089-8.
Jump up ^ McClinton, J. Timothy; White, Scott M. (2015-03-01). "Emplacement of submarine lava flow fields: A geomorphological model from the Niños eruption at the Galápagos Spreading Center". Geochemistry, Geophysics, Geosystems. 16 (3): 899–911. doi:10.1002/2014gc005632. ISSN 1525-2027.
Jump up ^ Makris, J.; Ginzburg, A. (1987-09-15). "Sedimentary basins within the Dead Sea and other rift zones The Afar Depression: transition between continental rifting and sea-floor spreading". Tectonophysics. 141 (1): 199–214. doi:10.1016/0040-1951(87)90186-7.
Jump up ^ Bastow, Ian D.; Keir, Derek; Daly, Eve (2011-06-01). "The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading". Geological Society of America Special Papers. 478: 51–76. doi:10.1130/2011.2478(04). ISSN 0072-1077.
Jump up ^ Grandin, R.; Socquet, A.; Binet, R.; Klinger, Y.; Jacques, E.; Chabalier, J.-B. de; King, G. C. P.; Lasserre, C.; Tait, S. (2009-08-01). "September 2005 Manda Hararo-Dabbahu rifting event, Afar (Ethiopia): Constraints provided by geodetic data". Journal of Geophysical Research. 114 (B8). doi:10.1029/2008jb005843. ISSN 2156-2202.
Jump up ^ Bhagwat, S.B. (2009). Foundation of Geology Vol 1. Global Vision Publishing House. p. 83. ISBN 9788182202764.
Jump up ^ Vine, F. J.; Matthews, D. H. (1963). "Magnetic Anomalies Over Oceanic Ridges". Nature. 199: 947–949. doi:10.1038/199947a0.
Jump up ^ Forsyth, Donald; Uyeda, Seiya (1975-10-01). "On the Relative Importance of the Driving Forces of Plate Motion". Geophysical Journal International. 43 (1): 163–200. doi:10.1111/j.1365-246x.1975.tb00631.x. ISSN 0956-540X.
Jump up ^ Patriat, Philippe; Achache, José (1984). "India–Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates". Nature. 311 (5987): 615. Bibcode:1984Natur.311..615P. doi:10.1038/311615a0.
Jump up ^ Sclater, John G.; Anderson, Roger N.; Bell, M. Lee (1971-11-10). "Elevation of ridges and evolution of the central eastern Pacific". Journal of Geophysical Research. 76 (32): 7888–7915. doi:10.1029/jb076i032p07888. ISSN 2156-2202.
Jump up ^ Parsons, Barry; Sclater, John G. (1977-02-10). "An analysis of the variation of ocean floor bathymetry and heat flow with age". Journal of Geophysical Research. 82 (5): 803–827. doi:10.1029/jb082i005p00803. ISSN 2156-2202.
^ Jump up to: a b Davis, E.E; Lister, C. R. B. (1974). "Fundamentals of Ridge Crest Topography". Earth and Planetary Science Letters. North-Holland Publishing Company. 21: 405–413. Bibcode:1974E&PSL..21..405D. doi:10.1016/0012-821X(74)90180-0.
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